DESIGN OF LOW COST RADIOTHERAPY FACILITIES

DESIGN OF LOW COST RADIOTHERAPY FACILITIES FOR BANGLADESH USING LOCAL RESOURCES

ANDDEVELOPMENT OF A COMPUTER PROGRAM FOR

SHIELDING CALCULATION PURPOSES.

M.Sc. THESISPARTIAL FULFILLMENT OF M. Sc DEGREE FOR MEDICAL PHYSICS

DECEMBER, 2005

SupervisorProfessor Dr. G.A. ZakariaDepartment of Medical PhysicsAnd Biomedical Engineering,Gono Bishwabidyalay (University),Dhaka, Bangladesh.

Performed ByMohammad Anwarul IslamDepartment of Medical Physics and Biomedical EngineeringGono Bishwabidyalay(University), Dhaka, Bangladesh.

Email: [email protected]

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DEDICATEDTO

MY PARENTS

mailto:[email protected]

ACKNOWLEDGEMENTS

It is a matter of great pleasure and privilege to record my deepest sense of gratitude, indebtedness and thankful acknowledgement to my respected teacher and supervisor Dr. Golam Abu Zakaria, Professor, Department of Medical Physics and Biomedical Engineering, Gono University, Savar, Dhaka and Head, Department of Medical Physics, Gummersbach Teaching Hospital, University of Cologne, Germany. His constant supervision, indispensable guidance, encouragement, inspiration and invaluable suggestions during the entire period of the research work enabled me to complete the study and to prepare the thesis paper.

I am immensely grateful to my respected teacher to Professor M. Ali Azgar, Head, Department of Medical Physics and Biomedical Engineering Gono Bishwabidyalay (University), Dhaka, Bangladesh, for suggesting me the problem and for his going through the thesis and making valuable corrections and affectionate guidance and inspiration throughout the progress of the work.

My special thanks and regards to M. Jahangir Alam, Senior Medical Physicist, Delta Medical Center, Dhaka, Bangladesh, for suggesting me the problem and for his constant supervision and affectionate guidance and inspiration throughout the progress of the work. I am thankful to prof. G. Hartmann from German Cancer Center, Heidelberg, Germany and Mr. Mansoor Naqvi from Aga Khan University Hospital, Karachi, Pakistan for their effort to go throw the work and for valuable suggestions

I am deeply indebted to Md. Nurul Islam, Asso. professor, Md. Mozibur Rahman, Hasin A. Anupama and Md. Mizanur Rahman, lecturers of Medical Physics and Biomedical Engineering, Department of Gono Bishwabidyalay.

I want to express my sincere thanks to all of my friends, especially to Masud Rana, Dulal Ahmed, Md.faruk hossain and Mahbubur Rahman, lecturer, Kholna Engineering University who helped me and inspired me during the work.

I am also grateful to all the members of my family without whose care and support this work would not be possible.

Moham m ad Anwarul Islam

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ABSTRACT

Radiotherapy is an important method of cancer treatment and is used world wide. During radiotherapy procedures the radiation hazard from the accelerator is an important problem. To overcome this problem, a shielded radiotherapy room is very essential. However, it is very expensive to build this type of shielded room. It is expected that the number of radiotherapy treatment facilities in Bangladesh will be increased soon. Bangladesh is a developing small country with many lacking in resources. This characteristics lead to the demand of low cost treatment facilities. Considering the situation of our country, development of a low cost radiotherapy facility is therefore a timely demand of our people.

The aim of this work is to calculate the basic layout and the dimension of a treatment room, to design protective walls, to calculate the barrier thickness for two typical linear accelerator of a modern radiotherapy department and finally to show low cost but a high dense material from local resources.

For the development of a low cost radiotherapy facility, have been investigated local resources, that means locally available shielding materials such as bricks, gray stone, black stone, steel and lead.

As a first step, cubic size concrete slabs have been made and their shielding characteristics have been determined. For this purpose three materials have been used: bricks, gray stone, and black stone. The density of these concrete materials were measured. Tenth value layers are determined for various types of photon energy. By using this tenth value layer data, values for the shielding thickness were determined. Following this, shielding volume of primary and secondary barrier have been estimated.

In a second step, the cost of concrete in taka per unit volume are taken from local market and with that the costing were analyzed with various room dimensions, various materials and design purposes. Considering and observing all costing factors, a radiotherapy department is designed which offers low cost but maximum facilities. The shielding thickness of 6 MV and 10 MV linac rooms are calculated with the calculation of maze door shielding. After all 6 MV and 10 MV linac rooms are designed and the this costing is compared with that of traditionally designed rooms.

For the calculation of shielding thickness for different energies and materials a computer program was developed, which is practically applicable for shielding calculation. This program takes into account two parts: primary shielding calculation and secondary shielding calculation. All criteria’s of IAEA and ICRP are followed for radiotherapy department designing and computer program development.

In conclusion, it has been shown that the use of a locally available concrete material with density of 2.36 g/cm3 and an appropriate design has a realistic cost advantage.

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Content

1 INTRODUCTION ...................................................................................................... 7 1.1 General ................................................................................................................ 7 1.2 Radiation ............................................................................................................. 8 1.3 Linear Attenuation Coefficients and Exponential Attenuation .......................... 10

2 RADIATION AND DOSE LIMITS ............................................................................. 14 2.1 Primary beam ................................................................................................... 14 2.2 Scattering Radiation .......................................................................................... 14 2.3 Leakage Radiation ............................................................................................ 14 2.4 Background Radiation ....................................................................................... 14 2.5 The Linac X-Ray Beam ..................................................................................... 15 2.6 The Electron Beam ........................................................................................... 15 2.7 Target and Flattening Filter ............................................................................... 16 2.8 Beam Collimation and Monitoring ..................................................................... 16 2.9 Electron therapy mode ...................................................................................... 17 2.10 Types of Radiation Exposure .......................................................................... 18 2.11 Public Dose Limit Recommendation ............................................................... 18

3 RADIATION PROTECTION FUNDAMENTALS ..................................................... 20 3.1 Introduction ........................................................................................................ 20 3.2 International safety standards and their application ......................................... 20 3.3 Basic Framework of Radiation Protection ......................................................... 22 3.4 Barrier ................................................................................................................ 23 3.5 Work Load ......................................................................................................... 23 3.6 Use Factor ......................................................................................................... 24 3.7 Occupancy Factor ............................................................................................. 24 3.8 Barrier Transmission Factor .............................................................................. 24 3.9 Shielding Materials ............................................................................................ 25 3.10 Nature of Area ................................................................................................. 25

4 THEORETICAL FORMULATION OF SHIELDING CALCULATION ....................... 26 4.1 Primary Barrier Transmission Factor Bx ........................................................... 26 4.2 Dose Rate Measurement .................................................................................. 26 4.3 Secondary Barrier Transmission Factor Bp ...................................................... 27 4.4 Leakage Barrier Transmission Factor Bleak ..................................................... 27 4.5 Calculation of Barrier Thickness ....................................................................... 28 4.6 TVL Calculation for New Observed Materials ................................................... 30 4.7 Width and Length of Primary Barrier ................................................................. 34 4.8 Dose Rate ......................................................................................................... 35 4.9 Maze door shielding calculations ...................................................................... 35 4.10 Materials for Neutron Shielding for High Energy Room ................................ 39 4.11 Properties of Shielding Materials .................................................................... 40 4.12 Calculation for Neutron Shielding ................................................................... 40 4.13 Capture Gamma Ray Shielding ...................................................................... 42 4.14 Shielding of Simulator Room .......................................................................... 43 4.15 CT Simulator Room ......................................................................................... 46

5 PROPERTIES OF SHIELDING MATERIALS AND COST ANALYSIS ................... 50 5.1 Introduction ........................................................................................................ 50 5.2 Aim of this work ................................................................................................. 50

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5.3 Essential Properties for Shielding Materials ..................................................... 50 5.4 A shielding material and room design requirements: ....................................... 51 5.5 Conventional Shielding Materials for Gamma Rays ......................................... 51 5.6 Study of various self-fabricated materials of concrete and density measurement ......................................................................................................... 53 5.7 Area of Cost Analysis ........................................................................................ 56 5.8 Shielding Cost Analysis with Considering Various Materials ............................ 60 5.9 Cost analysis for design purposes .................................................................... 62

6 ROOM DESIGN AND SHEILDING CALCULATION ............................................... 64 6.1 Introduction ........................................................................................................ 64 6.2 Design of the Installation ................................................................................... 64 6.3 The basic layout and the dimensions of the treatment room ............................ 65 6.4 Data to calculating wall thickness for primary beam of 6 MV photon ............... 67 6.5 Data to calculating wall thickness for secondary beam of 6 MV photon .......... 69 6.6 Data to calculating wall thickness for primary beam of 10 MV photon ............. 71 6.7 Data to calculating wall thickness for secondary beam of 10 MV photon ........ 72 6.8 Data for maze door calculations of 6 MV photon .............................................. 75 6.9 Conventional room design ................................................................................ 80

7 OBSERVATIONS AND RESULS ........................................................................... 80 7.1 Introduction ........................................................................................................ 81 7.2 Density of the local concrete ........................................................................... 81 7.3 Cost analysis for various room dimensions ...................................................... 81 7.4 Cost analysis for using various shielding materials .......................................... 83 7.5 Model design of a low cost radiotherapy department ....................................... 84 7.6 Shielding thickness for radiotherapy room ........................................................ 86 7.7 Maze door calculation for 6 MV linac ............................................................... 87 7.8 TVL values for various materials and energies ................................................. 87 7.9 Software development ...................................................................................... 88 7.10 Overall findings of this work ............................................................................ 89

8 DISCUSSIONS ........................................................................................................ 90 8.1 About this work .................................................................................................. 90 8.2 Shielding Materials ............................................................................................ 90 8.3 Room dimension and design ............................................................................ 90 8.4 Shielding thickness ........................................................................................... 91 8.5 Software development ...................................................................................... 91 8.6 Maze door calculation ...................................................................................... 91

References ................................................................................................................. 93 Appendix .................................................................................................................... 95

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Chapter 1

1 INTRODUCTION

1.1 GeneralIn Bangladesh, there is no population based cancer registry system. It is estimated that, the incidence of cancer in Bangladesh is about 2,00,000 new cases per year and the total incident of cancer is about 1 million and from its 1,20000 number of patient incidence of breast cancer [1] and the total annual cancer death is about 1,50,000. For cancer treatment 60Co teletherapy unit and Linear Accelerator are being used in different medical hospitals (private and government). Recently an established cancer research organization suggested that 1 linac is essential for every 10 lac people where 200 cancer is incident as 1 lac people in a country. According to these statistics and concept 100 linac machines are essential for Bangladesh. Medical linear accelerator (Linac) machine first developed in Great Britain was followed closely by developments in the United States [3]. Linac are available with energies ranging from 4 to 35 MV. Linac works as dual mode such as electron mode and photon mode.

A radiotherapy department is a special structural building construction to protect people from radiation hazard. Radiotherapy treatment involves doctor, medical physicist, nurse, technician regularly and occasionally involves patient and general public. Radiation are harmful to health, although they are useful in some important cases for mankind. For such use one has to ensure protection against the radiation. The primary aim of radiological protection is to provide an appropriate standard of protection for people without unduly limiting the beneficial practices giving rise to radiation exposure. All those who are concerned with radiological protection have to make value judgments about the relative importance of different kinds of risk and about balancing of risks and benefits.

The risk of hazards from radiation can be minimized by following the three cardinal principles of radiation protection, which are:

a. Maximize the distance from the radiation sourceb. Minimize the time andc. Maximize the shielding effect

A shielding arrangement is a protective barrier against radiation sources, made of different types of materials in accordance with their cost and availability in the country that reduce the dose rate of radiation to an acceptable level recommended by the national and international societies. Shielding needs to meet the demand to contain and limit the radiation such that the people outside the shielding enclosure do not receive the doses beyond the acceptable limit [2]. This has given the following three recommendations for protection against ionizing radiation:

a. No practice shall be adopted unless its introduction produces a net benefit.b.All exposure shall be kept as low as reasonably achievable (ALARA),

economic and social factors being taken into account.

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c. The dose equivalent to individuals shall not exceed the limit recommended by ICRP for appropriate circumstance.

Shields are radiation dependent; hence shield discussion should be preceded by discussion about radiation types. Most radiation fields of interest are the combinations of different kinds of radiation. The most significant radiations to be considered for shielding designing are the fast and thermal neutrons and primary and secondary gamma rays.

1.2 Radiation When the energy transmitted from one place to another without medium in the form of wave, it is called the radiated energy. Heat, Sunlight, Microwave, X-ray, Gamma ray etc are the examples of radiation. There are two types of radiation-

i) Ionizing radiation ii) Non-ionizing radiation

In this work radiation with energy equal to or more than 6 MV are considered.

1.2.1 Sources of Radiation

There are two types of radiation sources, natural radiation source & artificial radiation source:

Natural radiation source: The nuclear particles (protons & neutrons) within the nucleus are in continual motion. As a result of this motion, collisions occur and energy is transferred back and forth from one particle to another. Where it not for the strong forces of attraction that exist between the nuclear species would be formed. In a stable nucleus no particle ever acquires enough energy to escape; however, in a radioactive nucleus, it is possible for a particle, by a series of chance encounters, to gain enough energy to escape from the nucleus. The ejection of a nuclear particle is pure chance guided by a definite statistical probability, and there is no way to decide when any particular nucleus will disintegrate. In this disintegration process of heavy particle, alpha particles, beta particles or gamma rays may be ejected.

Artificial radiation source: X-ray machine, Linear accelerator with high energy devices such as the cyclotron, the betatron, the Van de Graaff generator, and the nuclear reactor produce high energy ionizing radiation.

1.2.2 Radiation Measurement and Units Electron Volt (eV): This is the special unit of energy which is the energy required when an electron of charge falls through 1 volt [3]. Thus, 1 eV = 1.602 10-19 C 1 volt = 1.602 10-19 J.And 1 MeV = 106 eV = 1.602 10-13 J.

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Exposure: It is defined as the charge liberated by the ionizing radiation per unit mass of air and SI unit is expressed in C kg-1[3]. This quantity is used to describe the output of an x-ray generator.

Energy Transfer: The transfer of energy from a photon beam to the medium takes place in two stages, kerma and absorbed dose.

Kerma: In the process of energy transfer, the first stage involves the interaction of the photon with an atom, causing an electron or electrons to be set in motion. The transfer of energy is called kerma. The units of kerma are joules/kilogram, the same as those of absorbed dose, but there is no special unit for kerma.

Absorbed dose: The second stage, involves the transfer of energy from the high energy electron to the medium through excitation and ionization. This type of transfer of energy is called absorbed dose. This quantity that is of more interest in radiotherapy and radiobiology. The absorbed dose is the energy actually retained in the medium and it will be brought about by the ionizations and excitations. Kerma and absorbed dose do not take place at the same location. This quantity is defined as the energy deposited by ionizing radiation per unit mass of material and is expressed in J/kg. This is such an important quantity in radiological science that a special unit called rad. Recently there in more use of SI unit, called gray (Gy). The relation between them 1 Gy = 1 J/kg = 100 rad.

Equivalent dose: A dose of one type of radiation may produce a much larger biological effect than the same dose of a different type of radiation. Therefore, to obtain a quantity that expresses on a common scale the damage incurred by an exposed person the concept of dose equivalent has been introduce:

H (dose equivalent) = D Q Nwhere D is the absorbed doseQ is the numerical quality factor determine by the type of radiationN is the product of other modifying factors that determine the radiobiological damage.

Activity: The activity defined as the number of disintegration per unit time. The activity is the quantity that may be measured directly with a Geiger or scintillation counter. The special unit of activity is Becquerel (Bq) [11].

1 Bq = 1disintegation /second (dps).

The number of atoms present and the activity of the source are related by the transformation constant λ, some times it is called decay constant. The mathematical equation is given by

A= Aoe-λt , (1.1)

Where,

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Ao is the initial activityA is the activity after the time tλ is the decay constant

λ may be defined as another mathematical form such as λ = 0.693/T1/2 where T1/2 is the half life of this radioactive material.}

1.2.3 The interaction of ionizing radiation with matter

When an X- ray beam or photon beam passes into an absorbing medium such as body tissues, some of the energy carried by the beam is transferred to the medium where it may produced biological damage. The energy deposited per unit mass of the medium is known as the absorbed dose and is a very useful quantity for the estimation of biological effect. The biological events that result due to absorbed are quite complicated. The initial step in the process usually involves the interaction between a photons and electrons in the body, resulting in the scattering. Electrons produced due to scattering do ionizations and exitations, which may results in biological damage. Most of the energy however is converted into heat, producing no biological effect. Some of the high-speed electrons may suffer a collision with a nucleus and produce bremsstrahlung. This bremsstrahlung, as well as the scattered radiation, can then undergo interactions in the same way as the original photon. Usually, some 30 interactions are required before all the energy of the photon is converted into electronic motion we need to consider the interactions of high energy photons >10 MV as that results in the production of neutrons and this is another highly important area for considering shielding [3]

X ray photons may interact with the absorber to produce high-speed electrons by important mechanisms known as the photoelectric process, Compton scattering, pair production and photodisintegration processes. A less important process called coherent scattering also takes place.

1.3 Linear Attenuation Coefficients and Exponential Attenuation

Suppose a detector that can record the number of photons that pass through it is placed in an x ray beam at point P in figure(1.1). Let the number of photons recorded be N. If a slab of material of thickness ∆x is placed in the path of the x rays, a number, ∆N of the photons will interact with the attenuator and be removed from the beam. A photon cannot be partially stopped by the atoms in the slab of material[3].

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Fig. (1.1): Diagram illustrating how an attenuator attenuates radiation beam.

The ∆N varies as the product of N and ∆x, and so may be written as this relation

∆N = - μN ∆x (1.2)Where,μ is called the linear attenuation coefficient

This equation can be solved by the calculus and this form is as

N = N0 e-μx (1.3)

Where

N is the transmitted by any thickness xNo is the number incidentx thickness of the barrier

1.3.1 Tenth Value Layer

The thickness of absorber that attenuates the beam to 10% is called the tenth value layer or TVL. The relation between TVL and HVL is given below:

1 HVL = 0.31 TVL or 1 TVL = 3.2 HVL [3].

1.3.2 Half Value Layer

The thickness of absorber that attenuates the beam to 50% is called the half value layer or HVL [11]. Substituting N = 0.5 in equation 1.2 this equation stands,

HVL = x1/2 = 0.693/μ (1.4)

Where

X1/2 is the thickness of the barrier that attenuate 50% of the beam

This equation (1.3) can be written in any of the following ways

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N = No e-0.693 x/x1/ 2 = No

2 –x/x 1/ 2

(1.5)

This equation is valid only if the attenuation coefficient is actually a constant and this is only true if the photons in the incident beam all have the same energy and if the beam is narrow.

Exponential decrease is conveniently plotted on semi log graph paper because a straight line is obtained. It should be noted that the scale goes from 100 to 10. This is called a cycle. If a greater range of values is to be represented, double cycle paper and then from 10 to 1 should be used. It is noted that 10 half value layer gives an attenuation factor of almost exactly 1000 (210 = 1024 ≅1000).

The attenuator, due to thickness ∆x, has been extended in the plane of its cross section and now some photons may be scattered by it so that they reach the detector, with the result that the transmitted number of photons appear to be larger than before. The attenuation coefficient depends on thickness of the attenuator, its area & shape and on the distance between the attenuator and the detector as well as on the photon energy. Considering this factor, equation (1.3) can be written as

N = No e-μx B (x, hν, A,L), (1.6)

WhereB is a rather complicated factor, some times called a photon buildup

factorThe symbols in parentheses indicate the dependence on thickness, energy, area and distance. B can range in magnitude from 1 to 100. It is usually obtained experimentally. It is important in shielding for calculations.

Scattered beam

Incident beam

Fig.(1.2): has been altered to illustrate the effect of a broad beam on a transmission measurement.

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Attenuator

1.3.3 Various Types of Coefficient

The attenuation produced by a layer, ∆x, depends upon the number of electrons and atoms present in the layer. If the layer was compressed to half the thickness, it would still have the same number of electrons and still attenuate the x rays by the same fraction, but of course, its linear attenuation coefficient would be twice as great. Linear attenuations will, therefore, depend upon the density of the material. A more fundamental attenuation coefficient is the mass coefficient, which is obtained from the linear coefficient by dividing by the density, ρ. This coefficient represented by (µ / ρ), is independent of the density. The mass coefficient has the dimension m2/kg. The corresponding attenuation coefficient would now be the electronic attenuation coefficient, represented by eµ or µe, with the dimensions cm2 per electron or m2 per electron.

In a similar way one may compute the number of atoms in a foil and express the thickness of the foil in atoms per m2. The corresponding attenuation coefficient is now the atomic one with dimension cm2 per atom or m2 per atom. The atomic coefficient is Z times as large as the electronic one, since there are Z electrons in each atom. The relations between these coefficients are summarized in the Table (1.1)

Table (1.1): Relation Between Attenuation Coefficient and density [3]

Coefficient Symbol Relation Between Coefficients

Unit of Coefficient

Unit in Which Thickness is Measured

Linear µ -- m-1 mMass µ⁄ρ µ⁄ρ m2/kg kg/m2

Electronic eµ µ⁄ρ* 1/1000Ne m2/el el/m2

Atomic aµ µ⁄ρ* Z/1000Ne m2/al al/m2

whereis the density of the material Ne is the number of electron per g Z is the atomic number of material

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Chapter 2

2 RADIATION AND DOSE LIMITS

2.1 Primary beam

Primary radiation or beam is the radiation directly emitted from the treatment machine through the collimator opening in the case of external sources, and from the radioactive source in the case of brachytherapy and Co-60 units.

