Interaction of Radiation with Matter

INTERACTION OF RADIATION INTERACTION OF RADIATION WITH MATTER WITH MATTER

DR ARNAB BOSEDR ARNAB BOSEDept. of Radiotherapy Dept. of Radiotherapy

NRS Medical College, KolkataNRS Medical College, Kolkata

Part 1 : IntroductionPart 1 : Introduction

Importance of the knowledge of the Importance of the knowledge of the fundamentals of interaction with matterfundamentals of interaction with matter

1.1. Forms the basis of RadiobiologyForms the basis of Radiobiology

2.2. Forms the basis of Radiation protectionForms the basis of Radiation protection

3.3. Forms the basis of Radiation detectionForms the basis of Radiation detection

4.4. Ensures safe and effective methodologies in Ensures safe and effective methodologies in Radiology and RadiotherapyRadiology and Radiotherapy

Radiation

The term The term radiation radiation applies to the emission and applies to the emission and propagation of energy through space or a propagation of energy through space or a material medium . material medium .

Radiation may be Radiation may be

Electromagnetic RadiationElectromagnetic Radiation

Particle RadiationParticle Radiation When radiation passes through matter it may When radiation passes through matter it may

interact with the material , transferring some or interact with the material , transferring some or all of its energy to the atoms of that material.all of its energy to the atoms of that material.

Electromagnetic RadiationElectromagnetic Radiation

Constitutes the mode of energy propagation for Constitutes the mode of energy propagation for such phenomena as light waves, heat waves, such phenomena as light waves, heat waves, radio waves, u v rays, x rays and γ rays .radio waves, u v rays, x rays and γ rays .

Spectrum of electromagnetic radiation ranges Spectrum of electromagnetic radiation ranges from from 101077 m (radio waves) to 10 m (radio waves) to 10-13-13 m (ultra high m (ultra high energy X rays) .energy X rays) .

X rays and γ rays are the two major forms of X rays and γ rays are the two major forms of electromagnetic radiation used in modern day electromagnetic radiation used in modern day radiotherapy.radiotherapy.

An X ray beam emitted from a target or a γ ray An X ray beam emitted from a target or a γ ray emitted from a radioactive source consists of a emitted from a radioactive source consists of a large number of large number of photonsphotons , usually with a variety , usually with a variety of energies .of energies .

Electromagnetic Wave with respect to Electric & Magnetic Field

Particulate RadiationParticulate Radiation

Refers to the energy propagated by traveling Refers to the energy propagated by traveling corpuscles – which have definite rest mass , corpuscles – which have definite rest mass , definite momentum and a defined position at any definite momentum and a defined position at any instant .instant .

Elementary atomic particles are electrons (charge Elementary atomic particles are electrons (charge – 1) , protons (charge + 1) and neutrons (zero – 1) , protons (charge + 1) and neutrons (zero charge) .charge) .

Some common subatomic particles are positrons Some common subatomic particles are positrons (charge + 1) , neutrinos (zero charge) and (charge + 1) , neutrinos (zero charge) and mesons .mesons .

Part 2 : Interaction of Photons with Part 2 : Interaction of Photons with MatterMatter

When an X ray or γ ray beam passes through a When an X ray or γ ray beam passes through a medium , interaction occurs between the photon medium , interaction occurs between the photon and the matter and energy is transferred to the and the matter and energy is transferred to the medium . medium .

If the absorbing medium consists of body tissues If the absorbing medium consists of body tissues sufficient energy may be deposited within the sufficient energy may be deposited within the cells destroying their reproductive capacity .cells destroying their reproductive capacity .

Fate of the Photon BeamFate of the Photon Beam The photon beam may undergo the following four The photon beam may undergo the following four

processes – Attenuation , Absorption , Scattering and processes – Attenuation , Absorption , Scattering and Transmission .Transmission .

AttenuationAttenuation refers to the removal of radiation from the refers to the removal of radiation from the beam by the matter . Attenuation may occur due to beam by the matter . Attenuation may occur due to scattering and absorption .scattering and absorption .

AbsorptionAbsorption refers to the taking up of energy from the beam refers to the taking up of energy from the beam by the irradiated material .It is the absorbed energy which by the irradiated material .It is the absorbed energy which is important in producing the radiobiological effects .is important in producing the radiobiological effects .

