INTRODUCTION: Modern-Day Linear Accelerator Acceptance Testing and Commissioning

The delivery of radiation treatments is reaching new pinnacles with continued

advancement in accelerator and computer control technology. Computer-controlled

linear accelerators (linacs) are increasingly being used clinically in small, as well as big,

institutions.

There is a complete shift in the paradigm of the treatment delivery process. Historically,

linear accelerators have been used to deliver radiation of uniform intensity through field

apertures shaped by blocks. Now the emphasis is to shape the field apertures with a

multileaf collimato rsystem and vary the radiation intensities with dynamic motion of the

collimator system to deliver conformal radiation to the target volume. The fundamental

premise is that the high-dose volume is restricted to the shape of the target tissue while

excluding as much normal tissue from the high-dose volume as possible. Therefore, the

acceptance testing and commissioning of a computer-controlled linac can be quite

complex and may vary from institution to institution depending on its anticipated use.

The process of purchase, acceptance testing, and commissioning of a computer-

controlled linac is a major undertaking that can take up a considerable amount of time,

effort, and expense. Therefore, it is crucial that a great deal of thought and care go into

the initial planning. The primary objective is that the accelerator specifications must

meet the clearly defined needs of the facility over the projected lifetime of the

accelerator, which can be up to 10 years. It is very important that the selection process

for the equipment includes input from radiation oncologists, physicists, therapists, and

facility engineers. The selection, acceptance testing, and commissioning of a linac

involves;

• Evaluation of clinical needs

• Review of specifications and purchase agreement

• Design and construction of the facility to house the new machine

• Installation of the machine, safety checks, and initial radiation survey

• Acceptance testing of the machine

• Commissioning of the machine for clinical use

• Final report and documentation

• Training of the staff in the safe and efficacious use of the accelerator

• Establishment of the baseline quality assurance parameters and schedule

Criteria for Linac Selection:

The selection of a linac is critically dependent on its clinical utilization. Fortunately, the

choice of commercially available, FDA-approved medical linacs is primarily limited to

three major linac manufacturers: Elekta, Siemans, and Varian. Each of these

manufacturers offers linacs that are capable of delivering both uniform and modulated

intensities of radiation under computer control. Therefore, the task is limited to selecting

the most appropriate machine from those commercially available and developing the

purchase specifications to meet the clinical needs. This task is best accomplished by

the formation of an ad hoc committee in the department that includes at least a

physicist, radiation oncologist, therapist, and engineer. The charge of this

committee should include

• A systematic review of current and projected clinical needs and types of patients who

will be treated on the machine

• A careful review of deliverables, functionality, technical and physical specifications,

and cost of all commercially available linacs

• Review of available space, available funds, available or needed support staff, and

available in-house technical support and expertise

• Evaluation of future upgrades, warranties, and maintenance contracts

• Evaluation of the quality of the manufacturer’s service and technical support

• Final recommendation for the linac

Travelling Wave Linear Accelerators:

Periodic RF structures can be operated in two different ways, as a traveling-wave(TW)

accelerator or a standing-wave (SW) accelerator.

In a TW-structure, the fields build up in space with the wave front traveling with the

group velocity. The output of the structure is matched to a load where the left-over

energy is dissipated.

We consider the case where the power transferred to the beam is small compared to

the power dissipated in the structure walls–i.e. normal conducting Cu accelerators.

Two different types of structures are usually distinguished.

Full dispersion curve (Brillouin diagram) for a loaded waveguide with iris spacing

(period) equal to d. For comparison, and indicated with the dotted curve, is the

hyperbola corresponding to the uniform waveguide of the same diameter. Multiple pass-

bands are illustrated.

Some Historical Milestones:

The history of linear accelerators is briefly described by J. Blewett in the book L.A.

A technical discussion of some early projects is contained in the “Linear Accelerator

I+sue” of the Review of Scientific Instruments (February 1955).

Here we have chosen to enumerate only some of the important milestones. The

choice has led them to be spread more or less uniformly-in time, emphasizing the

remarkable--continuity of progress over a period of more than half a century which

promises to continue into the future. This uniform spread results perhaps in the

slighting of some of the early work when parallel progress was being made at many

laboratories.

The history of linacs can be viewed as a sequence of attempts to fool charged

particles so that they see cumulatively acting voltages across linear arrays of gaps. A

chronology follows. References can be traced through L.A.

1924: A theoretical paper by G. Ising, Stockholm, describes a method for accelerating

positive ions (canal rays) by applying the electrical wavefront from a spark discharge

to an array of drift tubes via transmission lines of successively greater lengths.

An experimental paper (including the theory of the betatron) by R. Wide roe,

Switzerland, describes the successful acceleration of potassium ions to 50 kV.