2.2 Scattering Radiation

The radiation, which is produced by the scattering of the primary beam from patient, collimator, barrier or air, is called the scattering radiation [4]. The amount of scattered radiation depends on the beam intensity incident on the scatterer, the quality of radiation, the beam size and the angle of incidence.

2.3 Leakage Radiation

Leakage radiation is the radiation that escapes through the shielded head of the therapy unit. Although x ray emission for megavoltage radiation is primarily in the direction of the incoming electrons, there is a limited emission of radiation in other directions. The level of this radiation has to be controlled, partly to protect the patient from unwanted radiation outside the useful field and partly to protect the amount of shielding required from the walls of the treatment room. The specification of the amount of unwanted radiation is normally given in term of leakage radiation, that is, the radiation measured when the radiation beam through the beam defining system is blocked. Leakage radiation of the linac present only “ON” condition, for cobalt unit’s leakage radiation is always present [4].

The average leakage radiation in the region which is shielded only by the movable collimators should be less than 0.5% and the maximum leakage radiation should be less than 2%. The second region comprises a surface, surrounding the accelerator and flight tubes which is 1 m from the electron path, the leakage radiation should not exceed 0.1% of the radiation level on the central axis of the beam and the maximum leakage should not exceed 0.2% [9].

2.4 Background Radiation Radiation is a part of the natural environment. This background radiation is contributed principally by three sources such as the terrestrial radiation, cosmic radiation and radiation from radioactive elements in our bodies [3].

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2.5 The Linac X-Ray Beam

Bremsstrahlung x-rays are produced when the electrons are incident on a target of a high –Z material such as tungsten. The target is water-cooled and is thick enough to absorb most of the incident electrons. As a result of bremsstrahlung-type interactions, the electron energy is converted into a spectrum of x-ray energies with maximum energy equal to the incident electron energy. The average photon energy of the beam is approximately one-third of the maximum energy. It is customary for some of the manufacturers to designate their linear accelerators that have both electron and x-ray treatment capabilities by the maximum energy of the electron beam available. For example, the Varian linac 18 unit produces electron beams of energy 6, 9,12,15 and 18 MV and x-rays of energy 10 MV. It should be noted that the electron beam is designated by million electron volts because it is almost monoenergetic before incidence on the patient surface. The x-ray beam, on the other hand, is heterogeneous in energy and is designated by mega volts, as if applying that voltage across an x-ray tube produced the beam.

Fig. 2.1: Component of treatment head for x-ray therapy mode.

2.6 The Electron Beam

The electron beam, as it exits the window of the accelerator tube is a narrow pencil of about 3 mm in diameter. In the electron mode of linac operation, this beam, instead of striking the target, is made to strike an electron scattering foil in order to spread the beam as well as get uniform electron fluence across the treatment field. The scattering foil consists of a thin metallic foil, usually of lead. The thickness of the foil is such that most of the electrons are scattered instead of suffering breamsstrahlung. However, a small fraction of the total energy is still converted into breamsstrahlung and appears as x-ray contamination of the electron beam. In some linacs, the broadening of the electron beam is accomplished by electromagnetic scanning of the electron pencil beam over a large area. Although this minimizes the

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x-ray contamination, some x-ray are still produced by electrons striking the collimator walls or other high atomic number materials in the electron collimation system.

Fig. 2.2: Component of treatment head for electron therapy mode

2.7 Target and Flattening Filter

Target is the rotating window generally made by tungsten which placed on the transmission path of the electron. The efficiency of x-ray production increases very rapidly with increasing electron energy for machines operating in the megavoltage range, Target heating is not problem and can be controlled by cooling water flowing through a copper block into which the target is fitted. For a given electron energy the photon spectrum generated depends on the atomic number and the thickness of the target, thickness being expressed in terms of electron range in the material of the target. Linear accelerators produce electrons in the megavoltage range, the x-ray intensity is peaked in the forward direction. To make the beam intensity uniform across the field, a flattening filter is inserted in the beam. This flattening filter substantially reduces the dose rate at the beam center [4].

2.8 Beam Collimation and Monitoring

The treatment beam is first collimated by a fixed primary collimator located immediately beyond the x-ray target. In the case of x-rays, the collimated beam then passed through the flattening filter. In the electron mode, the filter is moved out of the way. The flattened x-ray beam or the electron beam is incident on the dose monitoring chambers. The monitoring system consists of several ion chambers or a single chamber with multiple plates. Although the chambers are usually transmission type, i.e., flat parallel plate chambers to cover the entire beam, cylindrical thimble chambers have also been used in some linac. This filter is usually made of lead, although tungsten, uranium, steel, aluminum, or a combination has also been used or suggested. The function of the ion chamber is to monitor dose rate, integrated dose, and field

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symmetry. Since the chambers are in a high-intensity radiation field and the beam is pulsed, it is important to make sure that the ion collection efficiency of the chambers remains unchanged with changes in the dose rate. Bias voltages in the range of 300 to 1000 V are applied across the chamber electrodes, depending on the chamber design. Contrary to the beam calibration chambers, the monitor chambers in the treatment head are usually sealed so that their response is not influenced by temperature and pressure of the outside air. However, these chambers have to be periodically checked for leaks.

2.9 Electron therapy mode

After passing through the ionization chambers, the beam is further collimated by a continuously movable x-ray collimator. This collimator consists of two pairs of lead or tungsten blocks, which provide a rectangular opening from 0×0 to the maximum field size (40×40 cm2 or a little less) projected at a standard distance such as 100 cm from the x-ray source. The collimator blocks are constrained to move so that the block edge is always along a radial line passing through the target. The field size definition is provided by a light localizing system in the treatment head. A combination of mirror and a light source located in the space between the chambers and the jaws projects a light beam as if emitting from the x-ray focal spot. Thus the light field is congruent with the radiation field. Frequent checks are required to ensure this important requirement of field alignment. Whereas the x-ray collimation systems of most medical linacs are similar, the electron collimation systems vary widely. Since electrons scatter readily in air, the beam collimation must be achieved close to the skin surface of the patient. There is a considerable scattering of electrons from the collimator surfaces including the movable jaws. Dose rate can change by a factor of two or three as the collimators are jaws are opened to maximum field size limited. If the electrons are collimated by the same jaws, as for x-rays, there will be an extremely stringent requirement on the accuracy or the jaw opening, output so critically depends on the surface area of the collimator. This problem has been solved by keeping the x-ray collimator wide open and attaching an auxiliary collimator for electrons in the form of trimmers extended down to the skin surface. In other systems, the auxiliary electron collimator consists of a set of attachable cones of various sizes. It should be mentioned that due to the electron scattering the dose distribution in an electron field is significantly influenced by the collimation system provided with the machine. Treatment Head (Gantry) The treatment head (Fig 2.1.8.) consists of a thick shell of high-density shielding material such as lead, tungsten, or lead-tungsten alloy. It contains an x-ray target, scattering foil, flattening filter, ion chamber, fixed and movable collimator, and light localized system. The head provides sufficient shielding against leakage radiation in accordance with radiation protection guidelines. Most of the linear accelerators currently produced are constructed so that the source of radiation can rotate about a horizontal axis (Fig.2.1.9). As the gantry rotates, the collimator axis moves in a vertical plane. The point of intersection of the collimator axis and the axis of rotation of the gantry is known as the isocenter. The isocentric mounting of the radiation machines has advantages over the units that move only up and down. The latter units are not suitable for isocentric treatment techniques in which beams are directed from different directions but intersect at the same point, the isocenter, placed inside

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the patient. However, the nonisocentric units are usually swivel mounted, i.e., the treatment head can be swiveled or rotated in any direction while the gantry can move only upward or downward. Although these units are not as flexible, they are mechanically simpler, more reliable, and less expensive than the isocentric models.

Fig.2.8: Photograph of linear accelerator.

2.10 Types of Radiation Exposure

The radiation exposure are, therefore, divided into three categories:

Occupational exposure is defined as all exposures of workers incurred in the course of their work.

Medical exposure is defined as exposure incurred:by patient as part of their own medical or dental diagnosis or treatment; by persons, other than those occupationally exposed, knowingly while voluntarily helping in the support and comfort of patients;

Public exposure is defined as exposure incurred by members of the public from radiation sources, excluding any occupational or medical exposure and the normal local natural background radiation but including exposure from authorized sources and practices and from intervention situations.

2.11 Public Dose Limit Recommendation

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The annual radiation dose limit recommended for individual members of the public from all radiation sources other than natural background and the individual’s medical care is as follows-For members of the public who are exposed continuously or frequently, the recommended annual effective dose limit is 1 mSv [10].

On an infrequent basis, a member of the public may receive more than 1mSv y–1. In such a case, the annual effective dose may exceed 1 mSv up to a value of 5 mSv. This Statement recommends that the term “infrequent,” in the context used here, should refer to a justified exposure that is not likely to occur often in an individual’s lifetime, with each occurrence justified independently of any other.

2.11.1 Dose Limit for Pregnant Women

The pregnant women who are a radiation worker can be considered as an occupationally exposed individual, but the fetus cannot. The total dose equivalent limit to an embryo- fetus is 1mSv or public dose limit 1 mSv/ year.

2.11.2 Occupational radiation Dose Limit

The people who directly work in radiotherapy department and involved in radiotherapy such as medical physicist, radiotherapy technician, radiotherapist, nurse are called occupational radiation exposure. The radiation dose limit of this type of worker is 20mSv/year.

2.11.3 Nonradiational worker Dose Limit

In our context, these are persons who work in a hospital but do not work with radiation. The radiation dose limit in this type of worker is 2mSv/year.

19

Chapter 3

3 RADIATION PROTECTION FUNDAMENTALS

3.1 Introduction

Soon after the discovery of x rays by Roentgen in 1895 and of natural radioactivity by Bequrel in 1896 it became apparent that ionizing radiation was not only useful for diagnosis and treatment of disease but also harmful to human tissues. It has been recognized since early studies on x rays and radioactive minerals that exposure to high levels of radiation can cause clinical damage to tissue of the human body.

Ionizing radiation and radioactive substances are natural and permanent features of the environment, and thus the risks associated with radiation exposure can only be restricted, not eliminated entirely. Additionally, the use of human made radiation is now widespread. Sources of ionizing radiation are essential to modern health care: disposable medical supplies sterilized by intense radiation have been central to combating disease; radiology and nuclear medicine are a vital diagnostic tool; and radiotherapy is commonly part of the treatment of malignancies. Application of ionizing radiation are growing in industry, agriculture, medicine and many other fields of industry and research, benefiting the humanity.

The acceptance by society of risks associated with radiation is conditional on the benefits to be gained from the use made of radiation. Nonetheless, the risks must be restricted and protected against by the application of radiation safety standards. It is therefore essential that activities involving radiation exposure be subject to certain standards of safety in order to protect the individuals that are exposed to radiation, be it:

(i) Occupational(ii) For medical diagnostic or therapeutic purposes(iii) As a member of the public.

3.2 International safety standards and their application

Establishing nuclear safety standards and providing for their application are statutory functions of the Agency, essential for a global safety regime that provides for protection of people and the environment. Notable achievements have been made in 2003 using the Agency standards to enhance nuclear safety in Member States. Central to the worldwide outreach and application of the agency safety standards is, however, the need to ensure an effective process to take into account the practical experience feedback of the application of Agency safety standards in Member States. In November 2001, the Commission on Safety Standards proposed to the Director General a strategy for the safety standards programme aimed at enhancing the standards and their global application. The strategy was prepared in consultation

20

with the various safety committees and submitted to the Board of Governors in September 2003 and to the 2003 session of the General Conference. The relevant documents are:

1. An overview of the IAEA safety standards: A brochure explaining the philosophy, structure, scope and means of application of the corpus of safety standards.

2. Overall structure of the IAEA safety standards: A document showing that all necessary activities and issues are appropriately covered in the standards and that there is an appropriate combination of ‘thematic’ and ‘facility specific’ standards.

3. An action plan for the development and application of safety standards is being submitted to the Board of Governors in March 2004.1 The action plan pays special attention to collecting experience feedback on the use of safety standards and to the review of Agency safety related publications developed outside the Agency’s safety standards programme.

6. The overview and the overall structure of the safety standards were presented at the Scientific Forum held during the 2003 session of the General Conference. There was general agreement that the Agency safety standards reflect a high level of safety and should serve as the global reference for the protection of people and the environment. Many regulatory bodies of Member States are using the Agency standards as reference for national regulations. In other Member States, regulators are called upon to ensure that their regulations are in agreement with the Agency standards and the levels of safety expressed in them.

7. The Agency continued to place considerable emphasis on pursuing the world wide application of the IAEA Safety Standards. The Agency’s safety standards are being used by some counties as a reference in the preparation of national reports and for the peer review under the Convention on Nuclear Safety and the Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management.

8. International and national standards organizations develop industrial standards that complement the Agency safety standards by specifying detailed requirements for design and operation of components and for procedures. Arrangements exists between the IAEA and standards organizations such as the International Organization for Standardization (ISO), and the International Electro technical Commission (IEC) to use a common structure and share glossaries of terms.

9. Professional societies, as for example, those in the medical area, are also involved in the development and review of Agency safety standards.

10.Among the safety standards published in 2003, were two safety requirements entitled Site Evaluation for Nuclear Installations (Safety Standards Series No.NS-R-3) and Remediation of Areas Contaminated by Past Activities and Accidents (Safety Standards Series No. WS-R-3). The safety requirements on

21

site evaluation for nuclear installations is an update of an earlier publication on site selection. The focus of the new publication is primarily on the evaluation of existing sites rather than on the selection of new ones. Requirements for site evaluation are intended to ensure adequate protection of site personnel, the public and the environment from the effects of ionizing radiation arising from nuclear installations. It provides specific requirements for the evaluation of external natural events (such as earthquakes, flooding, extreme meteorological conditions and geotechnical hazards) and human induced events (such as aircraft crashes and chemical explosions). The publication also covers the potential effects of the installation on the region (such as uses of land and water, population distribution in the region and dispersion of radioactive material in the atmosphere, surface water and through ground water). The safety requirements on remediation of areas contaminated by past activities and accidents establishes, for the first time, requirements in relation to protective actions and remedial measures intended to reduce actual prolonged exposure, o avert potential prolonged exposure or to reduce the likelihood of occurrence of such exposures due to contamination. It includes remedial measures such as removal or reduction of the source of exposure as well as other long-term protective actions such as restrictions on the consumption of foodstuff, grazing by livestock and the use of fodder, and restrictions on access or on land use. In addition, nine Safety Guides were published in 2003: seven on various aspects of nuclear power plants; and two on management of radioactive waste.

3.3 Basic Framework of Radiation Protection

The principle of protection and safety upon which the safety standard are based are those developed by the ICRP. The detailed formulation of these principles can be found in the ICRP publications and they cannot easily be paraphrased without losing their essence. However, a brief, although simplified, summary of the principles is as follows[3]:

1. A patient that entails exposure to radiation should only be adopted if it yield sufficient benefit to the exposed individuals or to the society to outweigh the radiation determent it cause.

2. Individual doses due to the combination of exposures from all relevant practices should not exceed specified dose limit for occupational and for public exposure; dose limit are not applicable to medical exposure.

3. Radiation sources and installations should be provided with the best available protection and safety measure under the prevailing circumstances, that the magnitudes and likelihood of exposure and the number of individuals exposed be as reasonably, achievable, economic and social factors being taken into account, and the doses they deliver and the risk they entail be constrained.

22

3.4 BarrierProtective barriers are deigned to ensure that the dose equivalent received by any individual does not exceed the applicable maximum permissible value. Protection is required against three types of radiation: the primary radiation, the scattered radiation, and the leakage radiation through the source housing.

3.4.1 Primary Barrier

A barrier sufficient to attenuate the useful beam to the required degree is called the primary barrier. Primary barrier attenuate the primary, scatter, and leakage radiation with acceptable level.

3.4.2 Secondary Barrier

The required barrier against stray radiation (leakage and scatter) is called the secondary barrier.

3.5 Work Load

Workload means the total absorbed dose delivered by the radiation device per interval of time. It is depends on the working time of the machine. Total workload is the sum of the three categories of workload such as:

Clinical workload (Wclin), which depends on the number of patients take radiotherapy per day and the average amount of dose, used per patient.

Wclin = Diso * Np* Nd*Ny (3.1) Where

Diso is the delivered dose at the isocenter per patient (average 3.3 Gy)Np is the number of patient per working dayNd is the number of working day per weekNy is the total number of working day per year

Physics workload Wphys includes use of the linac for calibration, quality assurance, phantom measurements servicing and maintenance.

Research workload Wrech is the important for developing country because there are no radiotherapy facilities available for the students. They collect data by the use of radiotherapy machine for research purpose. In my investigation, Bangladeshi students working here normally off day of the week in the private radiotherapy department.

Total linac workload Wtot at the machine isocenter is

Wtot = Wclin + Wphys + Wrech (3.2) 23

When selecting the workload, one chooses the maximum expected value of the dose of 1 meter from the target in place of an average value. Most shield designs seem to be based on the value given in 1000 gy/week from x-ray beam in the range 0.5 to 10 MV and the value of 500 Gy/week for higher energy machines [5].

3.6 Use Factor

Use factor U is the fraction of the BEAM ON time during which the primary beam is directed toward a particular barrier. The following primary beam use factors are usually assumed for external beam machines: U (floor) = 1; U (walls) = 0.25; U (ceiling) 0.25;. For all secondary barriers U is always equal to 1, since secondary radiation is always present [4].

3.7 Occupancy Factor

Occupancy factor T is a factor with which workload is multiplied to account for the degree of occupancy of the area in question. The fraction of the time that a person will be in the area out side the barrier. Typical values: T (offices) = 1; T (corridors) = 0.25; T (waiting) = 0.125 [3].

Table 3.1: Occupancy factors (T) for non-occupationally exposed personnel

Occupancy factor Type of Area

1 Full occupancy: Work area such as offices, shops, laboratories, children’s play areas, occupied nearby buildings, living quarters, wards, nurses stations

1\4 Partial Occupancy: corridors, rest room, elevators using operators, unattended parking lots

1\16 Occasional occupancy: waiting rooms toilets, stairways, unattended elevators, janitors closets, outside areas used only for pedestrians or vehicular traffic.

3.8 Barrier Transmission FactorBarrier transmission factor B provides the fraction of the incident beam air-karma in air transmitted through a given thickness of shielding material. Primary, scatter, and leakage barrier transmission factors Bpri , Bscat , and Bleak respectively , must be calculated and the required barrier thickness is then determined using published graphs of transmission factors against shielding material thickness for various beam energies and shielding materials [2].

24

3.9 Shielding Materials

Shielding materials are materials used in primary and secondary barriers to provide shielding against the primary, scatter, and leakage radiation produced in the radiotherapy treatment room.

3.10 Nature of Area

There are two types of area out side of the barrier – Controlled area and Uncontrolled area. To calculate the barrier thickness of primary or secondary barrier it is very important to know the nature of the area. Because permissible dose is different for controlled area and uncontrolled area. Controlled area means restricted zone for unauthorized public and Uncontrolled area means free accessible for any public.

25

Chapter 4

4 THEORETICAL FORMULATION OF SHIELDING CALCULATION

4.1 Primary Barrier Transmission Factor Bx

The transmission of the barrier (Bx) required to reduce the primary radiation field to an acceptable level outside the barrier is given by the equation 4.1. The thickness of the barrier is next determined from curves of Bx versus the shield thickness or calculation using Tenth Value Lengths (TVL) based on the energy of the beam and the type of shielding material.

( 4.1)

whereP is the dose per week required outside the barrier for protectiondpri is the distance from the x ray target to the point protected in

metersW is the workload per week at 1 m from the targetU is the barrier use factor or fraction of the time per week that the

primary beam falls on the barrierT is the occupancy factor or fraction of the time that a person will

be in the area outside the barrier

4.2 Dose Rate Measurement

The dose rate (Dp) outside a primary barrier can be calculated equation (4.2). A similar equation can be used for secondary barriers.

Dp = Diso . Bx . / (dpri)2 (4.2)

Where

Dp is the dose rate at point protected

Dios is the dose rate at the isocenter

Bx is the barrier transmission factor

dpri is the distance from the isocenter to the point interested out side the barrier

26

4.3 Secondary Barrier Transmission Factor Bp

For secondary barriers, the use factor (U) is always one, and both the head leakage and scattered radiation shielding requirements must be considered. The amount of scattered radiation depends on the beam intensity incident on the scatterer, the quality of radiation, the beam size and the angle of incidence. The barrier transmission needed for radiation scattered from the patient (Bp) is given by equation (4.3)

( ) ( )F

ddTWa

PB scap40022

sec⋅⋅= (4.3)

wheredsec is the distance from the scattering surface to the point to be

protected (m) dsca is the distance from the x ray target to the patient (m)a is the ratio of scattered radiation at 1 m from the scattering

object to the primary radiation at 1 m from the x ray targetF is the beam size at patient (cm2)

Table 4.1:Scatter-fractions (a) at 1 m from a human-size phantom, target to phantom distance of 1 m and field size of 400 cm2 is given below by different angle.