ScatteringScattering refers to the change in the direction of photons refers to the change in the direction of photons and it contributes to both attenuation and absorption .and it contributes to both attenuation and absorption .

Any photon which does not suffer the above processes is Any photon which does not suffer the above processes is transmittedtransmitted . .

Attenuation Coefficient (1)Attenuation Coefficient (1)

Fraction of photons removed from a mono Fraction of photons removed from a mono energetic beam of x-ray or gamma ray per unit energetic beam of x-ray or gamma ray per unit thickness of material is called thickness of material is called linear attenuation linear attenuation coefficientcoefficient ( (), typically expressed in ), typically expressed in cmcm-1-1 ..

Number of photons removed from the beam Number of photons removed from the beam traversing a very small thickness traversing a very small thickness x:x:

where where nn = number removed from beam, = number removed from beam, NN = number of photons incident on the material, = number of photons incident on the material, and minus sign is placed before μ to indicate that and minus sign is placed before μ to indicate that

no. of photons decreases as the absorber no. of photons decreases as the absorber thickness increases.thickness increases.

xNn


For a mono energetic beam of photons incident For a mono energetic beam of photons incident on either thick or thin slabs of material, an on either thick or thin slabs of material, an exponential relationship exists between number exponential relationship exists between number of incident photons (of incident photons (NN00 ) and those transmitted ) and those transmitted

(N) through thickness (x) without interaction:(N) through thickness (x) without interaction:

The number of photons indicate the Intensity of The number of photons indicate the Intensity of the beam and can also be written as ( I ).the beam and can also be written as ( I ).

xeNN 0


TotalTotal Linear attenuation coefficientLinear attenuation coefficient is the sum of is the sum of individual linear attenuation coefficients for each individual linear attenuation coefficients for each type of interaction:type of interaction:

For a given thickness of material , probability of For a given thickness of material , probability of interaction depends on number of atoms the x interaction depends on number of atoms the x ray or gamma ray encounter per unit distance. ray or gamma ray encounter per unit distance. The density (ρ ) of material affects this number.The density (ρ ) of material affects this number.

Linear attenuation coefficient is proportional to Linear attenuation coefficient is proportional to the density of the material.the density of the material.

pairComptonphotoRayleigh

Mass Attenuation CoefficientMass Attenuation Coefficient For a given thickness probability of interaction is For a given thickness probability of interaction is

dependent on the number of atoms per volume.dependent on the number of atoms per volume. This dependency can be overcome by This dependency can be overcome by

normalizing linear attenuation coefficient for normalizing linear attenuation coefficient for density of material – density of material –

Mass Attenuation CoefficientMass Attenuation Coefficient (μ / ρ ) = (μ / ρ ) = Linear attenuation coefficient Linear attenuation coefficient Density of the materialDensity of the material Mass attenuation coefficient is independent of Mass attenuation coefficient is independent of

density of the material.density of the material.

Half Value Layer (1)Half Value Layer (1)

Half value layerHalf value layer (HVL) defined as thickness of (HVL) defined as thickness of material required to reduce intensity of an x-ray material required to reduce intensity of an x-ray or gamma-ray beam to one-half of its initial value.or gamma-ray beam to one-half of its initial value.

It is an indirect measure of the photon energies It is an indirect measure of the photon energies (also referred to as (also referred to as qualityquality or penetrability) of a or penetrability) of a beam of radiation.beam of radiation.

For mono energetic photons, the probability of For mono energetic photons, the probability of attenuation remains the same for each additional attenuation remains the same for each additional HVL thickness placed in the beam.HVL thickness placed in the beam.

Relationship between μ and HVL:Relationship between μ and HVL:

HVL = 0.693/μHVL = 0.693/μ

Half Value Layer (2)Half Value Layer (2)

List of InteractionsList of Interactions

Attenuation of a photon beam by an absorbing Attenuation of a photon beam by an absorbing material is caused by 5 major types of material is caused by 5 major types of interactions – interactions –

1.1. Coherent Scattering Coherent Scattering

2.2. Photoelectric EffectPhotoelectric Effect

3.3. Compton EffectCompton Effect

4.4. Pair ProductionPair Production

5.5. Photonuclear EffectPhotonuclear Effect

Coherent ScatteringCoherent Scattering

X-rays passing close to the atom cause the bound X-rays passing close to the atom cause the bound electrons to vibrate momentarily at a frequency electrons to vibrate momentarily at a frequency equal to that of the radiation. These in turn emit equal to that of the radiation. These in turn emit radiation of the same frequency in all directions . radiation of the same frequency in all directions .