The first and last are grounded, the center one is attached to a 1 MHz

Oscillator with a voltage of 25 kV. The distance d between gaps is adjusted so that

where j is the frequency and X0 the free space wavelength at that frequency. The

potassium ions travel from one gap to the next in one-half an RF period. Since higher

frequency oscillators did not exist at the time, lighter particles traveling faster could

not be accelerated.

1931-35: K. Kingdon (G.E.), L. Snoddy (U niv. of Virginia) et al., accelerate electrons

from 28 keV to 2.5 MeV by applying progressive wavefronts to a drift tube array.

1931-34: E. Lawrence, D. Sloan et al., (U.C. at Berkeley) build a Wider&type linac

with 30 drift tubes, oscillating to 42 kV, driven by 7 MHz oscillators.

Mercury ions are accelerated to 1.26 MeV. Oscillators of high enough frequency for

protons are still not available. Similar work continues at Cornell, in Japan and in

England.

1937-45: W. Hansen and the Varian brothers invent the klystron (at first a low

power device) at Stanford. Subsequently, the high power magnetron (2 MW pulsed) is

developed in Great Britain for radar purposes as part of the war effort.

1945-47: L. Alvarez, W. Panofsky et al. (U. C. Berkeley) build a 32 MeV proton drift

tube linac three-feet in diameter, forty feet long, powered by 200 MHz war

surplus radar equipment. As indicated, the Alvarez structure difers from the Wideriie

structure in that all tubes are contained in one large cylindrical tank and are powered

at the same phase: the distance between drift tubes is arranged so that the particles

are shielded from the fields when they are in the decelerating phase. Adequate beam

acceptance required that the accelerating field not have much variation with radius,

thus precluding operation at higher frequency. As will be explained later, longitudinal

phase stability turned out to be satisfactory but transverse focusing was problematical.

1947-48: At Stanford, W. Hansen, E. Ginzton, W. Kennedy et al., build the Mark I

disk-loaded linac yielding 4.5 MeV electrons in a nine-foot structure powered to 1 h4W

at 2856 MHz. It is the first of a series: Mark II (40 MeV); Mark III (1.2 GeV); and

SLAC (30 GeV). Parallel efforts take place in Great Britain, France and the USSR,

and at M.I.T. and Yale in the U.S.A.

1953: Henry Kaplan and Edward Ginzton begin building a medical linear accelerator.

1952 : J. Blewett at BNL shows that alternating-gradient focusing works with quadrupole

coils inserted in the drift tubes, solving the transverse focusing problem for

protons. The Alvarez linac (with some modifications) from then on serves as the

model for most subsequent proton and ion linacs (up to 200 MeV).

1956: The first medical linear accelerator in the Western Hemisphere is installed at

Stanford Hospital in San Francisco.

1959: Stanford medical school and hospital move to the Palo Alto campus, bringing the

medical linear accelerator.

1962:Kaplan and Saul Rosenberg begin trials using the linear accelerator with

chemotherapy to treat Hodgkin's disease, an approach that dramatically improves

patient survival.

1965-66: P. Wilson, A. Schwettman et al., at HEPL, Stanford, report the first successful

operation of a superconducting linac producing 500 keV electrons with three

lead-coated cells.

1967-69: V. Sarantsev et al., at Dubna, USSR, build a linear induction accelerator,

as do D. Keefe et al., at LBL, shortly thereafter, both groups with the intent of

accelerating electron rings.

1971: R. Koontz, G. Loew, and R. Miller at SLAC for the first time accelerate a single

electron bunch through the 3-km linac and show experimentally that beam loading is

energy independent.

1972: D. Nagle, E. Knapp et al., at LASL, Los AIamos, successfully operate their

800-MHz side-coupled cavity linac, LAMPF, and produce 800 MeV protons.

1973: P. Wilson, D. Farkas and H. Hogg at SLAC invent the rf energy compression

scheme called SLED which in the next five subsequent years gets installed on the 3-km

linac, boosting its energy up to 30 GeV.

1980: R. Stokes et al., at LASL, successfully test RF quadrupoles (up to 2 MeV)

following a 1970 suggestion of I. Kapchinskii and V. Teplyakov, ITEP, Moscow.

1982: H. Grunder, F. Selph, et al., at-LBL, use the HILACand Bevatron to accelerate

U238 with charge state - 69.

1994: First use of the CyberKnife, invented at Stanford, which uses sophisticated

computerized imaging to aim a narrow X-ray beam precisely.

1997: Stanford pioneers the use of intensity-modulated radiation therapy, which

combines precise imaging with linear accelerators that deliver hundreds of thin beams

of radiation from any angle.

2004: Implementation of four-dimensional radiotherapy, which accounts for the motion

of breathing during imaging and radiation delivery.