4.4 Leakage Barrier Transmission Factor Bleak

The leakage barrier transmission factor B leak is calculated beam attenuation due to linac head shielding transmission of 0.1%. The energy of the leakage radiation is assumed the same as that of the primary radiation. Thus,

(4.4)where

P is the dose per week required outside the barrier for protectiondleak is the distance from isocenter to point of interestW is the workload per week at 1 m from the target

T is the occupancy factor

Angle(deg)

6MV 10 MV 18 MV 24 Mv

10 1.04×10-2 1.66×10-2 1.42×10-2 1.78×10-2

20 6.73×10-3 5.79×10-3 5.39×10-3 6.32×10-3

30 2.77×10-3 3.18×10-3 2.53×10-3 2.74×10-3

45 1.39×10-3 1.35×10-3 8.64×10-4 8.30×10-4

60 8.24×10-4 7.46×10-4 4.24×10-4 3.86×10-4

90 4.26×10-4 3.81×10-4 1.89×10-4 1.74×10-4

135 3.00×10-4 3.02×10-4 1.24×10-4 1.20×10-4

150 2.87×10-4 2.74×10-4 1.20×10-4 1.13×10-4

27

4.5 Calculation of Barrier Thickness

After the calculation of the barrier transmission factor, the thickness of the required shielding material can be determine based on broad beam transmission data. However, NCRC Report No. 49 and 51 are more convenient sources of graphical data for primary and secondary barriers composed of concrete, steel, and lead. When data are not available for leakage radiation, it is recommended that the primary curve for 60% of the nominal accelerating potential be used.

A second approach based on TVLs is given in NCRP Report No. 51. Both the first TVL (T1) and equilibrium, or subsequent, TVL (Te) are given in NCRP Report No. 51 for lead, concrete, and steel.

Table 4.2: Tenth Value Layers in ordinary concrete, steel, and lead [5]

Photon energy (MV)

Shield Material TVL 1 (m) TVLe (m)

6 MV Concrete 0.35 0.35

Steel 0.099 0.099

Lead 0.055 0.057


Steel 0.104 0.104

Lead 0.057 0.056


Steel 0.108 0.108


Steel 0.108 0.108


Steel 0.108 0.109


Steel 0.109 0.109

The number (n) of TVLs required for the shield can be obtained from equation (4.5) using the value of the barrier transmission factor (Bx)

n = log10 (1/Bx) (4.5)

The thickness (S) of the shield is then given by equation (4.6)

28

S = T1 + (n-1)Te (4.6)

Varian Corporation, a major manufacturer of medical linear accelerators, provides TVL data for primary beam and 900 head leakage radiation. A single TVL value, which appears to be based on the average value of the first and third TVL, is used for barrier thickness determination. The average TVL values were obtained form the data given by Nelson and LaRiviere (1984). The TVL values apply for concrete, steel, lead, and earth and are tabulated based on the megavoltage of the accelerators as shown in table 4.3 [2].

Table 4.3:Tenth Value Layers for Primary and Secondary Leakage Radiation at 900

X ray megavoltage Shielding Material TVL primary (m) TVL leakage (m)

6 Mv

Concrete 0.343 0.279

Earth 0.572

Steel 0.098 0.080

Lead 0.055 0.045

10 MV


Earth 0.648

Steel 0.105 0.085

Lead 0.056 0.046

15 MV


Earth 0.720

Steel 0.108 0.087

Lead 0.057 0.047

18 MV


Earth 0.740

Steel 0.111 0.087

Lead 0.056 0.047

20 MV


Steel 0.111 0.088

Lead 0.055 0.049

29

24 MV

Steel 0.107 0.089


Lead 0.052 0.051

4.6 TVL Calculation for New Observed Materials

There are no available TVL data for available densities. TVL value varies for different energy and materials with its densities. Plotting a graph by density vs TVL for different energies, it is possible to know average TVL values for these densities. The Varian TVL (primary) data from table 4.1 are used for calculation of TVL of new observed materials.

Table 4.4: TVL value for 6 MV linac

X ray megavoltage

Shielding Material

Density g/cm3

TVL primary (m)

TVL leakage (m)

6 MV

Concrete 2.35 0.343 0.279

Earth 1.5 0.572

Steel 7.87 0.098 0.080

Lead 11.35 0.055 0.045

TVL VS Density graph (6 MV)

0.343

0.572

0.098 0.055

y = 0.9082x-1.1243

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 5 10 15Density g/cm3

TVL

in m

eter

TVL 90 deg. leakage VS Density

graph (6 MV)

0.279

0.0450.08

y = 0.7468x-1.1273

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 1 2 3 4 5 6 7 8 9 10 11 12

Density g/cm3

TV

L le

akag

e (m

)

Fig. 4.1: Graphical representation of TVL for 6 MV linac

30


X ray megavoltage

Shielding Material

Density g/cm3

TVL primary (m)

TVL leakage (m)

10 MV

Concrete 2.35 0.389 0.305

Earth 1.5 0.648

Steel 7.87 0.105 0.085

Lead 11.35 0.056 0.046

TVL 90 deg. leakage Vs Density graph (10 MV)

0.305

0.085 0.046

y = 0.846x-1 .1652

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7 8 9 10 11 12

Density g/cm3

TVL

leak

age

in m

eter

TVL Vs Density graph (10 MV)

0.389

0.648

0.105 0.056

y = 1.0632x-1.1751

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 5 10 15Density g/cm 3

TVL

in m

eter

rFig. 4.2: Graphical presentation of TVL for 10 MV linac


X ray megavoltage

Shielding Material

Density g/cm3

TVL primary (m)

TVL leakage (m)

15 MV

Concrete 2.35 0.432 0.330

Earth 1.5 0.720 -

Steel 7.87 0.108 0.087

Lead 11.35 0.057 0.047

31

TVL 90 deg. leakage VS Density graph (15 MV)

0.33

0.087 0.047

y = 0.9446x-1.204

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7 8 9 10 11 12

Density g/cm 3

TVL

leak

age

in m

eter

Fig.4.3: Graphical representation of TVL for 15 MV linac


X ray megavoltage

Shielding Material

Density g/cm3

TVL primary (m)

TVL leakage (m)

18 MV

Concrete 2.35 0.444 0.330

Earth 1.5 0.740 -

Steel 7.87 0.111 0.087

Lead 11.35 0.056 0.047


0.74

0.0560.111

0.444

y = 1.264x-1.2397

0

0.1

0.2

0.30.4

0.50.6

0.7

0.8

0.9

0 5 10 15

Density g/cm 3

TVL

in m

eter

TVL 90 deg. leakage Vs Density graph (18 MV)

0.33

0.087 0.047

y = 0.9446x-1 .204

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6 7 8 9 10 11 12

Density g/cm3

TVL

leak

age

in m

eter


32


X ray megavoltage

Shielding Material

Density g/cm3

TVL primary (m)

TVL leakage (m)

20 MV

Concrete 2.35 0.457 0.343

Earth 1.5 0.111 0.088

Steel 7.87 0.055 0.049

Lead 11.35 0.457 0.343


0.457

0.1110.055

y = 1.4309x-1.3011

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 5 10 15

Density g/cm 3

TVL

in m

eter


0.343

0.088 0.049

y = 0.9812x-1.2081

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3 4 5 6 7 8 9 10 11 12

Density g/cm3

TVL

leak

age

im m

eter



X ray megavoltage

Shielding Material

Density g /cm3

TVL primary (m)

TVL leakage (m)

24 MV

Concrete 2.35 0.470 0.356

Earth 1.5

Steel 7.87 0.107 0.089

Lead 11.35 0.052 0.051

33


0.356

0.089 0.051

y = 1.0179x-1 .2122

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3 4 5 6 7 8 9 10 11 12

Density g/cm3

TVL

leak

age

in m

eter


0.47

0.107 0.052

y = 1.5404x-1.3546

00.10.20.30.40.50.60.70.80.9

1

0 1 2 3 4 5 6 7 8 9 10 11 12

Density g/cm 3

TVL

prim

ary

in m

eter

Fig.4.6: Graphical representation of TVL for 24 MV linac.

The TVL data for the various materials are calculated from the above graphical equation and this data are represent in the table 4.10 below.

Table 4.10: Table for calculated TVL (primary) and TVL 900 leakage from above graphs and equations of graph.

X-ray Megavoltage Shielding materials TVL primary

in meterTVL 900 leakage

in meter

6 MVConcrete (ρ=2.18 g/cm3) 0.3781 0.3102Brick concrete (ρ=1.68 g/cm3) 0.5068 0.4161






4.7 Width and Length of Primary Barrier

The primary barrier width is made equivalent to the maximum beam size at the barrier plus 0.305 m on either side to prevent radiation from leaking through the secondary barrier that abuts the primary. This rule of thumb was evaluated by McGinley in 2000 and found to be suitable for high – energy medical accelerator facilities.

34

Fig:4.2: The maximum beam size for most accelerators at 1 m from the target is 40 x 40 cm2. However, by rotating the collimator 450 from the zero position, the maximum horizontal width of the beam is the diagonal dimension of the 40 x 40 cm2 beam (56.6 cm). If the beam is projected on a barrier X m away , the maximum beam width at the barrier (W′) is given by equation (4.7)

W′ = 0.566 X (4.7)

The required horizontal barrier width (W) in meter is given by the addition of 0.305 x 2 = 0.61m and thus

W = 0.566 X + 0.61 (4.8)

4.8 Dose Rate

The state of Michigan requires that the instantaneous dose rate for both controlled and uncontrolled areas be equal to or less than 13.6 nSv/s with the accelerator operated at 3.33cGy/s at the isocenter. In non occupiable areas such as a roof a dose rate of 278 nSv/s is allowed with the accelerator operated 3.33cGy/s at the isocenter. Pennsylvania has a more restrictive limit of less than or equal to 5.6 nSv/s for uncontrolled areas with the accelerator operated at the maximum dose rate at the isocenter. Equation 4.2 is used to evaluate the dose rate out side the barrier.

4.9 Maze door shielding calculationsA maze is typically used to reduce the radiation level at the entrance of the accelerator room so that a massive door is not required. This calculation only for low energy treatment room ( < 10MV) will be discussed.

35

The radiation reaching the maze door is due to scattering from the room surfaces and the patient, as will as direct penetration of radiation through the maze wall ( D′ of Fig: 4.3 ). The scattered radiation reaching the maze door is made up of the following components:

1. Primary beam scattered from room surfaces (Ss)2. Head leakage photons scattered by room surfaces (L) 3. Primary scattered from the patient (Sp)

Fig: 4.3. Treatment room floor plan for calculation

Primary beam scattered from room surfaces (Ss)The radiation scattered to the door when the primary beam strikes wall C of Fig: 4.3 can be calculated by using equation (4.9 ) [5]

The equation for the calculation of primary beam scatter is given below:

(4.9)

WhereSs is the dose at doorD0 is the workload of acceleratorα1 is the reflection coefficient at first reflection based on beam

energy of one half the megavoltage of the acceleratorA1 is the area at first reflection (m2)α2 is the reflection coefficient at second reflection based on

an energy of .05 MVA2 is the cross section of maze (m2)di is the distance form the target to the first reflection (m)dr1 is the centerline distance along first leg of maze (m)dr2 is the centerline distance along second leg of maze (m).

36

4.9.1 Dose Rate for Leakage Radiation

Head leakage radiation that strikes wall C as shown in Fig: 4.4 can undergo a single scatter and reach the maze door. This radiation is more penetrating than the double scattered primary beam and the patient scatter and has an average energy of approximately 0.3 MV at the maze door for a 6 MV accelerator.

Fig: 4.4: Maze design of a radiotherapy room

Equation 4.9 is modified as shown below for the calculation of the head leakage the door after a single scatter.

(4.10)

WhereL is the dose at the maze door due to head leakage

L0 is the ratio of dose due to head leakage at 1.0 m from the target to the dose at the isocenter

D0 is the workload of the acceleratorα1 is reflection coefficient for wall reflectionA1 is the area of wall C that can be seen from maze door (m2)d1 is the distance from the target to the maze centerline (m)ds is the centerline distance along the maze (m)

4.9.2 Dose Rate for the Patient Scatter

37

Patient scatter to the maze door is calculated by the use of the following equation for accelerators with energy < 10MV.When the energy of the accelerator is greater than 10 MV, patient scatter can be ignored [2].

Fig: 4.5 depict the parameters used in equation 4.11 to calculate the radiation scattered by the patient to the maze door.

(4.11) Where

Sp is the dose at maze door due to patient scattera is the reflection coefficient for patient from table ( )D0 is the workload of the acceleratorF is the field area at patient (cm)2 α1 is the reflection coefficient for wall reflection (E= 0.5 MV)A1 is the area (m2) of maze back wall that can be seen from outer

maze entrancedsca is the distance from the target to the patient (m)dsec is the distance from patient to maze centerline (m)dr1 is the centerline distance along maze (m).

4.9.3 Calculation of Radiation Passing through Maze Wall

The radiation transmitted through maze wall D’ of Fig: 4.3 to the treatment room door is evaluated by use of equation 4.12 and compared with the radiation scattered down the maze to the door.

(4.12)

where

38

B is the barrier transmission factor for wall D’d” is the distance from the target to the center of the maze doorD0 is the workload of the acceleratorL0 is the ratio of dose due to head leakage at 1.0 m from the target

to the dose at the isocenter

The maximum dose at the maze door is produced by aiming the beam toward wall C of Fig: 4.3 with the collimator fully open and a patient in the beam. The dose ( D c) from all components (Ss, Sp , L and T) of the radiation field at the maze door when the beam irradiates wall C is given by equation 4.13

Dc = Sp + f(Ss) + L + T (4.13)

where

f is the fraction of beam transmitted through patient.

The values of patient transmission factor (f) is 0.23 for 6 MV x ray beam and is 0.27 for 10 MV beam [15]

For the typical case in which the use factors U for the major beam directions (up, down, horizontal, on walls A and C) are each one fourth the total dose (D t) as the maze door, the following equation is given. This equation is to be used for rooms with a floor plan similar to that shown in Fig: 4.3

Dt = 2.64 Dc (4.14)

The transmission factor required for the door shielding is calculated by dividing D t

into the procedure level needed for the area out side the door. Once the transmission factor has been evaluated, the thickness of lead needed in the door for a 6 MV accelerator can be determined from the transmission vs thickness curves.

4.10 Materials for Neutron Shielding for High Energy Room Materials with high hydrogen content efficiently attenuate fast neutrons produced by medical accelerators operating above 10 MV [2]. Concrete, which is commonly used for photon shielding, contains relatively high hydrogen content. The photoneutrons from medical accelerators have a TVL of 21 cm in concrete and the primary x ray beam has a TVL of approximately 44 cm in concrete. This difference in TVL values for neutrons versus photons means that a concrete shield that produces adequate attenuation of the photoneutrons. Fast neutrons are moderated or reduced in energy by elastic scattering from hydrogen and after a number a collisions, become slow neutrons. Low energy neutrons undergo capture reactions with many materials and penetrating capture gamma rays are emitted.

A number of materials such as Boron and Cadmium have large cross sections for slow neutron capture and a few millimeters of the materials will absorb almost all of

39

the neutrons. Polyethylene with boron content of 5% by weight is commonly used in neutron-shielded doors for treatment rooms.

4.11 Properties of Shielding Materials

Table 4.10: Shielding properties of materialsMaterials Hydrogen

content atoms/cm3

TVL for Fast

neutrons (cm)

TVL for slowneutrons

(cm)

TVL capture gamma

(cm)

Neutron activation

Concrete 0.8 to 2.4 x 1022

21.0* 34.0 45.0 Low

Polyethylene 8 x 1022 4.5# 77.0 ----- Very low

5% Boron ------ ----- 1.27 ----- Very low

Steel ------ ----- 10.7 13.5 Medium

Lead ------- ------ 410 6.1 Low

*head leakage radiation#maze neutrons

4.12 Calculation for Neutron Shielding

Most medical accelerators operating above 10 MV use a maze with a door shielded for neutrons and photons at the outer maze entrance. The polyethylene is used to moderate the fast and intermediate energy neutrons, which then react with the boron and produce a 0.473 MV photon. The lead placed after the polyethylene, where it will attenuate the photons produced in the boron and any capture gamma rays generated in the maze by neutron capture in the concrete wall, ceiling and floor. The evaluation of the neutron dose equivalent at the outer maze entrance is carried out using the method developed by Kersey (1979).use of Kerseys technique the effective neutron source position is taken to be the isocenter of the accelerator and the neutron dose equivalent (H) at the entrance to the maze per unit dose of x ray at the isocenter is given by the equation below:

H = H0 (T/T0) (d0/d1)2 10-d2

/ 5 (4.15)

WhereH0 is the neutron dose equivalent at the distance d0 (=1.41 m) from the

targetT/T0 is the ratio of the outer maze areas to the inner maze entrance aread1 is the distance in meters from the isocenter to the point on the maze

center line which the isocenter is just visible

40

Fig: 4.6: Conventional floor plan for accelerator room and maze.

In the case of a maze with two bends, d2 is the distance from A to B´ plus length B´to C of Fig:5.6 . Note that the maze has a tenth value distance (TVD) of 5 m for the attenuation of neutrons. McGinley and Butker (1991) found that the TVD for maze neutrons was approximately 16% less than 5 m. Therefore, 5 m is a conservative value to use for the neutron TVD when the dose equivalent is determined at the maze outer entrance .It was also discovered that a second turn in the maze reduced the neutron level by a factor of about three as compared with the value obtained by Kersey’s equation.

For a maze with two bends, equation 5.15 is modified as shown below to account for the second turn.

H = H0 ( T/T0) (d0/d1)2 (10-d2

/ 5)(10-d3/ 5) ( 1/3) (4.16)

where

d3 is the distance between points B´ and C of the Fig: 4.6 expressed in meters

The neutron level at the maze entrance varies with the collimator setting, with the maximum value obtained by closing the collimators. The maximum increase in the neutron dose at the outer maze entrance is the order of 15% when the collimator size is reduced from 40 x 40 cm2 to 0 x 0 cm2. The neutron field is also a function of the gantry angle and location of the target rotational plane in the treatment room.

Relative neutron and photon dose equivalent outside the maze entrance as a function of beam direction based on the Fig: 4.6. All data have been normalized to 1.00 for the down beam direction.

41

Table 4.11: Dose equivalent with different direction

Stated energy (MV)

Relative neutron dose equivalent Relative photon dose equivalentBeam direction

1 to 3 3 to 1 Up Down 1 to 3 3 to 1 Up Down18 0.68 1.17 0.74 1.00 0.75 1.13 0.89 1.0015 0.83 1.20 1.09 1.00 0.88 1.25 0.90 1.00

4.13 Capture Gamma Ray Shielding

Most accelerator doors have a lead component that is designed to attenuate the x rays scattered from the treatment room in to the maze. The capture gamma dose (D) at the maze entrance per dose of x ray at the isocenter is given by the equation below.

D = kΦtotal 10-d2/ TVD

2 (4.17)

WhereK is the ratio of the capture gamma dose to the total neutron fluence at

point A of the Fig.5.6 (k = 6.9 x 10-12 cm2 Gy)

TVD2 is the tenth value distance (=5.5 m for 18, 20, 25MV linac) Φtotal is the

total neutron fluence (cm –2) at point A per unit dose (Gy) at the isocenter d2 is the distance from A to B (Fig. 4.6)

The total neutron fluence at the inner maze entrance per unit dose of x ray at the isocenter can be evaluated by the equation below.

Φtotal = a Q/ 4 пd12 + 5.4 a Q / 2п S + 1.26aQ / 2п S (4.17)

WhereQ is the neutron source strength per unit dose of x ray at the isocenter S is the surface area of the treatment room in cm2 a is the transmission factor (= 1 for lead, 0.85 for tungsten)

The total neutron dose at the door position for two bends is given by the equation,Hweek = W x H0 x ( T/T0) x (d0/d1)2 x (10-d

2/ 5) x (10-d

3/ 5) x ( 1/3) (4.18)

The total neutron dose at the door position for one bend is given by the equation,

Hweek = W x H0 x ( T/T0) x (d0/d1)2 x (10-d2/ 5) (4.19)

where

42

W is the workload at the isocenter per week The total Photon dose at the door position is the sum of the capture gamma dose¸ head leakage, and patient scatter dose given by the following equation,

For Capture Gamma,

Dweek = W x D (4.20)

whereW is the workload at the isocenter per week

For Head Leakage Dose,

L = W x L0 x 1/(d1+d2)2 (4.20)

whereW is the workload at the isocenter per weekL0 is the ratio of dose due to head leakage at 1.0 m from the target to

the dose at the isocenter

For Patient Scatter Dose

Dpscat = a W (F/400) 1/(d1+d2)2 (4.21)

Wherea is the scatter fraction has a value 5.39 x 10-3 for 18

MV linacF is the maximum field size (40 x 40 cm2)

So the total gamma dose at the position of the door is given by

Dtotal = Dweek + L+ Dpscat (4.22)

Neutron shielding material (Polyethylene) will attenuate gamma dose so it should be considered and subtracted this dose from Dtotal .

If the shielding thickness is S and the permissible dose outside the barrier Dpermi then given by the equation

Dpermi / Dtotal = 10 – S / TVL (4.23)

4.14 Shielding of Simulator Room

A simulator is an x-ray unit used to established a treatment procedure before radiation therapy is begun or to verify the treatment plan. An isocentric tube is used so the movement of the beam will be similar to the treatment unit, and both radiographic and fluoroscopic modes of operation are possible. Recently simulators

43

have become available that have computed topography (CT) capabilities. Fig. 4.7 shows a conventional simulator room and Fig.4.8 shows a conventional simulator control room.