The energy is taken up from the beam and The energy is taken up from the beam and scattered in all direction, but none of the energy scattered in all direction, but none of the energy is absorbed. Thus this is a form of attenuation is absorbed. Thus this is a form of attenuation without absorption . without absorption .

This interaction is of little importance in practical This interaction is of little importance in practical radiotherapy, but is important in X-ray radiotherapy, but is important in X-ray crystallography . crystallography .

Since it involves bound electrons, it occurs more Since it involves bound electrons, it occurs more in higher atomic number materials, and also more in higher atomic number materials, and also more with low-energy radiations. with low-energy radiations.

Photoelectric Effect (1)Photoelectric Effect (1)

All of the incident photon energy is transferred to All of the incident photon energy is transferred to an electron, which is ejected from the atom.an electron, which is ejected from the atom.

Kinetic energy of ejected electron called the Kinetic energy of ejected electron called the photoelectronphotoelectron ( (EECC ) is equal to incident photon ) is equal to incident photon

energy (energy (EEOO ) minus the binding energy of the ) minus the binding energy of the

orbital electron (orbital electron (EEBB ))

EECC = =EEOO - - EEBB

Photoelectric Effect (3)Photoelectric Effect (3) Incident photon energy must be greater than or equal to the binding Incident photon energy must be greater than or equal to the binding

energy of the ejected photon.energy of the ejected photon.

The ionized atom regains electrical neutrality by rearrangement of the other The ionized atom regains electrical neutrality by rearrangement of the other orbital electrons. The electrons that undergo these rearrangements surrender orbital electrons. The electrons that undergo these rearrangements surrender some of the energy in form of a photon known as the some of the energy in form of a photon known as the characteristic radiation characteristic radiation of the atom.of the atom.

Absorption of these characteristic radiation internally in the atom may result Absorption of these characteristic radiation internally in the atom may result

in emission of in emission of Auger electronsAuger electrons . These electrons are mono energetic in nature. . These electrons are mono energetic in nature.

Probability of characteristic x-ray emission decreases as Z decreasesProbability of characteristic x-ray emission decreases as Z decreasesDoes not occur frequently for diagnostic energy photon interactions in soft Does not occur frequently for diagnostic energy photon interactions in soft

tissuetissue

Photoelectric Effect (4)Photoelectric Effect (4)

Probability of photoelectric absorption per unit Probability of photoelectric absorption per unit mass is approximately proportional tomass is approximately proportional to

Energy dependence explains, in part, why image Energy dependence explains, in part, why image contrast decreases with higher x-ray energies.contrast decreases with higher x-ray energies.

Process can be used to amplify differences in Process can be used to amplify differences in attenuation between tissues with slightly different attenuation between tissues with slightly different atomic numbers, improving image contrast.atomic numbers, improving image contrast.

33 / EZ

Photoelectric Effect (5)Photoelectric Effect (5) Graph of probability of photoelectric effect, as a function Graph of probability of photoelectric effect, as a function

of photon energy, exhibits sharp discontinuities called of photon energy, exhibits sharp discontinuities called absorption edgesabsorption edges . .

Photon energy corresponding to an absorption edge is the Photon energy corresponding to an absorption edge is the binding energy of electrons in a particular shell or sub binding energy of electrons in a particular shell or sub shell .shell .

The phenomena of absorption edges is important for two The phenomena of absorption edges is important for two different reasons:1) At these absorption edges, low-different reasons:1) At these absorption edges, low-energy photons are less attenuated and therefore more energy photons are less attenuated and therefore more penetrating than high energy photons. 2)A substance is penetrating than high energy photons. 2)A substance is relatively transparent to its own characteristic radiation. relatively transparent to its own characteristic radiation. This effect is important when filters are considered as the This effect is important when filters are considered as the filters will be “transparent” to their own characteristic filters will be “transparent” to their own characteristic radiation. radiation.