ADVANCES IN MEDICAL LINEAR ACCELERATOR TECHNOLOGY Background:

- Radiation Oncology is the branch of medicine that uses various types of radiation to

treat and control cancer. The foundation of radiation oncology is based on the

interaction between matter and energy. Beginning with the discovery of x-rays by

Wilhelm Roentgen in 1895. In 1896 Henri Becquerel discovered radioactivity and in

1898 separation of radium by Marie and Pierre Curie, it known that certain materials

also emit radiation. When radiation does interact with medium, it produces ionization.

When cell get enough ionization, it dies, thus interaction between radiation and matter

be well understood and this translates the science of radiation physics into clinical

treatment of cancer. The transmission of radiation in the clinical environment depends

on very sophisticated technology, one of such device called Linac. These create the x –

rays treatment beam, these beam consist of much higher energy than a standard x –ray

machine and must be meticulously maintained in order to guarantee patients safety. In

1900’s treatment for cancer patients started with the KV X-rays machines which are

operated at 150 – 350 KV, having low depth penetration and excessive dose to the skin.

During 1930 – 40’s several types of equipments were introduced including 1 & 2 MeV

Van De Graff accelerator and 18 – 45 MeV Betatrons. The history of particle

accelerators for ion beams is often described in association with the development of

cyclotrons, primarily due to their wide-spread use in the medical field. However, what is

often not acknowledged is that ion linear accelerators ("linacs") were developed in

parallel with the cyclotron and other circular accelerators. While Lawrence and

Livingston designed the first small cyclotron in 1930, R. Wideröe had already published

a paper in 1928 on his results from an rf powered linear accelerator for ions

In mid 1950’s Linear Accelerators now popularly known as LINAC’s are evolved with the

earlier advent of microwave power tube such as Klystron developed during the Second

World War. These klystrons are a vital source of microwave power for radars during the

war. These linacs are in the range of 4 – 8 MeV. In 1952 , microwave electron based

linear accelerators which have made possible modern radiotherapy treatment of tumors

with mega voltage x rays. The first one was at HAMMERSMITH HOSPITAL, operational

with 8MV built by metropolitan Vickers, the first medical linear accelerator (Linac)

treated its first patient, in London, in 1953, so the use of these machines in clinical

practice has been almost co-existent with the lifetime of Physics in Medicine and

Biology

In 1956 Henry Kaplan utilized the linear accelerator used by the physicists of Stanford,

a fighting tool against cancer. 2 year old boy with a retinoblastoma is (the first patient

treated with his linear accelerator at Stanford, in western hemisphere) and he survived

with the vision intact for the rest of the life, since then the medical Linac have been in

vogue. Destroying the tumour while sparing the eye would have been impossible with

the earlier less focused radiation sources. This linacs has other medical uses too, its

radiation can quell the rejection of an organ transplant, suppress the immune system of

patients undergoing blood and marrow transplantation and correct certain neurological

and cardiovascular

One of the biggest challenges clinicians has aligning the radiation beam and the

tumour, The heart beating, the lungs inflating and deflating with by breathing , the blood

flow through the vein, subtle movement of skeleton and muscles all this motion adds up

to a moving target for a beam of radiation. A Linac was developed at Stanford that can

continuously tracking the position of tumour in real time during the treatment. Tracking

allows radiation oncologist to deliver a large doses of radiation precisely to the tumour

cells, the unit called cyber knife, first used for a patient in 1994. An added advantage of

linear accelerator is that it gives electron beams of various energies. These electron

beams offer the advantage of rapid and sharp fall of depth dose, variable depth

penetration, less bone absorption than x-ray beams and decreased radiation build up.,

and the superficial tumors are best treated with the electron beams. Since the advent of

linear accelerator for cancer treatment, the five year survival rate have been improved

from 39% to 54%, much of this improvement can be directly attributed to the capabilities

provided by linear accelerators. Basic of linear accelerator:- Linear Accelerators are the

linear devices used to accelerate atomic and subatomic particles to high velocities.

Accelerating employ electric and magnetic forces to accurate focus and steer the

particles. The electric and magnetic field exerts forces only in charged particles such as

ions , protons or electrons. They do not exert forces on neutral particles. The unit of

energy for these particle is electron volt, which is the energy imparted to an electron or

proton when it is accelerated through a potential difference of one volt. For most

applications involving particle energies of one MeV or higher, radiofrequency (RF linacs)

linear accelerator is employed. In these electric or magnetic fields oscillate at high

frequency, commonly known as radio frequency in the range of million to billion of

cycles per second In RF Linac very high electric or magnetic fields are produced by

injecting RF energy from a powerful RF system in to a confined cavity bounded by

conducting material ( copper) to keep the energy from radiating away. The particles to

be accelerated are injected to the Linac structure. In older linacs drift tubes distributed

along the axis of the structure. The particles are exposed to longitudinal electrical field,

when they are in acceleration and are hidden from the electric field by drift tube when

the electric fields are in opposite direction.