The techniques used for simulator shielding design are similar to those for medical accelerators with certain exceptions, noted below. Shielding is normally provided to a height of seven feet above the floor and in the ceiling [2] if the space above is occupied. Lead-backed gypsum board is used in the walls, and the room door is made of wood with a lead core [2]. A viewing window made of leaded glass is provided so the operator can see the patient during the simulation process. The workload (W) is expressed in terms of milliamperes multiplied by the exposure time in minutes (mA min),and values of 160, 300, and 3200 mA min per week are typical for radiography, fluoroscopy, and CT, respectively. Protection levels (P), use factor (U), and occupancy factors (T) for controlled and uncontrolled areas are similar to those other radiotherapy protection.

Fig.4.7: A schematic diagram of an x-ray simulator machine. For a primary barrier the exposure per mA minute (Kux) in seven consecutive days of operation is computed by use of equation 4.24[6].

Kux = Pd2 / WUT (4.24)

WhereP = protection levels (R wk-1)

44

D = distance from the x-ray target to the area being protected (m)W = workload in mA wk-1

U = use factor (1/4 for each wall, floor, and ceiling)T = occupancy factor.

The value of Kux is used with Fig.4.9 to determine the thickness of lead required for the primary barrier [2]. The fig.4.9 is based on 125 kv x-rays which is typical operating voltage for a radiotherapy simulator [2].

Fig.4.8: A schematic diagram of a simulator control room.

The transmission curve cannot be described by a single half value layer (HVL), and the thickness of lead needed must be determined graphically. The primary barrier will receive only scattered radiation 75% of the time. The scattered radiation can be ignored, since it will be a factor of 1000 less than the primary radiation. The width of the primary barrier is made 30 cm larger than the maximum beam size on all sides in order to reduce leakage caused by photons scattered from the main beam [2].

Radiation that reaches a secondary barrier is made up of patient scatter and head leakage. The barrier transmission factor (B) required for leakage is given by the equation (4.25).

B = P (dsec)2 × 600×I / WT (4.25)

WhereP = protection level for area protected (R wk-1)

45

dsec = distance in meters from radiation source to point protectedI = maximum tube current (mA)W = workload (mA min wk-1)T = occupancy factor.

In equation 4.25 it has been assumed that the tube head leakage at 1 m is 0.1 R min-1. The value of B is used to compute the number of half value layer (N) required for the shield as given by equation (4.26)

N = 3.32 log10 (1/B) (4.27)

The shield thickness for leakage (SL) is then calculated by the equation below,

SL = N× HVL (4.28)

The barrier transmission factor (Kux) for patient scatter is determined by the equation (4.29)

(4.29)

The patient scattering coefficients (a) for 125 kv x-ray are given by NCRP Report No. 49 in the table below.

Table 4.12: Patient scattering coefficient for various angleAngle 300 450 600 900 1200 1350

Patient scattering

Coefficient (a)0.0018 0.0015 0.0015 0.0015 0.0023 0.0025

4.15 CT Simulator Room

Conventional simulators are being replaced by CT unit that are used for three- dimensional (3D) treatment planning. This new device allows rapid determination of the treatment volume and optimization of the dose distribution so as to provide a uniform tumor dose and to avoid high dose to critical structures. When a conventional simulator is used for 3D planning, the patient must remain in the treatment position for long periods of time while a number of film radiographs are taken. The use of CT simulator requires that the patient spend very little time on the CT table.

The components of CT simulator are a CT scanner, a workstation for virtual simulation, and a laser system for use in marking the patient for positioning on the treatment table. Data to be used for set up of the treatment unit are transferred directly to the treatment machine over a network.

Design of room shielding for the CT simulator relies on isodose distributions, supplied by the manufacturer, to determine the dose at the shielding barriers. Fig.4.10 is an isodose for one of the commercial CT simulator [2]. The isodose curves have been normalized to the dose per scan (slice) and a set of curves has been determined for a body phantom. Scatter plots are also supplied by the

46

manufacturer for head phantoms. The x-ray tube voltage (kv) and current (mA) are indicated in the figure. In some cases, the isodose curves are normalized to the dose per mA second.

Fig.4.9: Isodose distribution of a CT simulator.

It should be noted that the primary x-ray beam cannot exit the CT scanner and strike any of the barriers. Therefore, only stray radiation (leakage and scattered) needs to be considered for the shielding determination (U=1). The design of the room shielding is similar to a conventional simulator in that the following parameters required:

1. Workload2. kV and mA3. Occupancy factor for each barrier (T)4. Type of area outside each barrier (controlled and uncontrolled).

The workload is expressed in mA s per week or the number of scan (slices) per week.

47

The dose per week (D) at the point protected without the shield in place is given by the equation below,

D = W × D0 × T (4.30)

WhereW = workload in units of the number of scans per week or the mA min per

weekD0 = dose per scan or dose per mA min obtained from the isodose curvesT = occupancy factor.

The required barrier transmission (TR) is calculated as shown below:

TR = P / D (4.31)

WhereP is the protection level (0.002 mrem per week for uncontrolled areas and 0.010 mrem per week for controlled areas)

The shielding curves for x-rays produced at various voltages (kV) given in NCRP report No. 49 are used to evaluate the shielding thickness required for protection. These curves are not transmission curves but have been normalized to the x-ray exposure per mA min at 1 m from the target (X0). The transmission value (TR) is related to the NCRP data as given by equation 4.32.

TR = Xs / X0 (4.32)

Where Xs = R per mA min at 1 m for shield thickness

X0 = R per mA min at 1 m for zero shield thickness

The value of Xs required for protection is given by equation 4.33 as below

Xs = X0 × TR (4.33)

The shielding thickness required can be read directly from the table 4.13 below.

Table 4.13: Exposure for CT simulator for various operating voltage CT simulator operating voltage, kV X0 (R per mA min at 1 m)

150 0.95

125 0.90

100 0.86

70 0.73

50 0.50

48

49

Chapter 5

5 PROPERTIES OF SHIELDING MATERIALS AND COST ANALYSIS

5.1 Introduction

It is actually the task of an architect to design a treatment room for a linear accelerator. However, he must be required to consult a medical physicist because the letter knows the provision of radiation protection and is responsible for the linear accelerator. Making cost of a well-protected radiotherapy department is very high so it is very sensitive to design of a radiotherapy department. Space area, space cost, material cost, very safely control design, operating facilities, everything must be considered to this design. Radiotherapy in limited number of facilities are in Bangladesh for cancer treatment, but it is expected that the number of facilities will increase soon. Considering this prospect, to design low cost radiotherapy department is a time demanded expectation for Bangladeshi people or such developing countries. I want to reach this point that fulfills the all expectations.

5.2 Aim of this work- To calculate the basic layout and the dimension of the treatment room- To design of the two radiotherapy room, one CT room and one simulator

room- To design of protective walls- To investigate low cost but high dense material from local resources.

- To calculate the barrier thickness for two typical linear accelerator of a modern radiotherapy department.

5.3 Essential Properties for Shielding Materials

Any shielding material must possess the following properties

a. Durability: This factor concerns the chance of its properties with use (e.g. exposure to atmosphere or under radiation), thus loosing the attenuating ability or strength. It should also not develop cracks, which will be dangerous for radiation leaks.

b. Heat transfer Properties: Radiation energy generated heat when it exposed to the walls or air or any absorbing materials. This heat cannot remove easily and this problem is usually important only for the inner layers.

c. Density: Closely pact, but light elements attenuate neutrons most, whereas the high atomic weight elements are best for gamma ray attenuation.

50

d. Fabricability: Shielding materials should have enough fabricability because it has a part of the building stricture.

e. Cost: It should be low cost and easy to collect.

5.4 A shielding material and room design requirements:

1. Design of the shield is to be done in such a way as to protect radiation workers and the general public at large from excessive radiation during radiotherapy procedure.

2. The operating personnel must not be exposed to radiation exceeding permissible levels for duration of time laid by ICRP recommendation.

3. The secondary barriers in the treatment rooms must erected to avoid the unwanted radiation level to the personnel in neighboring rooms.

5.5 Conventional Shielding Materials for Gamma Rays

The linear attenuation coefficient for the gamma ray shielding materials generally increases with the density, for photons of a given energy. Linear attenuation coefficient is more important for shielding narrow beam but mass attenuation is more important for attenuating the broad beam. The beam intensity reduces with the thickness of the material.

In order to decrease the total radiation to an acceptable level, the shield is designed of a suitable material. For purpose of absorbing gamma radiation, the shielding element having high atomic number is used. A brief description of shielding materials for shielding against gamma radiation is given below:

5.5.1 LeadLead is a soft, structurally weak, low melting point (327.4 0C) material with a density of 11.35-g/cm3. the gamma radiation attenuation property of lead is excellent. Lead is attractive as a shielding material against gamma radiation because of its low cost - to- density ratio comparatively with other high dense materials.

Unless alloyed, lead is a relatively weak structural material. In most cases, additional support is required to confine the lead because of its high creep and low melting point. Where stronger Lead is required, Antimony is the commonly used alloying agent. These alloys are created not only to change mechanical properties but also to attenuate a broad spectrum of various types and energies of radiation.

Lead is usually selected as the primary barrier against gamma radiation on the basis of comparison with other gamma shield materials. The comparison normally includes factors such as gamma energy spectra and intensities.

Another consideration, lead is mainly used when space or land is most costly, lead barrier reduces barrier space because lead barrier thickness is comparatively less than other low dense shielding materials.

Average price =5,10,750 Tk /m3. in local market (June’2005, Dhaka, Bangladesh)

51

5.5.2 Steel Steel is an excellent gamma ray shielding material. Normally its density is 7.8 g/cm3. Although not as effective as lead, the improved structural properties and economy of steel may be compensating factors; namely, it may be cheaper to use steel even though the volume requirement is larger than for lead.

Table 5.1: Number densities of the elements of steelSymbol Atomic No. Z Number Density [Atom/b-cm]

Cr 24 1.502491

Mn 25 0.138824

Fe 26 5.402856

Ni 28 0.833760

Steel is heavy strength, high melting point, better heat conductor and easy to construction. It can be used as primary shielding material, ceiling and as door.

Average price = 2,75,450 Tk / m3 in local market (June’ 2005, Dhaka, Bangladesh)

5.5.2 Ordinary concreteOrdinary concrete is an inexpensive and easy to handle. Concrete is the local resource of Bangladesh, as a result my interest is pointed here to collect exact density of this concrete which type of concrete available here. Measuring only rock density is not suitable for shielding calculations because rock is one of component of the concrete. Cement, sand, water are the component materials of the concrete. Mixing by the exact ratio of this component makes structural walls or slab. It is very important for shielding calculations of concrete barriers. The density of the ordinary concrete varies from 2.15 g / cm3 to 2.36 g / cm3. So it is very important to know exact density of the local concrete.

The standard ordinary concrete is composed of cement, sand and rock in the ratio of 1:2:4 respectively by volume and adding about 53 liters water per 100 kg cement since ordinary concrete is a combination of hydrogen and other nuclei of fairly high atomic number it is efficient both in absorbing gamma rays and slowing down of fast neutrons by elastic collision.

Table 5.2 The number densities for the elements of Ordinary Concrete Symbol Atomic No. Z Number Density [Atom/b-cm]

H 1 0.013098O 8 1.1645Na 11 0.039967Mg 12 6.0149x10-3

Al 13 1.0949x10-1

Si 14 0.736616 0.0028

52

K 19 4.4994x10-2

Ca 20 1.9393x10-1

Fe 26 2.9025x10-2

5.6 Study of various self-fabricated materials of concrete and density measurement

The following materials have been taken from local market:i) Gray color stone ii) Black color stoneiii) Bricksiv) Heavy sand (small parts of stone)v) Sandvi) Cement

Fig.5.1: Picture of Gray color stone

53

Gray color stone Black color stoneFig.5.2: Small part of gray color and black color stone.

5.6.1 Gray Concrete

Mixture of Gray color stone + Heavy sand + Cement according to the ratio of 4:2:1 respectively. This type of concrete was made and its density was empirically determined by measuring the weight and the volume.

Fig.5.3: Picture of a concrete that density are measured.

Calculated density is 2.18 g/cm3

Average price = 2,895 Tk / m3. in local market (June’2005, Dhaka, Bangladesh)

5.6.2 Black Concrete

Mixture of Black color stone + Heavy sand + Cement according to the ratio of 4:2:1 respectively. This type of concrete was also made and its density was again empirically determined by measuring the weight and the volume.

54

Fig.5.4: Picture of a concrete that density are measured

Calculated density is 2.36 g/cm3Average price = 3,178 Tk / m3. in local market (June’2005, Dhaka, Bangladesh)

5.6.3 Brick ConcreteMixture of Small parts of brick + Sand + Cement according to the ratio of 6:4:1 respectively. This type of concrete was also made and its density was again empirically determined by measuring the weight and the volume.

Fig.5.5: Picture of a concrete that density are measured.

Calculated density is 1.68 g/cm3

Average price =1271 Tk / m3. in local market (June’2005, Dhaka, Bangladesh)

55

5.7 Area of Cost Analysis

Cost analysis is an important work to design of a low cost radiotherapy facilities or any business implementation. Cost analysis apply in various parts of business function such as in the field of

i) Machinery, Raw materials cost ii) Office space, Building structure cost iii) Manpower and Manpower managementiv) Communications, Distributions, Carrying cost v) Dust, Biproducts management or purification cost vi) Office management, Data management cost etc.

In this present work, cost analysis limit to the material costing and deferential costing only [14]. Building structure is the important issue for to establishing a modern radiotherapy department because of its high shield properties. Authority of radiotherapy department invests high amount of capitals to build this structure to avoid any risk of radiation hazard. So this analysis area is limited into only building structure.

5.7.1 Method of Cost Analysis

a) Collection of shielding materials and analyze costing which discuss aboveb) Considering various room areas and analysis cost with considering space

costc) Cost analysis for using of high dense material for reducing land spaced) Cost analysis for architectural design purposes.

5.7.2 Cost Analysis of Various Room Dimensions

According to the inverse square law, dose rate reduces with the distance. Room area or distance from the x-ray target to the point protected area is the responsible for this inverse square law. Greater distance reduces dose rate but increases land space. We shall use this logic in situation where land cost is very cheap but it is need to know the variation of material volume with the variation of distance. To observe this variation few calculated data put into the Table 4.3 below.Flux I per unit surface area at distance r is

I=ξ/4πr2

Where ξ is the emission rate of the source

56

Fig.5.6: Reduces dose rate with inverse square law.

5.7.3 Analysis for Primary Shielding

Considering precondition for calculation of data

Energy : 6 MV photonBeam category : PrimaryMaterial : ConcreteDensity of material : 2.35 g/cm3Floor to ceiling height : 3.1 meterFloor to isocenter height : 1 meterPermissible dose at out side the barrier : 0.00002 Gy/week for uncontrolled areasPermissible dose for ceiling : 0.0001 Gy/week for controlled areas Use factor : 0.25Occupancy factor : 1Work load : 1000 Gy/weekPosition of dose rate measurement : 1 m outside the barrierPosition of isocenter : Center position of the room Room length (gantry rotation plane) : Outside the barrier to barrier

The above data are used as input data for the computer software. Then variable values for the length inside the room (the room dimension) as given as the first column of the table 5.3 were studied. The results of the calculation are shown in the table below.

57

Table 5.3 Data for primary barrier material volume for various room areas.Length inside the room (m)

Vertical primary wall thickness (m)

Width (m)

Minimum distance from isocenter to outside of ceiling (m)

Ceiling maximum thickness (m)

Ceiling width for primary

(m)

Total vertical primary wall volume (m3)

Total primary ceiling volume (m3)

Total primary barrier volume (m3)

4 2.06 2.874

4.1 1.81 2.930

36.71 21.21 57.92

5 2.02 3.157 39.54 23.84 63.37

6 1.99 3.440 42.44 26.50 68.94

7 1.96 3.720 45.20 29.15 74.35

8 1.93 4.006 47.93 31.80 79.73

910

1.911.89

4.2894.572

50.79 34.44 85.23

53.57 37.10 90.67

11 1.87 4.855 56.28 39.75 96.03

12 1.85 5.138 58.93 42.40 101.33

Shielding volume Vs room dimension graph

y = -0.0103x2 + 5.5359x + 52.377

40

50

60

70

80

90

100

110

1 2 3 4 5 6 7 8 9

Room dimension in m

Shie

ldin

g vo

lum

e in

m3

Fig: 5.7: Relation with primary shielding volume vs room dimension Analysis: Primary shielding volume i.e. primary shielding cost increases with polynomial character with the increasing of the room dimension.

58

Increasing 1 meter of length (gantry rotation plane) increases averagely 7% of total cost without considering space cost. After a certain limit of increasing length, the shield volume may be decreases with tend to 0.

5.7.4 Analysis for Secondary Shielding

Considering precondition for calculation of data

Energy : 6 MV photonBeam category : SecondarySecondary barrier material : ConcreteDensity of material : 2.35 g/cm3Floor to ceiling height : 3.1 meterSecondary ceiling thickness : 0.90 m Dose rate in protected area : 0.00002 Gy/week for uncontrolled areaPermissible dose for ceiling : 0.0001 Gy/week for controlled areas Use factor : 1Occupancy factor : 1Work load : 1000 Gy/weekPosition of dose rate measurement : 1 m outside the barrierPosition of isocenter : Center position of the room Room length (gantry rotation plane) : Outside the barrier to barrierPrimary barrier width for ceiling : 2.874 mThe above data and the first column of the table 5.3 are used with computer software and calculate the data in table below.

Table 5.4: Data for secondary barrier material volume for various room areas.Lengt

h inside

the room(m)

Width inside

the room

(m)

Primary barrierwidth(m)

Area of vertical

secondary barrier(m2)

Secondary barrier

thickness(m)

Secondaryverticalwalls

volume(m3)

Secondaryceilingvolume

m3

Total secondary

barrier volume(m3)

4 4 3.68 8.64 1.02 4.50 13.31

5 5 3.92 12.16 1.00 12.16 10.63 22.79

6 6 4.17 15.66 0.97 15.19 18.75 33.94

7 7 4.40 19.20 0.95 18.24 28.88 47.12

8 8 4.68 22.64 0.93 21.05 41.00 62.05

9 9 4.94 26.12 0.91 23.76 55.13 78.89

10 10 5.20 29.60 0.89 26.34 71.26 97.6

11 11 5.46 33.08 0.87 28.77 89.38 118.15

12 12 5.73 36.54 0.86 31.43 109.51 140.94

59

Secondary shielding volume vs room area

y = 0.9417x2 + 6.5116x + 5.9329

40

60

80

100

120

140

160

16 25 36 49 64 81 100 121 144

Room area im m2

Shie

ldin

g vo

lum

e in

m3

Fig: 5.8: Relation with secondary shielding volume vs room dimension

Analysis: Secondary shielding volume i.e secondary shielding cost increases with polynomial character with the increasing of the room area.

Increasing 1 m2 of area increases about 3% (averagely) of total cost without considering space cost. After a certain limit of increasing area, the shield volume may be decreases with tend to 0.

5.8 Shielding Cost Analysis with Considering Various Materials

Bricks concrete (ρ=1.68 g/cm3), ordinary concrete (ρ=2.18 g/cm3), ordinary concrete (ρ=2.36 g/cm3), lead (ρ=11.35 g/cm3) etc are locally available shielding materials. These materials have different densities and prices. So shielding thickness and cost will be varied with the using of different materials. To analyzing this problem, the data Table 5.5 is considered.

5.8.1 Considering precondition for calculation of data for material vs cost

Energy : 6 MV photonBeam category : PrimaryFloor to ceiling height : 3.1 meterFloor to isocenter height : 1 meterDose rate in protected area : 0.00002 Gy/week for uncontrolled

60

areasDose rate in protected area : 0.0001 Gy/week for controlled areasUse factor : 0.25Occupancy factor : 1Work load :1000 Gy/weekPosition of dose rate measurement : 1 m outside the barrierPosition of dose rate measurement for ceiling : 0 m outside the barrierPosition of isocenter : Center position of the room Room length (gantry rotation plane) : Outside the barrier to barrierInner length (without primary barrier) : 5.5 meterAverage space cost (Dhaka city corp.) : 9,000 Tk./m2

5.8.2 Total cost calculationTotal cost calculated separately with deferent materials but same place. All price considered in approximate and relative value. The labour cost, iron rod cost, construction tools etc are not considered for simplicity. The total cost relative data for primary beam shielding with ceiling are included in table 5.5.

Table 5.5: Total primary shielding cost for various materials

Mat

eria

l

Shi

eldi

ng ty

pe

Shi

eldi

ngTh

ickn

ess

m

Shi

eldi

ng w

idth

m Shie

ldin

g vo

lum

em

3

Tota

l shi

eldi

ng v

olum

em

3

Cos

t per

uni

tSh

ield

ing

volu

me

m3 /T

k.

Tota

l mat

eria

l cos

tTk

.

Use

d sp

ace

m2

Tota

l Spa

ce c

ost

Tk.

Tota

l cos

tTk

.

Bric

ks c

oncr

ete

1.68

g/cm

3 Vertical wall 2.86 3.58 63.48

105.54 1271 134141 31.46 283140 417281

Ceiling 2.61 2.93 42.06

Con

cret

e2.

18g/

cm3

Vertical wall 2.13 3.58 47.27

78.69 2895 227807 23.43

210870 438677

Ceiling 1.95 2.93 31.42

Vertical wall 1.97 3.58 43.72 72.88 3178 231612 21.6

719503

0 426642

61

Con

cret

e2.