Compton Effect (1)Compton Effect (1)

Photon interacts with an atomic electron as Photon interacts with an atomic electron as though it were a though it were a free free electron. Practically this electron. Practically this means that energy of the incident photon must means that energy of the incident photon must be large compared with the electron binding be large compared with the electron binding energy.energy.

The electron receives some energy from the The electron receives some energy from the photon and is emitted at an angle Φ while the photon and is emitted at an angle Φ while the photon with reduced energy is scattered at an photon with reduced energy is scattered at an angle θ .angle θ .



As incident photon energy increases, scattered As incident photon energy increases, scattered photons and electrons are scattered more toward photons and electrons are scattered more toward the forward direction.the forward direction.

Probability of interaction increases as incident Probability of interaction increases as incident photon energy increases. photon energy increases.

Probability also depends on electron density ( no. Probability also depends on electron density ( no. of electrons per gram of matter ).of electrons per gram of matter ).

Electron density fairly constant in tissue.Electron density fairly constant in tissue.

Probability of Compton scatter/unit mass Probability of Compton scatter/unit mass independent of Z.independent of Z.

Compton Effect (5)Compton Effect (5) Maximum energy of photons with 90° scatter is 0.511 Maximum energy of photons with 90° scatter is 0.511

M e V while that for 180° scatter ( i.e.. Back scatter) is M e V while that for 180° scatter ( i.e.. Back scatter) is 0.255 M e V.0.255 M e V.

Energy of the photons scattered at angles <90 ° will Energy of the photons scattered at angles <90 ° will be more than 0.511 M e V and will gradually approach be more than 0.511 M e V and will gradually approach the incident photon energy.the incident photon energy.

Energy of the scattered radiation is independent of Energy of the scattered radiation is independent of the incident beam energy .This implies that as the the incident beam energy .This implies that as the photon energy increases there is a corresponding photon energy increases there is a corresponding increase in the forward scatter of the beam. This increase in the forward scatter of the beam. This results in better dose distribution.results in better dose distribution.

Direction of the scatter depends on the energy of the Direction of the scatter depends on the energy of the incident photon beam . This means that higher beam incident photon beam . This means that higher beam energies allow greater absorption of the dose in the energies allow greater absorption of the dose in the body with less scattering of energy.body with less scattering of energy.

Compton Effect (6)Compton Effect (6) If the angle by which the electron is scattered is θ If the angle by which the electron is scattered is θ

and the angle by which the photon is scattered is and the angle by which the photon is scattered is Φ, then the following formula describes the Φ, then the following formula describes the change in the wavelength ( δ λ )of the photon: change in the wavelength ( δ λ )of the photon:

λ 1 – λ 2 = δ λ = 0.024 ( 1- c o s Φ) Å λ 1 – λ 2 = δ λ = 0.024 ( 1- c o s Φ) Å Thus the wavelength change depends neither on Thus the wavelength change depends neither on

the material being irradiated nor on the radiation the material being irradiated nor on the radiation energy, but only upon the angle through which energy, but only upon the angle through which the radiation is scattered. the radiation is scattered.

The Compton effect results in both attenuation The Compton effect results in both attenuation and absorption . and absorption .


Laws of conservation of energy and momentum place Laws of conservation of energy and momentum place limits on both scattering angle and energy transfer.limits on both scattering angle and energy transfer.

Maximal energy transfer to the Compton electron Maximal energy transfer to the Compton electron occurs with a 180-degree photon backscatter.occurs with a 180-degree photon backscatter.

Scattering angle for ejected electron cannot exceed 90 Scattering angle for ejected electron cannot exceed 90 degrees.degrees.

Energy of the scattered electron is usually absorbed Energy of the scattered electron is usually absorbed near the scattering site.near the scattering site.

Pair Production (1)Pair Production (1) When the photon with energy in excess of 1.02 M When the photon with energy in excess of 1.02 M

e V passes close to the nucleus of an atom, the e V passes close to the nucleus of an atom, the photon disappears, and a positron and an electron photon disappears, and a positron and an electron appear. This effect is known as pair production. appear. This effect is known as pair production.

Pair production results in attenuation of the beam Pair production results in attenuation of the beam with absorption. with absorption.

The positron created as a result loses its energy by The positron created as a result loses its energy by interaction with an electron to give rise to two interaction with an electron to give rise to two annihilation photons, each having 0.511 M e V annihilation photons, each having 0.511 M e V energy. Again because momentum is conserved in energy. Again because momentum is conserved in the process two photons are rejected in opposite the process two photons are rejected in opposite directions. This reaction is known as an directions. This reaction is known as an annihilation annihilation reaction. reaction.

Pair Production (3)Pair Production (3)

Pair production results from an interaction with Pair production results from an interaction with the electromagnetic field of the nucleus and as the electromagnetic field of the nucleus and as such the probability of this process increases such the probability of this process increases rapidly with the atomic number (rapidly with the atomic number (ZZ 22 ).).

In addition, the likelihood of this interaction In addition, the likelihood of this interaction increases as the photon energy increases, in increases as the photon energy increases, in contrast to the Compton effects and the contrast to the Compton effects and the photoelectric effect. photoelectric effect.

Photonuclear ReactionPhotonuclear Reaction

This reaction occurs when the photon has energy This reaction occurs when the photon has energy greater than the binding energy of the nucleus greater than the binding energy of the nucleus itself. In this case, it enters the nucleus and ejects itself. In this case, it enters the nucleus and ejects a particle from it. The photon disappears a particle from it. The photon disappears altogether, and any energy possessed in excess altogether, and any energy possessed in excess of that needed to remove the particle becomes of that needed to remove the particle becomes the kinetic energy of escape of that particle. the kinetic energy of escape of that particle.

The threshold energy for thisThe threshold energy for this effect is 10.8 M e V.effect is 10.8 M e V. The main use of this reaction is for energy The main use of this reaction is for energy

calibration of machines producing high energy calibration of machines producing high energy photons. photons.

Relative Importance of the Relative Importance of the Various ProcessesVarious Processes

The relative importance of the 3 principal modes of The relative importance of the 3 principal modes of interaction pertinent to radiation therapy- the interaction pertinent to radiation therapy- the Photoelectric , Compton and Pair production processes - Photoelectric , Compton and Pair production processes - as a function of Incident beam energy and Atomic as a function of Incident beam energy and Atomic number of absorber matter shows -number of absorber matter shows -

For an absorber with Z approximately equal to that of For an absorber with Z approximately equal to that of soft tissue - 7 , and for mono energetic photons , soft tissue - 7 , and for mono energetic photons , Photoelectric effect is the dominant interaction below Photoelectric effect is the dominant interaction below about 30 k e v. about 30 k e v.

Above 30 k e v Compton effect remains dominant and Above 30 k e v Compton effect remains dominant and remains so, remains so,

Until about 24 M e v , after which Pair Production effect Until about 24 M e v , after which Pair Production effect becomes dominant .becomes dominant .

Relative ImportanceRelative Importance

Relative Importance of the Relative Importance of the Various Processes (3)Various Processes (3)

In a graph plotted for total mass attenuation In a graph plotted for total mass attenuation coefficient vs. photon energy it is seen that -coefficient vs. photon energy it is seen that -

The μ /ρ is large for low energies and high Z media The μ /ρ is large for low energies and high Z media (eg. Lead ) because of the predominance of (eg. Lead ) because of the predominance of Photoelectric interactions under these conditions. The Photoelectric interactions under these conditions. The μ /ρ decreases rapidly with energy until the photon μ /ρ decreases rapidly with energy until the photon energies far exceed the electron binding energies and energies far exceed the electron binding energies and Compton effect becomes the predominant mode of Compton effect becomes the predominant mode of interaction.interaction.

In the range of Compton effect the μ /ρ of lead and In the range of Compton effect the μ /ρ of lead and soft tissues do not vary greatly as Compton effect is soft tissues do not vary greatly as Compton effect is independent of Z .The μ /ρ however decrease with independent of Z .The μ /ρ however decrease with energy until Pair production becomes important .energy until Pair production becomes important .