36g/

cm3 Ceiling 1.81 2.93 29.16

Ste

el7.

87 g

/cm

3

Vertical wall

0.56 3.58 12.43

20.65 275450 5688042 6.16 55440 5743482Ceiling 0.51 2.93 8.22

Lead

Vertical wall 0.32 3.58 7.10

11.77 510750 6011527 3.52 31680 6043207

Ceiling 0.29 2.93 4.67

Material Vs cost graph

0

2000000

4000000

6000000

Bricksconcrete

1.68g/cm3

Concrete2.18g/cm3

Concrete2.36g/cm3

Steel Lead

Materials

Cos

t in

Tk.

Fig: 5.8: Graphical representation of relative cost for various materials

Analysis: It has calculated for primary barrier shielding cost with various materials. If the cost use of ordinary concrete with density 2.35 g/cm3 is as standard cost then use of bricks concrete decreases 2.19 %, concrete with density 2.18g/cm3 increases 2.82 % , steel 1246% and lead 1316%.

5.9 Cost analysis for design purposes

The propose design for 6 MV and 10 MV linac rooms are present in figure 6.2 with one common primary barrier i.e. number of 3 primary barriers and dose rate at the maze door is calculated. In this investigation, no shielded door is needed. The conventional room design for 6 MV and 10 MV linac rooms are present in figure 6.8 with same room dimension and same facilities. Shielded door is needed for this type of design. The number of 4 primary barriers are present in this type of design. One primary barrier with two sided secondary walls are extra wall structure than propose

62

design structure. This causes extra costing than proposed design. The proposed design cost reduces about 25% .

63

Chapter 6

6 ROOM DESIGN AND SHEILDING CALCULATION

6.1 Introduction

The purpose of shielding design for therapeutic installations such as linear accelerator, brachytherapy, Co-60 unit, simulator, CT simulator which can be achieved by selection appropriate location, proper equipment, optimum protection using the lowest cost materials, appropriate design, better utilizing space, scope to installation modern facilities and employing the least possible amount of labor. Thus the appropriate location for a therapeutic installation is very important.

The optimum conditions for the installation are described below [4].1. Location should be in a corner of the building.2. Location should be in the ground floor.3. No basement beneath the installation should be allowed.4. There should not be any room in constant use near the installation

meaning that there should be lowest possible occupancy factor, T.

This work not only designs a single room, a modern radiotherapy department also. This department contains minimum one 6 MV linac, one 10 MV linac, CT simulator room and its control room, etc. To established a modern radiotherapy department for such developing country, as Bangladesh is not very easy because it is more expensive. As a result, designing of a low cost radiotherapy department is very important for improve cancer treatment facilities in Bangladesh.

6.2 Design of the Installation

In the present study, one type of structural design considered and there are different types of shielding materials such as bricks concrete (p=1.68g/cm3), ordinary concrete (p=2.18 g/cm3), ordinary concrete (p=2.36g/cm3) will be used. After making the specific shield design, it may be compared with wall thickness, material cost, land cost, labor cost etc and decision will be taken which shielding material is preferable for Bangladesh. According to the part of the total relative cost analysis from table 5.5, it is clear that bricks concrete is the lowest cost material for designing of a radiotherapy department. I find out that the density of this bricks concrete is 1.68 g/cm3 which is not actually constant because it is depend upon its row materials, burning time etc. Moreover bricks wall may introduce extra humidity. Considering this problems, second lowest cost material is selected and its name is ordinary concrete which density is 2.36 g/cm3.

64

6.3 The basic layout and the dimensions of the treatment room

Mainly the dimension of the linear accelerator and its efficient operation and different therapy utilization determines the dimension of the treatment room. The dimension of patient couch and its movement in different directions must be considered. Provision has to be made for gaining all the components make up of the accelerator into the room and out of the room as well, when an old machine is to be replaced against a new one. Any larger room demands unnecessary cost and space.

The following Table 6.1 shows the dimension of a standard accelerator, and a patient couch[16]. Table 6.1: Standard machine (Mevatron) and patient couch dimension. Dimension Height in cm Length in cm Width in cm

Mevatron 260.4 283.7 131

Patient couch 69-134 270 61

Considering above data the minimum general treatment room dimensions are designed which shows in fig. 6.1.

Fig. 6.1: Basic deign of a radiotherapy room for high-energy photon.

According to the figure 6.1 and Table 6.1

Length (gantry rotation plane) = 7.59 meterWidth (patient cough parallel) = 5.80 meterHeight (minimum floor to ceiling height) = 3.10 meterMaze width = 1.80 meter

6.3.1 Basic design fundamentals

Before the planning stage for the radiation therapy facility development, many parameters need to know by a qualified medical physicist such as [ 2]

65

1. Radiation source- manufacturer and model of accelerator, megavoltage of x-ray beam, method of specifying the megavoltage and the energy of electron beams.

Head leakage- value of the neutrons and x-ray head leakage.3. Work load (W)- cGy at the isocenter in seven days of operation and cGy at

the isocenter in any one hour of operation of the x-ray beam.4. Use factor (U)- use factor of each wall for primary beam shielding and

secondary beam shielding. Typically ¼ is used for primary beam shielding and 1 is used for secondary beam shielding.

5. Occupancy factor (T)- It depends upon the design of the installations and the probability of public presence out side of the department.

6. Shielding materials- list density and other shielding properties of the various materials used to construct the treatment vault.

6.3.2 Optimum design

The protective barriers for high energy machines are massive can cost very large sum to install. In new installations it is wise to place these machines far enough underground to avoid radiation problems through the ceiling or the walls and also to avoid floorloading problems that exist on higher level floors. If the protection has to be achieved in a completed hospital or in a new hospital a deep basement with a very thick ceiling is required. If a number of units are to be installed they should be clustered together so that the walls of one unit protect the next. To minimize cost [3], the rooms should be made as small as possible consistent with allowing enough space for setting up patient and servicing the machine.

6.3.3 Propose design fundamentals

A model design is proposed for shielding calculations and cost analysis. Considering 6 MV and 10 MV of energy linac for a modern radiotherapy department. Work load of 6 MV linac is considering 1000 cGy/week and 600 cGy/week for 10 MV linac. Work load is always 1 for primary beam shielding wall and 1/4 for secondary beam shielding wall. Occupancy factor considering 1 for outside the department which area considering uncontrolled area. Occupancy factor for inside the department considering according to this type of areas.

Drawing of the treatment room and maze scaled to 1 cm = 1 m. Both mazes are designed with two bends and a primary beam shielding wall is always common for both energy machine.

66

Fig. 6.2: Proposed room design for a modern radiotherapy department.

6.4 Data to calculating wall thickness for primary beam of 6 MV photon

Table 6.2: Applicable data for shielding calculation Location Parameters Variable Value Nature of Area

Left

side

prim

ary

barri

er(P

3)

Permissible dose(out side the barrier) P 0.00002Gy/week Uncontrolled

Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1 UncontrolledUse factor U 1/4Distance isocenter to point protected (I1P3)

dpri 6.0 m

Measured thickness 2.18 mMeasured width 4.00 m

67

Table 6.3: Applicable data for shielding calculation


Location Parameters Variable Value Nature of Area

Com

mon

prim

ary

wal

l sid

e(P

4)

Permissible dose (out side the barrier) P 0.0001Gy/week Controlled

Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1 ControlledUse factor U ¼Distance isocenter to point protected (I1P4)

dpri 4.50 m

Measured thickness 1.78 m

Measured width 3.20 m


Prim

ary

beam

shi

eldi

ng fo

r cei

ling Permissible dose (out

side the barrier) P 0.0001Gy/week Controlled

Workload at isocenter W 1000 Gy/WeekOccupancy factor T ¼ ControlledUse factor U ¼Distance isocenter to point protected dpri 4.00 m

Measured thickness 1.60 mMeasured width

2.9 m

68

Fig.6.3: General primary shielding designs for the ceiling.6.5 Data to calculating wall thickness for secondary beam of 6 MV photon

The scatter fraction (a) is always considering 0.000462 for 90o scatter to calculate secondary wall thickness which indicates as a default value on the software and the maximum radiation field (F) always considering 1600 cm2.



S4 a

nd S

5 sid

es

Permissible dose (out side the barrier) P 0.00002Gy/week Uncontrolled

Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1/4 UncontrolledUse factor U 1Distance from isocenter to point protected (I1S5, I1S5)

dsec 6.80 m

Distance from target to point protected dl 6.80 m

Distance from target to the patient dsca 1 m




S3 s

ide


Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1/4 UncontrolledUse factor U 1Distance from isocenter to point protected (I1S3)

dsec 5.40 m




69



Com

mon

sec

onda

ry w

all

(S7 a

nd S

9 sid

es)


Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1/4 ControlledUse factor U 1Distance from isocenter to point protected (I1S7, I1S9)

dsec 5. 0 m



Measured thickness

0.61 m

Table 6.8: Applicable data for shielding calculationLocation Parameters Variable Value Nature of

Area

S11

sid

e


Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1/4 ControlledUse factor U 1Distance from isocenter to point protected (I1S11)

dsec 5.50 m



Measured thickness 0.59 mTable 6.9: Applicable data for shielding calculation


S12

sid

e Permissible dose (out side the barrier) P 0.0001Gy/week Controlled

Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1 Controlled

70

Use factor U 1Distance from isocenter to point protected (I1S12)

dsec 5.50 m



Measured thickness 0.75 mTable 6.9: Applicable data for shielding calculation


M1s

ide


Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1 ControlledUse factor U 1Distance from isocenter to point protected (I1M1)

dsec 7.0 m






Sec

onda

ry w

all f

or c

eilin

g


Workload at isocenter W 1000 Gy/WeekOccupancy factor T 1/4 ControlledUse factor U 1Distance from isocenter to point protected dsec 4.0 m

Distance from target to point protected dl 4.0m


Measured thickness 0.67 m6.6 Data to calculating wall thickness for primary beam of 10 MV photonTable 6.11: Applicable data for shielding calculation: Location Parameters Variable Value Nature of Area

Permissible dose (out side the barrier) P 0.0001Gy/wee

k Controlled

Workload at isocenter W 600 Gy/Week

71

Com

mon

prim

ary

wal

l sid

e(P

2)

Occupancy factor T 1 ControlledUse factor U 1/4Distance isocenter to point protected (I2P2)

dpri 4.80 m


Measured width

3.33 m

Table 6.12: Applicable data for shielding calculationLocation Parameters Variable Value Nature of

Area

Rig

ht s

ide

prim

ary

barr

ier

(P1)

Permissible dose(out side the barrier) P 0.00002Gy/we

ek Uncontrolled

Workload at isocenter W 600 Gy/WeekOccupancy factor T 1 UncontrolledUse factor U 1/4Distance isocenter to point protected (I2P1)

dpri 5.60 m


Measured width 3.77 m



Prim

ary

beam

shi

eldi

ng fo

r ce

iling


Workload at isocenter W 600 Gy/WeekOccupancy factor T 1/4 ControlledUse factor U 1/4Distance isocenter to point protected dpri 4.00 m

Measured thickness 1.72 mMeasured width 2.9 m

6.7 Data to calculating wall thickness for secondary beam of 10 MV photon

The scatter fraction (a) is always considering 0.000381 for 90o scatter to calculate secondary wall thickness which indicates as a default value on the software and the maximum radiation field (F) always considering 1600 cm2.Table 6.14: Applicable data for shielding calculation


72

Com

mon

sec

onda

ry w

all

(S8

and

S 6 s

ides

)


Workload at isocenter W 600 Gy/WeekOccupancy factor T 1/4 ControlledUse factor U 1Distance from isocenter to point protected (I2S8, I2S6)

dsec 5. 0 m



Measured thickness

0.54 m



Rig

ht s

ide

seco

ndar

y w

alls

(S

2 an

d S

10 s

ides

)


Workload at isocenter W 600 Gy/WeekOccupancy factor T 1/4 UncontrolledUse factor U 1Distance from isocenter to point protected(I2S2, I2S10)

dsec 6.4 m



Measured thickness

0.69 m



Permissible dose (out side the barrier) P 0.0001Gy/wee

k Controlled

Workload at isocenter W 600 Gy/WeekOccupancy factor T 1/4 ControlledUse factor U 1

73

Sec

onda

ry w

alls

for

S13

and

S14

sid

es Distance from isocenter to point protected (I2S13, I2S14)

dsec 5. 5 m



Measured thickness

0.52 m



Sec

onda

ry w

all f

or S

1 sid

e Permissible dose (out side the barrier) P 0.00002Gy/week Uncontrolled

Workload at isocenter W 600 Gy/WeekOccupancy factor T 1/4 UncontrolledUse factor U 1Distance from isocenter to point protected(I2S1)

dsec 5.4 m






Sec

onda

ry w

all f

or M

2 si

des Permissible dose (out

side the barrier) P 0.0001Gy/week Controlled

Workload at isocenter W 600 Gy/WeekOccupancy factor T 1/4 ControlledUse factor U 1Distance from isocenter to point protected (I2M2)

dsec 7.0 m




74



Sec

onda

ry w

all f

or c

eilin

g


Workload at isocenter W 600 Gy/WeekOccupancy factor T 1/4 ControlledUse factor U 1Distance from isocenter to point protected dsec 4.0 m

Distance from target to point protected dl 4.0m



6.8 Data for maze door calculations of 6 MV photonTwo bends mazes are designed for both 6 MV and 10 MV linac rooms. Four steps are taken for maze door calculations such as calculation for wall scatter, leakage radiation, patient scatter and maze wall transmission. The calculating formulae of leakage and patient scatter is not available for two bends maze design of low energy (≤10 MV) treatment room. For this reasons, calculated this value with considering one bends.

6.8.1 Data for wall scatterWall scatter data for door shielding of 6 MV photon energy calculated from the figure 6.4 below.

75

Fig. 6.4: Wall scatter data measurement The wall scatter data are calculated from fig. 6.4 and this data are included into the table 6.20

Table 6.20: Applicable data for maze door shielding calculationsParameters Symbol Value

Workload at accelerator Do 2.5 × 104 cGy

Reflection coefficient at first reflection ∝1 0.005

Beam area at first reflection A1 3.87 m

Reflection coefficient at second reflection ∝2 0.02

Cross section of maze (m2) A2 5.58 m

Distance from the target to the first reflection (I1A1) di 3.0 m

Centerline distance along first leg of maze (I1m1) dr1 8.7 m

Centerline distance along second leg of maze (m1D1)

dr2 3.5 m

Calculated dose at door Ss 0.0014 cGy/week

76

6.8.2 Data for patient scatterPatient scatter data for door shielding of 6 MV photon energy calculated from the figure 6.5 below.

Fig. 6.5: Patient scatter data measurement

The wall scatter data are calculated from fig. 6.5 and this data are included into the table 6.21.



Reflection coefficient for wall reflection ∝1 0.02

Area of maze back wall that can be seen from outer maze entrance

A1 2.17 m2

Distance from target to patient (m) dsca 1.0 m

77

Distance from patient to maze centerline(I1m1) dsec 5.0 m

Centerline distance along maze (A1D1) dr1 13.2 m

Reflection coefficient for patient a 0.00075

Field area at patient (cm2) F 1600 cm2

Calculated dose at door Sp 0.00074cGy/week

6.8.3 Data for head leakage radiation Leakage radiation data for door shielding of 6 MV photon energy calculated from the figure 6.6 below.

Figure 6.6: Leakage radiation data measurementLeakage radiation data are calculated from fig. 6.6 and this data are included into the table 6.22.



Reflection coefficient for wall reflection ∝1 0.005

Area of primary or secondary wall that can be seen A1 2.48 m2

78

from the maze door

Distance from target to maze centerline (I1A1) di 3.50 m

Centerline distance along the maze (A1D1) ds 11.20 m

Ratio of dose due to head leakage at 1.0 m from the target to the dose at the isocenter

Lo 0.0003

Dose at the maze door due to head leakage L 0.00006cGy/week

6.8.4 Data for maze wall transmissionMaze wall transmission data for door shielding of 6 MV photon energy calculated from the figure 6.7 below

Fig. 6.7: Applicable data for maze door shielding calculationsMaze wall transmission data are calculated from fig. 6.7 and this data are included into the table 6.23Table 6.23: Applicable data for maze door shielding calculations

Parameters Symbol ValueWorkload at accelerator Do 2.5 × 104 cGy/weekSecondary wall thickness in font of maze (m)

0.60 m

79

Maze wall with TVL TVL 0.279Number of TVL #TVL 2.15Distance from the target to the maze door (m)

d 9.5 m

Head leakage Lo 0.0003Dose at door due to transmission T 0.00058

cGy /week

6.8.5 Total dose at maze door

Use the equations (4.13) and (4.14), the total dose at the maze door per week 0.0073 cGy or 7.3 mrem per week.

6.9 Conventional room design

The fig. 6.8 is represent the conventional room design with the 6 MV linac and 10 MV linac rooms. The main differences between proposed design and conventional design are the design of maze and common primary wall which reduced building cost for maze door and a vertical primary and secondary wall building cost. 6 MV and 10 MV linac rooms are separately designed in conventional.

Fig. 6.8: Conventional room design

Chapter 7

7 OBSERVATIONS AND RESULS

80

7.1 IntroductionThe present research work is an attempt to design low cost radiotherapy facilities using local resources and for these purposes various types of materials considered to study and calculated TVL for different energies. To calculation of TVL, density measurement is very essential and for these purposes different types of concretes were made and densities were measured. The market values of the materials were collected to measurement cost and searching cost benefits.

The results of the investigation are presented in this chapter that practically useful for the country to established radiotherapy department. The results and discussions have been arranged for:

a) Density of the local concreteb) Cost analysis for various room dimensionsc) Cost analysis for using various shielding materialsd) Model design of a low cost radiotherapy departmente) Comparison of costing with propose design and conventional design f) Shielding thickness for primary beamg) Shielding thickness for secondary beamh) Measurement of TVLi) Software development

7.2 Density of the local concrete

In my investigation, three types of concrete available in the local market with different densities. Mixture ratio and calculated densities are discussed in chapter 5.

Table 7.1: Measured materials densities

Materials Resource Major part of component

Measured density

Brick concrete Locally available small part of bricks 1.68 g/cm3

Ordinary concrete

Locally available at Sylhet, Sherpur, Coxbazar, Panchagar district.

Gray color stone, white color stone 2.18 g/cm3

Ordinary concreteSylhetmainly imported from India

Black color stonelocal name “Pakur stone”.

2.36 g/cm3

7.3 Cost analysis for various room dimensions

The cost of radiotherapy treatment room depends upon the dimension of the room. In this view primary and secondary beam shielding were analyzed in this various room dimension purpose.

81

7.3.1 Primary beam shielding analysis (6 MV)

Primary shielding volume i.e. primary shielding cost increases with polynomial characteristics with the increasing of the room dimension.

Increasing 1 meter of length (gantry rotation plane) increases 7% of total cost without considering space cost. After a certain limit of increasing length, the shield volume may be decreases with tend to 0.

Shielding volume Vs room dimension graph

y = -0.0103x2 + 5.5359x + 52.377

40

50

60

70

80

90

100

110

1 2 3 4 5 6 7 8 9

Room dimension in m

Shie

ldin

g vo

lum

e in

m3

Fig7.1: Relation between primary shielding volume and room dimension.

Room dimension is an one of the criteria to reduce costing. Room dimension should be minimum as possible with maximum facilities because of considering costing.

7.3.2 Secondary beam shielding analysis (6 MV)

Secondary shielding volume i.e. secondary shielding cost increases with polynomial characteristics with the increasing of the room area. Increasing 1 m2 of area increases about 3% (averagely) of total cost without considering space cost. After a certain limit of increasing area, the shield volume may be decreases with tend to 0.

82

Secondary shielding volume vs room area

y = 0.9417x2 + 6.5116x + 5.9329

40

60

80

100

120

140

160

16 25 36 49 64 81 100 121 144

Room area im m2

Shie

ldin

g vo

lum

e in

m3

Fig. 8.2: Relation between secondary shielding volume and room dimension.

7.4 Cost analysis for using various shielding materials

Primary shielding volume with 6 MV photon was calculated for various materials and observed that for low-density material need high shielding volume and more space area. Considering cost of space areas and cost of materials, the total primary shielding cost was calculated for a standard radiotherapy room and all parameters were constant for all calculation of materials. The relative cost are as follows:

Table 7.2: Primary shielding material vs costUse of Material Density in g/cm3 Total Cost (Taka)

Bricks concrete 1.68 4,17,281

Concrete(gray color stone) 2.18 4,38,677

Concrete(Black color stone) 2.36 4,26,642

Steel 7.8 57,43,482

Lead 11.35 60,43,207

83

Using Material Vs Relative Cost

Relative cost

Bricksconcrete

Concretep=2.36g/cc

Concretep=2.18g/cc

Lead Stee l

Fig.7.3: Graphical representation of material vs. cost.

7.5 Model design of a low cost radiotherapy department

A modern radiotherapy department consists of minimum one 6 MV photon therapy and 10 MV photon therapy facilities with associated control rooms are essential with one simulator room and its control room. The radiation therapy department design with very compactly because of to avoid public exposure with unwanted radiation. This whole department is possible to established in the corner of the hospital. The shielding calculations of this facilities are considered with some assumptions.

84

Fig.7.4: Model design of a low cost radiotherapy department.

7.5.1 Investigation for a proposed design

Table 7.3: Propose design advantage tableInvestigation field Investigation result

Costing for treatment rooms 25% less than other conventional rooms

Maze door No shielded maze door is needed

Closing of CT simulator roomThe secondary barriers are used for simulator shielding. It reduces simulator room costing.