Plot of total mass att. Coef.As a function of photon energy

Part 3 : Interaction of Particle Part 3 : Interaction of Particle Radiation with MatterRadiation with Matter

Interaction of Electrons with matter –Interaction of Electrons with matter –

The two different modes of interaction and energy The two different modes of interaction and energy transfer of electrons with matter include: transfer of electrons with matter include:

Collision between the particle and the electron Collision between the particle and the electron cloud resulting in ionization and excitation ( more cloud resulting in ionization and excitation ( more important in low atomic number elements). This important in low atomic number elements). This is called is called Collisional loss .Collisional loss .

Collision between the nucleus and the particle Collision between the nucleus and the particle resulting in bremsstrahlung radiation (more in resulting in bremsstrahlung radiation (more in high atomic number elements). This is called high atomic number elements). This is called Radiative loss . Radiative loss .

Electron InteractionsElectron Interactions The ionization pattern produced by a beam of The ionization pattern produced by a beam of

electrons is characterized by a constant value electrons is characterized by a constant value from the surface to a depth equal to about half from the surface to a depth equal to about half the range, followed by a rapid falling off to almost the range, followed by a rapid falling off to almost zero at a depth equal to the range . The zero at a depth equal to the range . The bremsstrahlung radiation produced when bremsstrahlung radiation produced when electrons slow down contributes to an electrons slow down contributes to an insignificant dose beyond the range of any insignificant dose beyond the range of any electron. This is specially seen in electrons in the electron. This is specially seen in electrons in the energy range of 6 -15 M e V .energy range of 6 -15 M e V .

These characteristics make electrons a useful These characteristics make electrons a useful treatment modality for superficial lesions. treatment modality for superficial lesions.

Proton & Pi Meson InteractionsProton & Pi Meson Interactions

Protons and pi mesons are charged particles that Protons and pi mesons are charged particles that are being used in experimental set-ups only. are being used in experimental set-ups only.

These particles have a very high These particles have a very high linear energy linear energy transfertransfer (LET) that is they have a very high (LET) that is they have a very high ionization density( Amount of energy deposited ionization density( Amount of energy deposited per unit path length is called the linear energy per unit path length is called the linear energy transfer (LET) ).transfer (LET) ).

Further, these charged particles also exhibit the Further, these charged particles also exhibit the phenomena of phenomena of Bragg’s peakBragg’s peak which refers to the which refers to the increased ionization occurring near the end of the increased ionization occurring near the end of the track with little effect beyond.track with little effect beyond.

Neutron InteractionsNeutron Interactions

• Neutrons are indirectly ionizing uncharged Neutrons are indirectly ionizing uncharged radiations, which interact only with the nucleus radiations, which interact only with the nucleus in two ways: in two ways:

By recoiling protons from hydrogen and the By recoiling protons from hydrogen and the nucleus in other elements. nucleus in other elements.

Nuclear disintegration , which contribute to Nuclear disintegration , which contribute to ~30% of the total dose in tissues.~30% of the total dose in tissues.

Part 4 : Practical ImplicationsPart 4 : Practical Implications

The three major forms of interaction of radiation The three major forms of interaction of radiation with matter, which are of clinical importance in with matter, which are of clinical importance in radiotherapy are: radiotherapy are:

Compton effect. Compton effect. Photoelectric effect. Photoelectric effect. Pair production. Pair production.

Out of these, the Compton effect is the most Out of these, the Compton effect is the most important in modern-day megavoltage important in modern-day megavoltage radiation therapyradiation therapy. .

The reduced scattering suffered by high-energy The reduced scattering suffered by high-energy radiation as well as the almost homogeneous radiation as well as the almost homogeneous tissue dosage is primarily due to the Compton tissue dosage is primarily due to the Compton effect. effect.

Practical Implications Practical Implications Coherent scattering is of little importance in Coherent scattering is of little importance in

practical radiotherapy, but is important in X-ray practical radiotherapy, but is important in X-ray crystallography .crystallography .

The photoelectric effect has several important The photoelectric effect has several important implications in practical radiology: implications in practical radiology:

In diagnostic radiology , the primary mode of In diagnostic radiology , the primary mode of interaction is photoelectric. It is also responsible interaction is photoelectric. It is also responsible for the contrast effect. for the contrast effect.

In therapeutic radiology , low-energy beams in In therapeutic radiology , low-energy beams in orthovoltage irradiation causes excessive orthovoltage irradiation causes excessive absorption of energy in bone. absorption of energy in bone.