Linac control room

Both linac control rooms are far distance to the treatment room, closely attached to the maze door and it is given more facilities to access treatment room and patient entering.

Position of the installation

It is possible to established the whole radiotherapy department in the corner of the hospital to reduce unwanted radiation for public.

85

7.6 Shielding thickness for radiotherapy room

The primary and secondary shield thickness was calculated according to the fig.7.4 and use as material ordinary concrete (ρ=2.36 g/cm3) because of its cost benefits. Shielding calculation will be easy to calculate for every material with the use of shielding calculation software. The above figure is the model design of a modern radiotherapy department and considering this imaginary design, shield thickness was calculated. The occupancy factor is dependable on the location and surrounding environment of the location so it does not properly use on a model design of a radiotherapy room.

7.6.1 Primary shielding thickness for 6 MV linac room

Table 7.4: Primary barrier thickness and width

Barrier Location Wall thickness in Meter

Barrier width in meter

Left side barrier 2.18 4.00Right side or common barrier 1.78 3.20Ceiling thickness 1.60 2.90

7.6.2 Secondary shielding thickness for 6 MV linac room

Table 7.5: Secondary thickness

Barrier Location Wall thickness in Meter

Linac based side or S3 point side 0.79S4 and S5 point side 0.73S7 and S9 point side or common wall side 0.61S11 point side 0.59S12 point side 0.75M1 point side 0.70Ceiling thickness 0.67

7.6.3 Primary shielding thickness for 10 MV linac room

Table 7.6: Primary barrier thickness and widthBarrier Location Wall

thickness in Meter

Barrier width in meter

Right side barrier or P1 point side

2.11 3.77

Light side or common barrier 1.63 3.33Ceiling thickness 1.72 2.90

86

7.6.4 Secondary shielding thickness for 10 MV linac room

Table 7.7: Secondary thicknessBarrier Location Wall thickness in

MeterLinac based side or S1 point side 0.75S2 and S10 point side 0.69S6 and S8 point side 0.54S13 point side 0.52S14 point side 0.52M2 point side 0.64Ceiling thickness 0.60

7.6.5 Thickness for common shielded wall

According to the figure 7.4, a common primary wall and two secondary walls beside the primary barrier are present. In this case maximum thickness and width are used. So the applicable results are given below:

Table 7.8: Value of common barrierField Wall thickness in meter

Thickness of common primary barrier

1.78

Width of common primary barrier 3.20

Thickness of common secondary barrier

0.61

7.7 Maze door calculation for 6 MV linac The dose rate at the maze door position for 6 MV linac room is calculated and this calculated value is 0.0073 cGy or 7.3 mrem per week. The recommended dose level at he maze door is 10 mrem per week [2]. So, no shielded door is needed.

7.8 TVL values for various materials and energies

The TVL values for various shielding materials and energies were not available at this moment for this reasons it was need to calculate and for this purposes, the relations were established between the densities and the TVL values for various materials and energies. This session discussed in the chapter 3 and calculated TVL from the graph or graphical equations

Table 7.9: Calculated TVL values for various material with different energies.

87

X-ray Megavoltage Shielding material TVL primary

in meterTVL 900 leakage

in meter







7.9 Software development

Shielding calculation is mainly based on mathematics including various data and using various parameters. This is the part of health physics and a person has to perform this work by a lot of effort. This job involves many categories of options. For example, room design, energy of beam, materials etc. Different options using different formulas and different data. As a result, we can save lot of time, that would otherwise be needed, by using this software. This software designed with the use of Microsoft Visual Basic 6.0 programming language. It has two parts –i) Primary beam shielding and ii) Secondary beam shielding, each part works independently.

The software was developed for primary beam and secondary beam shielding calculations. This window based software able to calculate primary wall and ceiling thickness of 6 different mega voltage energies such as 6, 10, 15, 18, 20, 24 MV photon for 5 different shielding materials such as brick concrete (ρ = 1.68 g/cm3), gray concrete (ρ = 2.18 g/cm3), black concrete (ρ = 2.36 g/cm3), steel and lead.

It is able to calculate secondary beam shield thickness of 6 different mega voltage energies such as 6, 10, 15, 18, 20, 24 MV photon and for 6 MV photon, the secondary wall and ceiling calculated for 3 different shielding materials such as brick concrete, gray concrete, black concrete. For other energies such as 10, 15, 18, 20, 24 MV photon, the software able to calculate secondary wall thickness and ceiling for all 6 materials.

It has not possible to compare another software with this software because this type of software is not available in the market but this software has compared to the

88

calculate with the different type of example calculations from the book of “ Shielding Techniques for Radiation Oncology Facilities”, second edition, by Patton H. McGinley, Ph.D.

7.10 Overall findings of this work

(i) Bricks concrete with density 1.68 g/cm3 and ordinary concrete with density 2.36 g/cm3 have an economic benefit for the construction of radiotherapy facilities. Bricks concrete has more benefit than ordinary concrete with density 2.36g/ cm3, but it has been proposed to use ordinary concrete for the design of the radiotherapy facilities in our country because bricks concrete may induce an extra humidity problem. It should be used in the less humidity zone in the country.

(ii) Room dimension of the radiotherapy facilities should be minimum as possible with maximum facilities for the minimize of the costing.

(iii) A realistic structural design is most important, considering the cost benefit facilities. One primary shielding barrier associated with secondary barrier should be common for two radiotherapy room facilities.

(iv) No shielded door is needed for two bends maze design for linac facilities with energy of 6 MV to 10MV.

(v) The TVL value of the shielding materials for its particular densities should be determined before the design of the radiotherapy room.

(vi) Software is suitable for the measuring of radiation shielding thickness with various energy and materials which is very helpful for the pre-design of the system.

89

Chapter 8

8 DISCUSSIONS

8.1 About this work

Radiation is harmful to health, although they are also useful in several ways to mankind. For such use one has to ensure due protection from the radiation. The primary aim of radiological protection is to provide an appropriate standard of protection for people without unduly limiting the beneficial practices associated with radiation therapy. All those concerned with radiological protection have to make value judgments about the relative importance of different kinds of risk and about balancing of risks and benefits.

To establish a radiotherapy department, considering low cost facilities, architectural design is important issue. It is actually the task of an architect to design a treatment room for a linear accelerator. However, he is required to consult a medical physicist because the later knows the provision of radiation protection and is responsible for the shielding design. Construction cost of a well-protected radiotherapy department is highly dependent on the shielding design. Space area, space cost, material cost, safe control design, operating facilities - everything must be considered for this design.

8.2 Shielding Materials

Locally available shielding materials are earth, bricks, and stone with different densities. Concrete was made by bricks, gray color stone, black color stone. Concrete density and cost was measured separately. In my investigation, use of black concrete, which made by black stone is more compatible than other with considering cost benefit and space cost. Though black stone is an imported item from India but is available in Bangladesh. Gray concrete is locally available and its advantage approximately same as black concrete without considering space cost. The average space cost considering 9000 taka per m2 of land in the divisional city of Bangladesh.

8.3 Room dimension and design

Room dimension is an important issue of radiotherapy treatment room, considering public and employee protection, optimum installation facilities and cost effectiveness. So a standard radiotherapy treatment room dimension must be as low as possible with maximum facilities. With this view, room dimension was observed and designed. The whole department designed such as that the direct primary beam could not reach the door and the door do not need heavy shielding. The CT simulator and another simulator room will be together with a linac room as a result radiation region

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more compact. This is an advantage to the protection of public from radiation exposure. Two radiotherapy treatment room were designed with a common primary wall and here was cost reduced. To reduce the wall thickness, occupancy factor is the important issue because it depends upon the occupational work area. To reduce the value of the occupancy factors, radiotherapy room should be a corner of the building.

8.4 Shielding thickness

Any shielding thickness depends on various parameters such as doom dimension, room design, workload, occupancy area, use factor etc. The above data will be collected before calculating the thickness. Occupancy factor depends on the location and surrounding condition of the zone where radiotherapy department will be established. So the shielding thickness was calculated by considering an imagine model treatment room. All thickness was calculated with considering photon energy but electron energy was not calculated because its low penetration ability and photon energy shielding is enough for electron energy shielding.

8.5 Software development

The software was developed with the help of the Microsoft Visual Basic 6.0 and it is an animated software and friendly with users. The software was only developed for the shielding thickness calculation of the primary and secondary beam with various energy and for different materials. New observed materials are included here and TVL data are calculated with the relation of the density of the materials. TVL data from Varian (1995) are used only where NCRP Report No. 51 TVL data are not available. In maximum cases, first and equilibrium TVL method is used when the first and equilibrium TVL is available, otherwise only first TVL method is used in this software.

In the present situation of Bangladesh, there are no available Medical Physicist or Health physicists who are well educated in this field. So, this software will be very helpful for a person who wants to establish a modern radiotherapy department or facilities. Using this software, a person will be taken economical and space estimate before execute his plane. This software is very important for survey measurement, reporting and quick calculation of shielding thickness.

8.6 Maze door calculation The dose rate at the maze door position for 6 MV linac room is calculated and this calculated value is 0.0073 cGy or 7.3 mrem per week. The recommended dose level at he maze door is 10 mrem per week. The reflection coefficient for photons with energy between 0.1 and 10 MeV in concrete is the same for same angle[2] but different in different angles. The proposed design of 6 MV and 10 MV linac rooms are the same structural layout with maze entrance. So, The calculation of maze door shielding of 6MV and 10 MV is the same for angles which associated with reflection coefficient. As a result, the dose

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rate at the maze door of 10 MV linac room is the approximately same as 6 MV linac room’s door.

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References

[1] Daily Jogantor: Page – 14, Date 16/5/2005

[2] McGinley, Patton H: Shielding Techniques for Radiation Oncology Facilities; Second Edition, Medical Physics Publishing, Madison, Wisconsin. 2002

[3] Cunnigham, Jone Robert and Johns, Harold Elford: The Physics of Radiology; Fourth Edition, Charles C Thomas Publisher, 1983.

[4] Greene, David and Williams, Peter C: Linear Accelerators for Radiation Therapy; Second Edition, Institute of Physics Publishing, Bristol and Philadelphia, 1997

[5] National Council on Radiation Protection and Measurements (NCRP); Report No. 51, Published in 1970


[7] McGinley, P.H; Butker, E: Phys. Med. Biol. 39: 1331-1336; 1994

[8] Nelson, W.R; LaRiviere, P.D: Health Physics, 47: 811-818; 1984

[9] International Electrotecnical Commission (IEC), 112, 1981


[11] Khan, Faiz M., Ph.D: The Physics of Radiation Therapy, Williams & Wilkins, 1984.

[12] Tylor, L, Rodgers, J.E: Medical Physics, 26: 1446-1446; 1999.

[13] IAEA Post Graduate Educational Course Training Material on Radiation Protection in Radiotherapy, Part 7, Practical 1

[14] Hosain, M. Moukbul: Cost and Management Accounting, 4th Edition, Deganta Printers, 1996.

[15] McGinley, P.H.; James, J.L: Dose levels in the maze of medical accelerator rooms. Midyear Topical Meeting of the Health Physics Society, Health Physics of Radiation Generating Machines. San Jose, Califonia, 1997

[16] Zakaria, Golam Abu: The design of Radiotherapy Treatment Rooms, Proceedings of the workshop on medical physics in radiotherapy and nuclear medicine, 5-10 December,1999, Dhaka, Bangladesh.

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Appendix

Example calculations with this software

1. Evaluate the thickness of concrete needed for the primary shield shown below. The beam is 20 MV, and the workload (W) is 500gy per week at the isocenter. Carry out the evaluation using the following methods [2]:

(a) Curve E.8 from NCRP Report No. 51(b) First and equilibrium TVL method (c) Varian TVL data

Results[2]: (a) 2.21 meter ordinary concrete(b) 2.16 meter ordinary concrete(c) 2.20 meter ordinary concrete

Software calculated result: (b) 2.16 meter ordinary concrete

(This software able to calculate shielding thickness only First and equilibrium TVL method.)

2. Find the width (W) of the primary barrier of example 1c. The primary barrier protrudes into the room, and the adjacent secondary wall is 0.91 m thick i.e. distance to the point protected is 3.2 m.

Result [2]:(i) Primary barrier width (W): 2.4 m

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Software calculated result: (i) 2.42 meter

3. Use the Varian 90o leakage TVLs for a 20 MV x-ray beam to determine the concrete thickness needed in the secondary wall shown below. Repeat the evaluation for an uncontrolled area.

Result [2]: (i) Secondary barrier thickness for controlled area: 0.79 meter concrete

(ii) Secondary barrier thickness for uncontrolled area: 0.823 meter concrete

Software calculated results:(i) 0.789 meter concrete for controlled area

(ii) 0.822 meter concrete for uncontrolled area

4. Repeat problem 3 for a 6 MV x-ray beam. Since the beam is less than 10 MV, both patient scatter and head leakage radiation must be considered when designing the secondary barrier. A workload of 1000 Gy per week will be used. Determine the concrete thickness for secondary barrier.

Result [2]:

(i) Secondary barrier thickness based on patient scatter: 0.491 meter concrete (ii) Secondary barrier thickness based on head leakage : 0.725 meter concrete

(iii) Finally decided secondary barrier thickness: 1.00 meter concrete(approx).

Software calculated results: (i) Secondary barrier thickness based on patient scatter: 0.4902 meter

concrete (ii) Secondary barrier thickness based on head leakage : 0.7259 meter

concrete (iii) Finally decided secondary barrier thickness: 0.78 meter concrete (exact).

5. Determine the primary beam shielding thickness for the assumptions of 10 MV linac,

dref= 1m (FAD = 1m), workload (W) = 40000 Gy/year, TVLconcrete = 40 cm, design constraint P = 0.3 mSv/year, occupancy factor T = 0.25, patient waiting distance d = 6m, use factor U = 0.25 [13].

Result [13]:(i) Shielding thickness approximately 2.2 m concrete.

Software calculated result:(i) Shielding thickness 2.112 m concrete.


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dref= 1m (FAD = 1m), workload (W) = 40000 Gy/year, TVLconcrete = 40 cm, design constraint P = 0.3 mSv/year for patient, P = 20mSv/y for staff, occupancy factor T = 0.05 for patient, occupancy factor T = 1 for staff, patient waiting distance d = 6m, use factor U = 0.25 [13].



SOFTWARE DESIGN

7.1 Introduction

Shielding calculation mainly based on mathematics including various data and using various parameters. This is the part of health physics and a person has to perform this work by a lot of study. In this job involves many categories of options and for example, room design, energy of beam, materials etc. For example, different options using different formulas and different data. As a result, it will be long time job which unnecessary with the help of this software. In the present situation of Bangladesh, there are no available Medical Physicist or Health physicists who are well educated in this field. So, this software will be very helpful for a person who wants to establish a modern radiotherapy department or facilities. Using this software, a person will be taken economical and space estimate before execute his plane. This software designed with the use of Microsoft Visual Basic 6.0 programming language. It has two parts –i) Primary beam shielding and ii) Secondary beam shielding, each part works independently with each other.

7.2 Programming Codes for Primary Beam Shielding Software

‘SELECT ENERGY SLIDE’Option ExplicitDim T As IntegerDim Mh, M1, M2 As StringPrivate Sub Combo1_Change()If Combo1.DataChanged = True ThenCommand1.Enabled = TrueElse

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Command1.Enabled = False

End If

End Sub

Private Sub Command1_Click()

If Combo1.DataChanged = False Then

M2 = “Please select Barrier type and click NEXT, or Help: [email protected]”MsgBox M2

End IfSelect Case Combo1.TextCase “6 MV Photon”Form2.Show 1Case “10 MV Photon”Form3.Show 1Case “15 MV Photon”Form4.Show 1Case “18 MV Photon”Form5.Show 1Case “20 MV Photon”Form6.Show 1Case “24 MV Photon”Form7.Show 1

End Select

End Sub

Private Sub Command2_Click()If Command2.Caption = “EXIT” ThenM1 = MsgBox(“Do You Want to Exit?”, vbYesNo + vbQuestion, “EXIT”)If M1 = vbYes ThenUnload MeElseExit SubEnd IfElseCombo1.Enabled = TrueCommand1.Enabled = TrueEnd IfEnd Sub

Private Sub Command3_Click()Mh = “In this section, thickness of the primary Barrier calculated with the photon energy 6MV, 10MV, 15 MV, 18 MV, 20 MV and 24 MV. Varian TVL data are used only energy ranges of 15MV to 24 MV for LEAD. Contact [email protected]”MsgBox MhEnd Sub

Private Sub Form_Load()T = 0Timer1.Interval = 100End Sub

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Private Sub Timer1_Timer()T = T + 1If T = 1 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\p001.BMP”)End If

‘Calculation of 6 MV Photon Energy Slide’Option Explicit

Dim T, T2, T3 As IntegerDim M1, M2, M3, M4, M5, M6, M7, M8, M9, M10, M11, M12, M13 As StringDim Bx, N, TVL1, TVLe, TVL, S, W As Double

Private Sub Command1_Click()Bx = Text1 * Text2 * Text2 / (Text3 * Text4 * Text5)N = Log(1 / Bx) / Log(10)

Select Case Combo1.Text

Case “Ordinary Concrete”TVL1 = 0.35TVLe = 0.35S = TVL1 + N * TVLe – TVLe

Case “Concrete (p=2.18 g/cm3)”TVL = 0.378S = N * TVL

Case “Lead”TVL1 = 0.055TVLe = 0.057S = TVL1 + N * TVLe – TVLe

Case “Steel”TVL1 = 0.099TVLe = 0.099S = TVL1 + N * TVLe – TVLe

Case “Bricks Concrete (p=1.68 g/cm3)”TVL = 0.506S = N * TVL

End Select

W = 0.566 * Text2 + 0.61If S < 0 ThenS = 0End If

Label13.Caption = “THE PRIMARY BARRIER THICKNESS:”Label12.Caption = (S)Label11.Caption = “Meter”

Label16.Caption = “THE PRIMARY BARRIER WIDTH:”Label15.Caption = (W)Label14.Caption = “Meter”Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Result.bmp”)End Sub

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Private Sub Command2_Click()If Command2.Caption = “EXIT” ThenM12 = MsgBox(“Do You Want to Exit?”, vbYesNo + vbQuestion, “EXIT”)If M12 = vbYes ThenUnload MeElseExit SubEnd IfElseCombo1.Enabled = TrueCommand1.Enabled = TrueEnd IfEnd Sub

Private Sub Command3_Click()M13 = “Use TVL data: It has been used First and Equilibrium TVL method for ordinary concrete, steel, and lead. Distance dp: Distance from the x-ray target to the point protected. Usually, this distance will be taken from x-ray target to the outside of the primary Barrier and plus 1 meter. Workload (W): It has been taken with the assumption with considering of busyness of the department. Usually, design purposes NCRP Recommended value is 1000Gy/week. Use Factor (U): It is a normalized value of radiation falling time on the Barrier. 1/4 is the NCRP recommended value for primary Barrier. Occupancy Factor (T): It depends upon the type of occupancy area outside the primary Barrier and its value varies to the ‘Control’ and ‘Uncontrolled’ areas. Three types of values are used such as 1, 1/4, 1/16 according to the type of occupancy areas, Full occupancy, Partial occupancy, and Occasional occupancy respectively. For more information request to [email protected] or [email protected] “MsgBox M13End Sub

Private Sub Form_Load()T = 0

Timer1.Interval = 400T2 = 0Timer2.Interval = 200End Sub

Private Sub Text1_Change()If Combo1.DataChanged = False ThenM1 = “Please Select Material before input value, or Help: [email protected]”MsgBox M1End If

If IsNumeric(Text1) = False Then

M6 = “Please Enter a NCRP recommanded value or Help: [email protected]”MsgBox M6ElseText2.Enabled = TrueEnd IfEnd Sub

Private Sub Text1_Click()

If Combo1.DataChanged = True ThenTimer2.Enabled = True

T2 = 1End IfIf T2 = 2 Then

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Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 3 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End If

If T2 = 3 ThenT2 = 0ElseTimer2.Enabled = True

End If

End Sub

Private Sub Text2_Change()

If IsNumeric(Text2) = False ThenM7 = “Please Enter an actual value or designed value or Help: [email protected]”MsgBox M7ElseText3.Enabled = TrueEnd If

If Text1 > 0 ThenText2.Enabled = TrueElseM2 = “This value of (P) is not possible, this value Might be greater than 0, Help [email protected]”MsgBox M2Text1.Text = 0.000000001End IfEnd Sub

Private Sub Text2_Click()If Text1.DataChanged = True ThenTimer1.Enabled = TrueEnd IfEnd Sub


If IsNumeric(Text3) = False ThenM8 = “Please Enter an assumed or designed value for workload, Help: [email protected]”MsgBox M8ElseText4.Enabled = True

End If

End Sub

Private Sub Text3_Click()If Text3.DataChanged = True ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test3.bmp”)End IfEnd Sub

Private Sub Text4_Change()If Text3.Text = True ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Occupancy.bmp”)

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End IfIf Text3 <= 0 ThenM3 = “This value of Workload(W) is not possible, this value M2ght be greater than 0”MsgBox M3

Text3.Text = 0.0000001

End IfIf IsNumeric(Text4) = False ThenM9 = “Please Enter a valid value according to design or Help: [email protected]”MsgBox M9ElseText5.Enabled = True

End IfEnd Sub

Private Sub Text4_Click()If Text3.DataChanged = True ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Occupancy.bmp”)End IfEnd Sub