Practical ImplicationsPractical Implications The attenuation produced by the Compton effect is The attenuation produced by the Compton effect is

described by the mass scattering coefficient ( σ / ρ ), and described by the mass scattering coefficient ( σ / ρ ), and is practically same for all substances except hydrogenous is practically same for all substances except hydrogenous material, like water and soft tissue, where the Compton material, like water and soft tissue, where the Compton effect is greater (because of the higher electron density).effect is greater (because of the higher electron density).

Attenuation does not depend on the atomic number of Attenuation does not depend on the atomic number of absorber matter in Compton effect. absorber matter in Compton effect.

Thus concrete is as good as lead in shielding of megavoltage Thus concrete is as good as lead in shielding of megavoltage equipment! equipment!

The absorption in bones does not exceed that produced in The absorption in bones does not exceed that produced in the soft tissues – unlike in PE effect seen in orthovoltage the soft tissues – unlike in PE effect seen in orthovoltage radiation era. radiation era.

Port films produced in megavoltage equipment have very Port films produced in megavoltage equipment have very little detail.little detail.

Practical ImplicationsPractical Implications

The low mass of the electron leads to greater The low mass of the electron leads to greater scattering. This is of practical importance as scattering. This is of practical importance as radioactive isotopes which are produce high radioactive isotopes which are produce high energy beta radiation are better stored in low energy beta radiation are better stored in low atomic number materials e.g. plastics as they will atomic number materials e.g. plastics as they will lead to lesser bremsstrahlung radiation. lead to lesser bremsstrahlung radiation.

Also higher atomic number elements are better Also higher atomic number elements are better for x ray production. The amount of radiative loss for x ray production. The amount of radiative loss is proportional to the square of the atomic is proportional to the square of the atomic number of the material This leads to the number of the material This leads to the phenomenon of greater ionization in soft tissues phenomenon of greater ionization in soft tissues relative to bones. Ionization and excitation are relative to bones. Ionization and excitation are more for low atomic materials.more for low atomic materials.

Practical ImplicationsPractical Implications Protons and Heavy particle beams have the ability to Protons and Heavy particle beams have the ability to

concentrate dose inside the target volume and minimize concentrate dose inside the target volume and minimize dose to surrounding normal tissues because of the Bragg dose to surrounding normal tissues because of the Bragg peak effect and minimal scattering. peak effect and minimal scattering.

However there are several practical and theoretical However there are several practical and theoretical difficulties with the use of these charged particles. Some difficulties with the use of these charged particles. Some of them include: of them include:

The narrow Bragg peak makes a homogenous Tumor The narrow Bragg peak makes a homogenous Tumor Dose difficult.Dose difficult.

Generation of these charged particles requires Generation of these charged particles requires expensive and large machines.expensive and large machines.

The method of the production ensures that the field size The method of the production ensures that the field size is very narrow. So, for treatment of cancers the beam is very narrow. So, for treatment of cancers the beam has to be scanned back and forth across the treatment has to be scanned back and forth across the treatment area.area.

Practical ImplicationsPractical Implications Hydrogenous materials like fats absorb neutrons Hydrogenous materials like fats absorb neutrons

more than heavier materials and thus there is a more than heavier materials and thus there is a 20% greater absorption in fat relative to muscle. 20% greater absorption in fat relative to muscle.

Lower atomic materials (e.g. fats and paraffin) Lower atomic materials (e.g. fats and paraffin) are better for neutron shielding as compared to are better for neutron shielding as compared to lead as greater absorption occurs. lead as greater absorption occurs.

Neutrons, being uncharged particles also Neutrons, being uncharged particles also penetrate deeply into matter.penetrate deeply into matter.

But neutrons are not commonly used in practical But neutrons are not commonly used in practical radiotherapy, because of technical difficulties in radiotherapy, because of technical difficulties in production of these beams as well as their production of these beams as well as their complicated dosimetry. complicated dosimetry.

Part 5 : ConclusionPart 5 : Conclusion Despite several decades of research, photon-Despite several decades of research, photon-

beam still constitute the main therapeutic beam still constitute the main therapeutic modality in radiotherapy, because of several modality in radiotherapy, because of several unresolved technical problems with the use of unresolved technical problems with the use of particulate radiation.particulate radiation.

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Interaction of Radiation with Matter

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