Private Sub Text5_Change()If Text4 <= 0 ThenM4 = “This value of Use Factor(U) is not possible, this value M2ght be greater than 0”MsgBox M4

Text4.Text = 0.00000001End IfIf IsNumeric(Text5) = False ThenM10 = “Please Enter a valid value according to NCRP report, Help: [email protected]”MsgBox M10ElseCommand1.Enabled = True

End IfEnd Sub

Private Sub Text6_Change()If Text5 <= 0 ThenM5 = “This value of Occupancy Factor(T) is not possible, this value M2ght be greater than 0”MsgBox M5

Text5.Text = 0.00000001End If

End Sub

Private Sub Timer1_Timer()T = T + 1If Text1.DataChanged = True ThenTimer1.Enabled = TrueEnd If

If T = 1 Then

Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Move.bmp”)End IfIf T = 2 Then

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Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Move1.bmp”)End If

If T = 2 ThenT = 0End IfIf Text3.DataChanged = True ThenTimer1.Enabled = FalseElseTimer1.Enabled = TrueEnd If

End Sub

Private Sub Timer2_Timer()

T2 = T2 + 1If Text1.DataChanged = True ThenT2 = 1End IfIf T2 = 2 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 3 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End IfIf T2 = 4 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 5 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End IfIf T2 = 6 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 6 ThenT2 = 0End If

If Text2.DataChanged = True ThenTimer2.Enabled = FalseElseTimer2.Enabled = TrueEnd If

End Sub

‘Calculation of 10 MV Photon Energy Slide’Option ExplicitDim T As IntegerDim T2 As IntegerDim M3 As StringDim M As StringDim Mn As StringDim Mn1 As StringDim Mn2 As StringDim M4 As StringDim M5 As StringDim M6 As StringDim M7 As String

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Dim M8 As StringDim M9 As StringDim M10 As StringDim M11 As StringDim Et As StringDim Bx As DoubleDim N As DoubleDim TVL1 As DoubleDim TVLe As DoubleDim TVL As Double

Dim S As DoubleDim Wdi As Double


Bx = Text1 * Text2 * Text2 / (Text3 * Text4 * Text5)N = Log(1 / Bx) / Log(10)




Case “Lead”TVL1 = 0.057TVLe = 0.056S = TVL1 + N * TVLe – TVLe


Case “Bricks Concrete (p=1.68 g/cm3)”TVL = 0.577S = N * TVLEnd Select

Wdi = 0.566 * Text2 + 0.61If S < 0 ThenS = 0End If

Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Result.bmp”)

Label13.Caption = “THE PRIMARY BARRIER THICKNESS:”Label12.Caption = (S)Label11.Caption = “Meter”

Label16.Caption = “THE PRIMARY BARRIER WIDTH:”Label15.Caption = (Wdi)Label14.Caption = “Meter”

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End Sub

Private Sub Command2_Click()If Command2.Caption = “EXIT” ThenM10 = MsgBox(“Do You Want to Exit?”, vbYesNo + vbQuestion, “EXIT”)If M10 = vbYes ThenUnload MeElseExit SubEnd IfElseCombo1.Enabled = True

Command1.Enabled = True

End If

End Sub

Private Sub Command3_Click()M11 = “Use TVL data: It has been used First and Equilibrium TVL method for ordinary concrete, steel, and lead. Distance dp: Distance from the x-ray target to the point protected. Usually, this distance will be taken from x-ray target to the outside of the primary Barrier and plus 1 meter. Workload (W): It has been taken with the assumption with considering of busyness of the department. Usually, design purposes NCRP Recommended value is 1000Gy/week. Use Factor (U): It is a normalized value of radiation falling time on the Barrier. 1/4 is the NCRP recommended value for primary Barrier. Occupancy Factor (T): It depends upon the type of occupancy area outside the primary Barrier and its value varies to the ‘Control’ and ‘Uncontrolled’ areas. Three types of values are used such as 1, 1/4, 1/16 according to the type of occupancy areas, Full occupancy, Partial occupancy, and OccaSonal occupancy respectively. For more information request to [email protected] or [email protected]”MsgBox M11End Sub

Private Sub Form_Load()T = 0Timer1.Interval = 200T2 = 0Timer2.Interval = 200End Sub



M3 = “Please Select Material before input value, or Help: [email protected]”MsgBox M3End If


M4 = “Please Enter a NCRP recommended value or Help: [email protected]”MsgBox M4ElseText2.Enabled = True

End If

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End Sub

Private Sub Text1_Click()If Combo1.DataChanged = True ThenTimer2.Enabled = True

T2 = 1End IfIf T2 = 2 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 3 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End If


End IfEnd Sub



If Text1 <= 0 ThenM = “This value of (P) is not possible, this value Might be greater than 0, Help [email protected]”MsgBox MText1.Text = 0.000000001End IfEnd Sub




End If

End Sub

Private Sub Text3_Click()

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If Text3.DataChanged = True ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test3.bmp”)End IfEnd Sub


If Text3 <= 0 ThenMn = “This value of Workload(W) is not possible, this value Might be greater than 0”MsgBox Mn

Text3.Text = 0.0000001


End IfEnd Sub


Private Sub Text5_Change()If Text4 <= 0 ThenMn1 = “This value of Use Factor(U) is not posSble, this value Mght be greater than 0”MsgBox Mn1

Text4.Text = 0.00000001End IfIf IsNumeric(Text5) = False ThenM8 = “Please Enter a valid value acording to NCRP report, Help: [email protected]”MsgBox M8ElseCommand1.Enabled = True

End IfEnd Sub

Private Sub Text6_Change()If Text5 <= 0 ThenMn2 = “This value of Occupancy Factor(T) is not posSble, this value Mght be greater than 0”MsgBox Mn2


End Sub


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If T = 1 Then

Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Move.bmp”)End IfIf T = 2 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Move1.bmp”)End If


End Sub




End Sub

‘Calculation of 15 MV Photon Energy Slide’Option ExplicitDim TVL, Bx, N, S, TVL1, TVLe, Wdi As DoubleDim T, T2 As Integer

Dim M3 As StringDim M As StringDim Mn As StringDim Mn1 As String

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Dim Mn2 As String

Dim M4 As StringDim M5 As StringDim M6 As StringDim M7 As StringDim M8 As StringDim M9 As StringDim M10 As StringDim M11 As StringDim Et As String





Case “Lead”TVL = 0.057S = N * TVL



End SelectWdi = 0.566 * Text2 + 0.61If S < 0 ThenS = 0End If

Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Result.bmp”)

Label13.Caption = “THE PRIMARY BARRIER THICKNESS”Label12.Caption = (S)Label11.Caption = “Meter”Label16.Caption = “THE PRIMARY BARRIER WIDTH”Label15.Caption = (Wdi)Label14.Caption = “Meter”

End Sub

Private Sub Command2_Click()If Command2.Caption = “EXIT” ThenM10 = MsgBox(“Do You Want to Exit?”, vbYesNo + vbQuestion, “EXIT”)If M10 = vbYes Then

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Unload MeElseExit SubEnd IfElseCombo1.Enabled = True


End If

End Sub

Private Sub Command3_Click()M11 = “Use TVL data: It has been used First and Equilibrium TVL method for ordinary concrete and steel but Varian data is used for only LEAD. Distance dp: Distance from the x-ray target to the point protected. Usually, this distance will be taken from x-ray target to the outside of the primary Barrier and plus 1 meter. Use Factor (U): It is a normalized value of radiation falling time on the Barrier. 1/4 is the NCRP recomended value for primary Barrier. Occupancy Factor (T): It depends upon the type of occupancy area outside the primary Barrier and its value varries to the ‘Control’ and ‘Uncontrol’ areas.Three types of values are used such as 1, 1/4, 1/16 according to the type of occupancy areas, Full occupancy, Partial occupancy, and Occasional occupancy respectively. For more information request to [email protected] or [email protected]”MsgBox M11End Sub

Private Sub Form_Load()T = 0Timer1.Interval = 200

Timer2.Interval = 200End Sub





M4 = “Please Enter a NCRP recommanded value or Help: [email protected]”MsgBox M4ElseText2.Enabled = True

End If

End Sub

Private Sub Text1_Click()If Combo1.DataChanged = True ThenTimer2.Enabled = TrueT2 = 1End If

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If T2 = 2 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 3 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End If


End IfEnd Sub







End If

End Sub


Private Sub Text4_Change()If Text3 <= 0 ThenMn = “This value of Workload(W) is not possible, this value Might be greater than 0”MsgBox Mn

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Text3.Text = 0.0000001


End IfEnd Sub


Private Sub Text5_Change()If Text4 <= 0 ThenMn1 = “This value of Use Factor(U) is not possible, this value Mght be greater than 0”MsgBox Mn1

Text4.Text = 0.00000001End IfIf IsNumeric(Text5) = False ThenM8 = “Please Enter a valid value acording to NCRP report, Help: [email protected]”MsgBox M8ElseCommand1.Enabled = True

End IfEnd Sub

Private Sub Text6_Change()If Text5 <= 0 ThenMn2 = “This value of Occupancy Factor(T) is not possible, this value Mght be greater than 0”MsgBox Mn2


End Sub


If T = 1 Then


If T = 2 ThenT = 0End If

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If Text3.DataChanged = True ThenTimer1.Enabled = FalseElseTimer1.Enabled = TrueEnd IfEnd Sub



If Text2.DataChanged = True ThenTimer2.Enabled = FalseElseTimer2.Enabled = TrueEnd IfEnd Sub

‘Calculation of 18 MV Photon Energy’

Option ExplicitDim TVL As DoubleDim S As DoubleDim TVL1 As DoubleDim TVLe As DoubleDim M3 As StringDim M As StringDim Mn As StringDim Mn1 As StringDim Mn2 As StringDim Bx As DoubleDim T As IntegerDim T2 As IntegerDim N As Double

Dim Wdi As DoubleDim M4 As StringDim M5 As StringDim M6 As String

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Dim M7 As StringDim M8 As StringDim M9 As StringDim M10 As StringDim M11 As StringDim Et As String








End Select

Wdi = 0.566 * Text2 + 0.61If S < 0 ThenS = 0End If

Label13.Caption = “THE PRIMARY BARRIER THICKNESS”Label12.Caption = (S)Label11.Caption = “Meter”

Label16.Caption = “THE PRIMARY BARRIER WIDTH”Label15.Caption = (Wdi)Label14.Caption = “Meter”Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Result.bmp”)End Sub

Private Sub Command2_Click()If Command2.Caption = “EXIT” ThenM10 = MsgBox(“Do You Want to Exit?”, vbYesNo + vbQuestion, “EXIT”)If M10 = vbYes ThenUnload MeElse

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Exit SubEnd IfElseCombo1.Enabled = True


End If

End Sub

Private Sub Command3_Click()M11 = “Use TVL data: It has been used First and Equilibrium TVL method for ordinary concrete and steel but Varian data is used for only LEAD. Distance dp: Distance from the x-ray target to the point protected. Usually, this distance will be taken from x-ray target to the outside of the primary Barrier and plus 1 meter. Use Factor (U): It is a normalized value of radiation falling time on the Barrier. 1/4 is the NCRP recommended value for primary Barrier. Occupancy Factor (T): It depends upon the type of occupancy area outside the primary Barrier and its value varies to the ‘Control’ and ‘Uncontrolled’ areas. Three types of values are used such as 1, 1/4, 1/16 according to the type of occupancy areas, Full occupancy, Partial occupancy, and Occasional occupancy respectively. For more information request to [email protected] or [email protected]”MsgBox M11End Sub


Timer2.Interval = 200End Sub






End If

End Sub


T2 = 1End If

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If T2 = 2 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 3 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End If


End IfEnd Sub







End If

End Sub



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Text3.Text = 0.0000001


End IfEnd Sub


Private Sub Text5_Change()If Text4 <= 0 ThenMn1 = “This value of Use Factor(U) is not possible, this value Might be greater than 0”MsgBox Mn1


End IfEnd Sub


If T = 1 Then


If T = 2 ThenT = 0End IfIf Text3.DataChanged = True ThenTimer1.Enabled = False

End If

End Sub

Private Sub Timer2_Timer()T2 = T2 + 1If Text1.DataChanged = True Then

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T2 = 1End IfIf T2 = 2 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 3 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End IfIf T2 = 4 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 5 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End IfIf T2 = 6 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 6 ThenT2 = 0End If


End Sub


Option ExplicitDim TVL As DoubleDim S As DoubleDim TVL1 As DoubleDim TVLe As Double

Dim M3 As StringDim M As StringDim Mn As StringDim Mn1 As StringDim Mn2 As StringPublic Bx As DoubleDim T As IntegerDim T2 As IntegerDim N As DoubleDim Wdi As DoubleDim M4 As StringDim M5 As StringDim M6 As StringDim M7 As StringDim M8 As StringDim M9 As StringDim M10 As StringDim M11 As StringDim Et As String


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Select Case Combo1.TextCase “Ordinary Concrete”TVL1 = 0.48TVLe = 0.44S = TVL1 + N * TVLe – TVLe





End Select

Wdi = 0.566 * Text2 + 0.61If S < 0 ThenS = 0End IfImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Result.bmp”)

Label13.Caption = “THE PRIMARY BARRIER THICKNESS”Label12.Caption = (S)Label11.Caption = “Meter”

Label16.Caption = “THE PRIMARY BARRIER WIDTH”Label15.Caption = (Wdi)Label14.Caption = “Meter”End Sub



End If

End Sub


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M11 = “Use TVL data: It has been used First and Equilibrium TVL method for ordinary concrete and steel but Varian data is used for only LEAD. Distance dp: Distance from the x-ray target to the point protected. Usually, this distance will be taken from x-ray target to the outSde of the primary Barrier and plus 1 meter. Use Factor (U): It is a normalized value of radiation falling time on the Barrier. 1/4 is the NCRP recommended value for primary Barrier. Occupancy Factor (T): It depends upon the type of occupancy area outside the primary Barrier and its value varies to the ‘Control’ and ‘Uncontrolled’ areas. Three types of values are used such as 1, 1/4, 1/16 according to the type of occupancy areas, Full occupancy, Partial occupancy, and Occasional occupancy respectively. For more information request to [email protected] or [email protected]”MsgBox M11End Sub







End If

End Sub




End If

End Sub

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End If

End Sub



Text3.Text = 0.0000001


End IfEnd Sub

Private Sub Text4_Click()If Text3.DataChanged = True Then

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Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Occupancy.bmp”)End IfEnd Sub



End IfEnd Sub


If T = 1 Then



End Sub

Private Sub Timer2_Timer()T2 = T2 + 1If Text1.DataChanged = True ThenT2 = 1End IfIf T2 = 2 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 3 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End IfIf T2 = 4 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 5 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)

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End IfIf T2 = 6 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 6 ThenT2 = 0End If


End Sub


Option ExplicitDim S As Double

Dim TVL As DoubleDim TVL1 As DoubleDim TVLe As Double

Dim M3 As StringDim M As StringDim Mn As StringDim Mn1 As StringDim Mn2 As StringPublic Bx As DoubleDim T As IntegerDim T2 As IntegerDim N As Double

Dim Wdi As DoubleDim M4 As StringDim M5 As StringDim M6 As StringDim M7 As StringDim M8 As StringDim M9 As StringDim M10 As StringDim M11 As StringDim Et As String




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End Select

Wdi = 0.566 * Text2 + 0.61If S < 0 ThenS = 0End IfLabel13.Caption = “THE PRIMARY BARRIER THICKNESS”Label12.Caption = (S)Label11.Caption = “Meter”

Label16.Caption = “THE PRIMARY BARRIER WIDTH”Label15.Caption = (Wdi)Label14.Caption = “Meter”

Image1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Result.bmp”)End Sub



End If

End Sub

Private Sub Command3_Click()M11 = “Use TVL data: It has been used First and Equilibrium TVL method for ordinary concrete and steel but Varian data is used for only LEAD. Distance dp: Distance from the x-ray target to the point protected. Usually, this distance will be taken from x-ray target to the outside of the primary Barrier and plus 1 meter. Use Factor (U): It is a normalized value of radiation falling time on the Barrier. 1/4 is the NCRP recommended value for primary Barrier. Occupancy Factor (T): It depends upon the type of occupancy area outside the primary Barrier and its value varies to the ‘Control’ and

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‘Uncontrolled’ areas. Three types of values are used such as 1, 1/4, 1/16 according to the type of occupancy areas, Full occupancy, Partial occupancy, and Occasional occupancy respectively. For more information request to [email protected] or [email protected]”MsgBox M11End Sub







End If

End Sub




End If

End Sub


If IsNumeric(Text2) = False ThenM5 = “Please Enter an actual value or designed value or Help: [email protected]”MsgBox M5

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ElseText3.Enabled = TrueEnd If





End If

End Sub



Text3.Text = 0.0000001


End IfEnd Sub


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End IfEnd Sub


If T = 1 Then



End Sub


T2 = T2 + 1If Text1.DataChanged = True ThenT2 = 1End IfIf T2 = 2 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 3 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End IfIf T2 = 4 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)End IfIf T2 = 5 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test.bmp”)End IfIf T2 = 6 ThenImage1.Picture = LoadPicture(“C:\Program Files\primary Shielding Calculation\Test1.bmp”)

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End IfIf T2 = 6 ThenT2 = 0End If


End Sub ‘About this Software Slide

Option Explicit

‘ Reg Key Security Options...Const READ_CONTROL = &H20000Const KEY_QUERY_VALUE = &H1Const KEY_SET_VALUE = &H2Const KEY_CREATE_SUB_KEY = &H4Const KEY_ENUMERATE_SUB_KEYS = &H8Const KEY_NOTIFY = &H10Const KEY_CREATE_LINK = &H20Const KEY_ALL_ACCESS = KEY_QUERY_VALUE + KEY_SET_VALUE + _KEY_CREATE_SUB_KEY + KEY_ENUMERATE_SUB_KEYS + _KEY_NOTIFY + KEY_CREATE_LINK + READ_CONTROL

‘ Reg Key ROOT Types...Const HKEY_LOCAL_MACHINE = &H80000002Const ERROR_SUCCESS = 0Const REG_SZ = 1 ‘ Unicode nul terminated stringConst REG_DWORD = 4 ‘ 32-bit number

Const gREGKEYSYSINFOLOC = “SOFTWARE\Microsoft\Shared Tools Location”Const gREGVALSYSINFOLOC = “MSINFO”Const gREGKEYSYSINFO = “SOFTWARE\Microsoft\Shared Tools\MSINFO”Const gREGVALSYSINFO = “PATH”

Private Declare Function RegOpenKeyEx Lib “advapi32” Alias “RegOpenKeyExA” (ByVal hKey As Long, ByVal lpSubKey As String, ByVal ulOptions As Long, ByVal samDesired As Long, ByRef phkResult As Long) As LongPrivate Declare Function RegQueryValueEx Lib “advapi32” Alias “RegQueryValueExA” (ByVal hKey As Long, ByVal lpValueName As String, ByVal lpReserved As Long, ByRef lpType As Long, ByVal lpData As String, ByRef lpcbData As Long) As LongPrivate Declare Function RegCloseKey Lib “advapi32” (ByVal hKey As Long) As Long

Private Sub cmdSysInfo_Click()Call StartSysInfoEnd Sub

Private Sub cmdOK_Click()Form1.Show 1Unload MeEnd Sub

Private Sub Form_Load()Me.Caption = “About “ & App.TitlelblVersion.Caption = “Version “ & App.Major & “.” & App.Minor & “.” & App.Revision

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End Sub

Public Sub StartSysInfo()On Error GoTo SysInfoErr

Dim rc As LongDim SysInfoPath As String

‘ Try To Get System Info Program Path\Name From Registry...If GetKeyValue(HKEY_LOCAL_MACHINE, gREGKEYSYSINFO, gREGVALSYSINFO, SysInfoPath) Then ‘ Try To Get System Info Program Path Only From Registry... ElseIf GetKeyValue(HKEY_LOCAL_MACHINE, gREGKEYSYSINFOLOC, gREGVALSYSINFOLOC, SysInfoPath) Then ‘ Validate Existance Of Known 32 Bit File Version If (Dir(SysInfoPath & “\MSINFO32.EXE”) <> “”) Then SysInfoPath = SysInfoPath & “\MSINFO32.EXE” ‘ Error – File Can Not Be Found... Else GoTo SysInfoErr End If ‘ Error – Registry Entry Can Not Be Found... Else GoTo SysInfoErr End If Call Shell(SysInfoPath, vbNormalFocus) Exit SubSysInfoErr: MsgBox “System Information Is Unavailable At This Time”, vbOKOnlyEnd Sub

Public Function GetKeyValue(KeyRoot As Long, KeyName As String, SubKeyRef As String, ByRef KeyVal As String) As Boolean Dim i As Long ‘ Loop Counter Dim rc As Long ‘ Return Code Dim hKey As Long ‘ Handle To An Open Registry Key Dim hDepth As Long ‘ Dim KeyValType As Long ‘ Data Type Of A Registry Key Dim tmpVal As String ‘ Tempory Storage For A Registry Key Value Dim KeyValSize As Long ‘ Size Of Registry Key Variable ‘------------------------------------------------------------ ‘ Open RegKey Under KeyRoot {HKEY_LOCAL_MACHINE...} ‘------------------------------------------------------------ rc = RegOpenKeyEx(KeyRoot, KeyName, 0, KEY_ALL_ACCESS, hKey) ‘ Open Registry Key If (rc <> ERROR_SUCCESS) Then GoTo GetKeyError ‘ Handle Error... tmpVal = String$(1024, 0) ‘ Allocate Variable Space KeyValSize = 1024 ‘ Mark Variable Size ‘------------------------------------------------------------ ‘ Retrieve Registry Key Value... ‘------------------------------------------------------------ rc = RegQueryValueEx(hKey, SubKeyRef, 0, _ KeyValType, tmpVal, KeyValSize) ‘ Get/Create Key Value If (rc <> ERROR_SUCCESS) Then GoTo GetKeyError ‘ Handle Errors

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If (Asc(Mid(tmpVal, KeyValSize, 1)) = 0) Then ‘ Win95 Adds Null Terminated String... tmpVal = Left(tmpVal, KeyValSize – 1) ‘ Null Found, Extract From String Else ‘ WinNT Does NOT Null Terminate String... tmpVal = Left(tmpVal, KeyValSize) ‘ Null Not Found, Extract String Only End If ‘------------------------------------------------------------ ‘ Determine Key Value Type For Conversion... ‘------------------------------------------------------------ Select Case KeyValType ‘ Search Data Types... Case REG_SZ ‘ String Registry Key Data Type KeyVal = tmpVal ‘ Copy String Value Case REG_DWORD ‘ Double Word Registry Key Data Type For i = Len(tmpVal) To 1 Step –1 ‘ Convert Each Bit KeyVal = KeyVal + Hex(Asc(Mid(tmpVal, i, 1))) ‘ Build Value Char. By Char. Next KeyVal = Format$(“&h” + KeyVal) ‘ Convert Double Word To String End Select GetKeyValue = True ‘ Return Success rc = RegCloseKey(hKey) ‘ Close Registry Key Exit Function ‘ Exit GetKeyError: ‘ Cleanup After An Error Has Occured... KeyVal = “” ‘ Set Return Val To Empty String GetKeyValue = False ‘ Return Failure rc = RegCloseKey(hKey) ‘ Close Registry KeyEnd Function

7.3 Programming Codes for Secondary Beam Shielding Software

‘SELECT ENERGY SLIDE’Option ExplicitDim T As IntegerDim M1 As String

Dim M2 As StringDim M3 As String

Private Sub Combo1_Change()If Combo1.DataChanged = True ThenCommand1.Enabled = TrueElseCommand1.Enabled = False

End If

End Sub



M3 = “Please select Barrier type and click NEXT, or Help: [email protected]”MsgBox M3

End IfSelect Case Combo1.Text

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Case “6 MV Photon”Form2.Show 1Case “10 MV Photon”Form3.Show 1Case “15 MV Photon”Form4.Show 1Case “18 MV Photon”Form5.Show 1Case “20 MV Photon”Form6.Show 1Case “24 MV Photon”Form7.Show 1

End Select

End Sub



End If

End Sub

Private Sub Command3_Click()M1 = “Help is not available now, contact [email protected]”MsgBox M1End Sub


End Sub

Private Sub Timer1_Timer()T = T + 1If T = 1 ThenImage1.Picture = LoadPicture(“C:\Program Files\Secondary Barrier Thickness Calculation\p001.BMP”)End IfIf T = 136 ThenT = 0End IfEnd Sub

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‘SLIDE FOR 6 MV PHOTON ENERGY’

Option ExplicitDim a As DoubleDim Bp As DoubleDim T As DoubleDim Mat As String

Dim Bl As DoubleDim M14 As StringDim M1 As StringDim M2 As StringDim M3 As StringDim M4 As StringDim M5 As String

Dim Np As DoubleDim Nl As DoubleDim TVL As DoubleDim S As DoubleDim TVL1 As DoubleDim HVL As DoubleDim Sl As DoubleDim Sp As Double

Dim M6 As StringDim M7 As StringDim M8 As StringDim M9 As StringDim M10 As StringDim M11 As StringDim M12 As StringDim M13 As String

Private Sub Command1_Click()Select Case Combo1.TextCase “Ordinary Concrete”TVL1 = 0.173TVL = 0.279Mat = “Ordinary Concrete”

Case “Bricks Concrete (p=1.68 g/cm3)”TVL1 = 0.242TVL = 0.412Mat = “Bricks Concrete”

Case “Concrete (p=2.18 g/cm3)”TVL1 = 0.186TVL = 0.307Mat = “Concrete(p=2.18 g/cm3)”End SelectSelect Case Combo2.TextCase “Default, 90 deg.”A = 4.26 * 10 ^ (-4)

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Case “10 degree”a = 1.04 * 10 ^ -2Case “20 degree”a = 6.73 * 10 ^ -3Case “30 degree”a = 2.77 * 10 ^ -3Case “45 degree”a = 1.39 * 10 ^ -3Case “60 degree”a = 8.24 * 10 ^ -4Case “90 degree”a = 4.26 * 10 ^ -4Case “135 degree”a = 3 * 10 ^ -4Case “150 degree”a = 2.87 * 10 ^ -4End Select

If Option1.Value = True ThenT = 1End If

If Option2.Value = True ThenT = 1 / 4End If

If Option3.Value = True ThenT = 1 / 16

End If

Bp = Text1 * Text3 ^ 2 * Text4 ^ 2 * 400 / (a * Text2 * Text5 * T)Np = Log(1 / Bp) / Log(10)Sp = Np * TVL1Bl = 1000 * Text1 * Text6 ^ 2 / (Text2 * T)

Nl = Log(1 / Bl) / Log(10)Sl = Nl * TVLHVL = 0.31 * TVL1

If (Sl – Sp) > 3 * HVL ThenS = Sl + HVLElseS = Sl + SpEnd If

If S < 0 ThenS = 0End If

Label17.Caption = “Barrier transmission for patient scattering:”Label18.Caption = (Bp)

Label26.Caption = “Thickness required for patient scattering:”Label27.Caption = (Sp)Label36.Caption = “m”

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Label28.Caption = “Barrier transmission for leakage radiation:”Label29.Caption = (Bl)

Label30.Caption = “Thickness required for leakage radiation:”Label31.Caption = (Sl)Label37.Caption = “m”

Label32.Caption = “Total Secondary Barrier Thickness”Label33.Caption = (S)Label34.Caption = “Meter”Label35.Caption = (Mat)End Sub



End If

End Sub

Private Sub Command3_Click()M13 = “Use TVL data: It has been used First and Equilibrium TVL method for ordinary concrete, steel, and lead, Varian TVL data are used for Leakage radiation. Distance (dp): Distance from the x-ray target to the point protected. Usually, this distance will be taken from x-ray target to the outside of the primary barrier and plus 1 meter. Workload (W): It has been taken with the assumption with considering of busyness of the department. Usually, a design purpose NCRP recommended value is 1000Gy/week. Use Factor (U): It is a normalized value of radiation falling time on the barrier. 1/4 is the NCRP recommended value for primary barrier. Occupancy Factor (T): It depends upon the type of occupancy area outside the primary barrier and its value varies to the ‘Control’ and ‘Uncontrolled’ areas. Three types of values are used such as 1, 1/4, 1/16 according to the type of occupancy areas, Full occupancy, Partial occupancy, and Occasional occupancy respectively. [email protected] “MsgBox M13End Sub

Private Sub Option1_Click()If Option1.Value = True And IsNumeric(Text6) = True ThenCommand1.Enabled = TrueEnd IfEnd Sub


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M6 = “Please Enter an NCRP recommended value or Help: [email protected]”MsgBox M6ElseText2.Enabled = True

End If

End Sub


If IsNumeric(Text2) = False ThenM7 = “Please Enter an actual numeric value, Help: [email protected]”MsgBox M7ElseText3.Enabled = TrueEnd If

If Text1 <= 0 ThenM2 = “This value of (W) is not possible, For design pupose, 1000 Gy/ week usually used. Please input numeric value. Help [email protected]”MsgBox M2Text1.Text = 0.000000001End IfEnd Sub


If IsNumeric(Text3) = False ThenM8 = “Please Enter an assumed or designed value, please input numeric value which is greater than 0. Help: [email protected]”MsgBox M8ElseText4.Enabled = True

End If

End SubPrivate Sub Text4_Change()If Text3 <= 0 ThenM3 = “This value is not possible, this value might be greater than 0”MsgBox M3

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Text3.Text = 0.0000001

End IfIf IsNumeric(Text4) = False And Text4 > 1.5 ThenM9 = “Please Enter a valid value acording to the NCRP report, default value is 1 meter. Help: [email protected]”MsgBox M9ElseText5.Enabled = True

End IfEnd Sub

Private Sub Text5_Change()If Text4 <= 0 ThenM4 = “This value of the beam size of a patient is not possible, please input correct value, otherwise defaultly programe will be closed.”MsgBox M4

Text4.Text = 0.00000001End IfIf IsNumeric(Text5) = False ThenM10 = “Please Enter a practical value, Help: [email protected]”MsgBox M10ElseText6.Enabled = TrueCombo2.Enabled = True

End IfEnd Sub

Private Sub Text6_Change()If Text5 <= 0 ThenM5 = “This value of Occupancy Factor(T) is not possible, this value might be greater than 0”MsgBox M5

Text5.Text = 0.00000001End IfIf IsNumeric(Text6) = False ThenM11 = “Please Enter a valid value to the collection or measurement or according to the design, Help: [email protected]”MsgBox M11End If

If Option1.Value = True And IsNumeric(Text6) = True ThenCommand1.Enabled = TrueEnd If

If Option2.Value = True And IsNumeric(Text6) = True ThenCommand1.Enabled = TrueEnd IfIf Option3.Value = True And IsNumeric(Text6) = True ThenCommand1.Enabled = TrueEnd If

End Sub

‘SLIDE FOR 10 MV PHOTON ENERGY’Option ExplicitDim a As Double

136




Dim Bp As DoubleDim T As DoubleDim Mat As String

Dim Bl As Double

Dim M1 As StringDim M2 As StringDim M3 As StringDim M4 As StringDim M5 As String

Dim Np As DoubleDim Nl As DoubleDim TVL As Double

Dim TVL1 As DoubleDim HVL As DoubleDim S As DoubleDim Sp As DoubleDim Sl As DoubleDim M14 As StringDim M6 As StringDim M7 As StringDim M8 As StringDim M9 As StringDim M10 As StringDim M11 As StringDim M12 As StringDim M13 As String

Private Sub Command1_Click()Select Case Combo1.Text

Case “Ordinary Concrete”TVL = 0.305Mat = “Ordinary Concrete”

Case “Bricks Concrete (p=1.68 g/cm3)”TVL = 0.452Mat = “Bricks Concrete (p=1.68)”

Case “Concrete (p=2.18 g/cm3)”TVL = 0.32878Mat = “Concrete (p=2.18 g/cm3)”

Case “Steel”TVL = 0.085Mat = “Steel”

Case “Lead”TVL = 0.046Mat = “Lead”

End SelectSelect Case Combo2.TextCase “Default, 90 deg.”A = 4.26 * 10 ^ (-4)Case “10 degree”

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a = 1.04 * 10 ^ -2Case “20 degree”a = 6.73 * 10 ^ -3Case “30 degree”a = 2.77 * 10 ^ -3Case “45 degree”a = 1.39 * 10 ^ -3Case “60 degree”a = 8.24 * 10 ^ -4Case “90 degree”a = 4.26 * 10 ^ -4Case “135 degree”a = 3 * 10 ^ -4Case “150 degree”a = 2.87 * 10 ^ -4End Select


If Option2.Value = True ThenT = 1 / 4End IfIf Option3.Value = True Then

T = 1 / 16End If

Bl = 1000 * Text1 * Text6 ^ 2 / (Text2 * T)

Nl = Log(1 / Bl) / Log(10)Sl = Nl * TVL

If Sl < 0 ThenSl = 0End If



Label32.Caption = “Total Secondary Barrier Thickness”Label33.Caption = (Sl)Label34.Caption = “Meter”Label35.Caption = (Mat)End Sub


138

If Command2.Caption = “EXIT” ThenM12 = MsgBox(“Do You Want to Exit?”, vbYesNo + vbQuestion, “EXIT”)If M12 = vbYes ThenUnload MeElseExit SubEnd IfElseCombo1.Enabled = True


End If

End Sub









139



End If

End Sub



If Text1 <= 0 ThenM2 = “This value of (W) is not possible, For design pupose, 1000 Gy/ week usually used. Please input numeric value. Help [email protected]”MsgBox M2Text1.Text = 0.000000001End IfEnd Sub


If IsNumeric(Text3) = False ThenM8 = “Please Enter an assumed or designed value, please input numeric value which is greater than 0. Help: [email protected]”MsgBox M8

End If


Text3.Text = 0.0000001

End IfIf IsNumeric(Text4) = False And Text4 > 1.5 ThenM9 = “Please Enter a valid value according to the NCRP report, default value is 1 meter. Help: [email protected]”MsgBox M9

End IfEnd Sub

Private Sub Text5_Change()If Text4 <= 0 ThenM4 = “This value of the beam size of a patient is not possible, please input correct value, otherwise defaultly program will be closed.”MsgBox M4

Text4.Text = 0.00000001

140






End IfIf IsNumeric(Text5) = False ThenM10 = “Please Enter a practical value, Help: [email protected]”MsgBox M10

End IfEnd Sub





End Sub

‘SLIDE FOR 15 MV PHOTON ENERGY’

Option ExplicitDim a As DoubleDim Bp As DoubleDim T As DoubleDim Mat As String

Dim Bl As Double



Dim TVL1 As DoubleDim HVL As DoubleDim S As DoubleDim Sp As Double

141



Dim Sl As DoubleDim M14 As StringDim M6 As StringDim M7 As StringDim M8 As StringDim M9 As StringDim M10 As StringDim M11 As StringDim M12 As StringDim M13 As String







End SelectSelect Case Combo2.TextCase “Default, 90 deg.”A = 4.26 * 10 ^ (-4)Case “10 degree”a = 1.04 * 10 ^ -2Case “20 degree”a = 6.73 * 10 ^ -3Case “30 degree”a = 2.77 * 10 ^ -3Case “45 degree”a = 1.39 * 10 ^ -3Case “60 degree”a = 8.24 * 10 ^ -4Case “90 degree”a = 4.26 * 10 ^ -4Case “135 degree”a = 3 * 10 ^ -4Case “150 degree”a = 2.87 * 10 ^ -4End Select

If Option1.Value = True Then

142

T = 1End If


T = 1 / 16End If









End If

End Sub

Private Sub Command3_Click()M13 = “Use TVL data: It has been used First and Equilibrium TVL method for ordinary concrete, steel, and lead, Varian TVL data are used for Leakage radiation. Distance (dp): Distance from the x-ray target to the point protected. Usually, this distance will be taken from x-ray target to the outside of the

143

primary barrier and plus 1 meter. Workload (W): It has been taken with the assumption with considering of busyness of the department. Usually, a design purpose NCRP recommended value is 1000Gy/week. Use Factor (U): It is a normalized value of radiation falling time on the barrier. 1/4 is the NCRP recommended value for primary barrier. Occupancy Factor (T): It depends upon the type of occupancy area outside the primary barrier and its value varies to the ‘Control’ and ‘Uncontrolled’ areas. Three types of values are used such as 1, 1/4, 1/16 according to the type of occupancy areas, Full occupancy, Partial occupancy, and Occasional occupancy respectively. [email protected] “MsgBox M13End Sub









End If

End Sub



If Text1 <= 0 Then

144




M2 = “This value of (W) is not possible, For design pupose, 1000 Gy/ week usually used. Please input numeric value. Help [email protected]”MsgBox M2Text1.Text = 0.000000001End IfEnd Sub



End If


Text3.Text = 0.0000001

End IfIf IsNumeric(Text4) = False And Text4 > 1.5 ThenM9 = “Please Enter a valid value acording to the NCRP report, default value is 1 meter. Help: [email protected]”MsgBox M9

End IfEnd Sub

Private Sub Text5_Change()If Text4 <= 0 ThenM4 = “This value of the beam size of a patient is not possible, please input correct value, otherwise defaultly programe will be closed.”MsgBox M4

Text4.Text = 0.00000001End IfIf IsNumeric(Text5) = False ThenM10 = “Please Enter a practical value, Help: [email protected]”MsgBox M10

End IfEnd Sub



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End Sub

‘SLIDE FOR 18 MV PHOTON ENERGY’Option ExplicitDim a As DoubleDim Bp As DoubleDim T As DoubleDim Mat As String

Dim Bl As Double







146







T = 1 / 16End If





Label30.Caption = “Thickness required for leakage radiation:”

147

Label31.Caption = (Sl)Label37.Caption = “m”




End If

End Sub






148





End If

End Sub



If Text1 <= 0 ThenM2 = “This value of (W) is not possible, For design purpose, 1000 Gy/ week usually used. Please input numeric value. Help [email protected]”MsgBox M2Text1.Text = 0.000000001End IfEnd Sub



End If


Text3.Text = 0.0000001


149







End IfEnd Sub



End IfEnd Sub





End Sub


Dim Bl As Double

Dim M1 As StringDim M2 As StringDim M3 As StringDim M4 As String

150



Dim M5 As String









End SelectSelect Case Combo2.TextCase “Default, 90 deg.”A = 4.26 * 10 ^ (-4)Case “10 degree”a = 1.04 * 10 ^ -2Case “20 degree”a = 6.73 * 10 ^ -3Case “30 degree”a = 2.77 * 10 ^ -3Case “45 degree”a = 1.39 * 10 ^ -3Case “60 degree”a = 8.24 * 10 ^ -4Case “90 degree”

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a = 4.26 * 10 ^ -4Case “135 degree”a = 3 * 10 ^ -4Case “150 degree”a = 2.87 * 10 ^ -4End Select



T = 1 / 16End If









End If

152

End Sub










End If

End Sub


If IsNumeric(Text2) = False ThenM7 = “Please Enter an actual numeric value, Help: [email protected]”MsgBox M7

153




ElseText6.Enabled = TrueEnd If




End If


Text3.Text = 0.0000001


End IfEnd Sub



End IfEnd Sub



154





If IsNumeric(Text6) = False ThenM11 = “Please Enter a valid value to the collection or measurement or according to the design, Help: [email protected]”MsgBox M11End If



End Sub


Dim Bl As Double





Case “Ordinary Concrete”TVL = 0.356

155


Mat = “Ordinary Concrete”








T = 1 / 16End If



156







End If

End Sub



Private Sub Option2_Click()If Option2.Value = True And IsNumeric(Text6) = True ThenCommand1.Enabled = True

157

End IfEnd Sub







End If

End Sub






End If

End SubPrivate Sub Text4_Change()If Text3 <= 0 ThenM3 = “This value is not possible, this value might be greater than 0”

158






MsgBox M3

Text3.Text = 0.0000001


End IfEnd Sub



End IfEnd Sub





End Sub

159




Example calculations with this software

1. Evaluate the thickness of concrete needed for the primary shield shown below. The beam is 20 MV, and the workload (W) is 500gy per week at the isocenter. Carry out the evaluation using the following methods [2]:

(a) Curve E.8 from NCRP Report No. 51(b) First and equilibrium TVL method (c) Varian TVL data

Results[2]: (d) 2.21 meter ordinary concrete(e) 2.16 meter ordinary concrete(f) 2.20 meter ordinary concrete

Software calculated result: (b) 2.16 meter ordinary concrete

(This software able to calculate shielding thickness only First and equilibrium TVL method.)

2. Find the width (W) of the primary barrier of example 1c. The primary barrier protrudes into the room, and the adjacent secondary wall is 0.91 m thick i.e. distance to the point protected is 3.2 m.

Result [2]:(i) Primary barrier width (W): 2.4 m

Software calculated result: (i) 2.42 meter

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3. Use the Varian 90o leakage TVLs for a 20 MV x-ray beam to determine the concrete thickness needed in the secondary wall shown below. Repeat the evaluation for an uncontrolled area.

Result [2]: (i) Secondary barrier thickness for controlled area: 0.79 meter concrete

(ii) Secondary barrier thickness for uncontrolled area: 0.823 meter concrete

Software calculated results:(i) 0.789 meter concrete for controlled area

(ii) 0.822 meter concrete for uncontrolled area

4. Repeat problem 3 for a 6 MV x-ray beam. Since the beam is less than 10 MV, both patient scatter and head leakage radiation must be considered when designing the secondary barrier. A workload of 1000 Gy per week will be used. Determine the concrete thickness for secondary barrier.

Result [2]:

(i) Secondary barrier thickness based on patient scatter: 0.491 meter concrete (ii) Secondary barrier thickness based on head leakage : 0.725 meter concrete

(iii) Finally decided secondary barrier thickness: 1.00 meter concrete(approx).

Software calculated results: (i) Secondary barrier thickness based on patient scatter: 0.4902 meter

concrete (ii) Secondary barrier thickness based on head leakage : 0.7259 meter

concrete (iii) Finally decided secondary barrier thickness: 0.78 meter concrete (exact).


dref= 1m (FAD = 1m), workload (W) = 40000 Gy/year, TVLconcrete = 40 cm, design constraint P = 0.3 mSv/year, occupancy factor T = 0.25, patient waiting distance d = 6m, use factor U = 0.25 [13].


Software calculated result:(ii) Shielding thickness 2.112 m concrete.


dref= 1m (FAD = 1m), workload (W) = 40000 Gy/year, TVLconcrete = 40 cm, design constraint P = 0.3 mSv/year for patient, P = 20mSv/y for staff, occupancy factor T = 0.05 for patient, occupancy factor T = 1 for staff, patient waiting distance d = 6m, use factor U = 0.25 [13].

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Date post:	29-Nov-2014
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DESIGN OF LOW COST RADIOTHERAPY FACILITIES

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