Lecture 7 Chapter 06 Photon Beams

8/10/2019 Lecture 7 Chapter 06 Photon Beams

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IAEAInternational Atomic Energy Agency

Slides based on the chapter authored byE.B. Podgorsak of the IAEA textbook (ISBN 92-0-107304-6): Radiation Oncology Physics: A Handbook for Teachers and StudentsObjective:

To familiarize the student with the basic principles of dosecalculations in external beam radiotherapy with photon beams.

Chapter 6: External Photon Beams: Physical Aspects

BME704: Radiation TherapyDevices - Lecture 6

Slide set prepared in 2006 by E.B. Podgorsak (Montreal, McGill University)

Comments to S. Vatnitsky:[email protected] 2012

Additional content provided by B.A Standish(Toronto, RyersonUniversity)

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IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.

CHAPTER 6. TABLE OF CONTENTS

6.1.6.2.6.3.6.4.6.5.6.6.6.7.6.8.6.9.6.10.6.11.

IntroductionQuantities used in describing a photon beamPhoton beam sourcesInverse square lawPenetration of photon beams into a phantom or patientRadiation treatment parametersCentral axis depth doses in water: SSD set-upCentral axis depth doses in water: SAD set-upOff-axis ratios and beam profilesIsodose distributions in water phantomsSingle field isodose distributions in patients

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IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.1 Slide 1

6.1 INTRODUCTION

! Radiotherapy also referred to as radiation oncology ortherapeutic radiology is a branch of medicine that usesionizing radiation in treatment of malignant disease.

! Radiotherapy is divided into two categories:

External beam radiotherapy Brachytherapy

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6.1 INTRODUCTION

! Ionizing photon radiation Gamma rays (originates in nuclear gamma decay)

Used in teletherapy machines

Bremsstrahlung (electron - nucleus Coulomb interaction)Used in x-ray machines and linacs

Characteristic x rays (electron - orbital electron interaction)Used in x-ray machines and linacs Annihilation radiation (positron annihilation)

Used in positron emission tomography (PET) imaging

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6.2 QUANTITIES USED IN DESCRIBING PHOTON BEAMS

! Radiation dosimetry deals with two distinct entities:

Description of photon radiation beam in terms of the number andenergies of all photons constituting the beam ( photon beamspectrum ).

Description of the amount of energy per unit mass (absorbeddose ) the photon beam may deposit in a given medium, such asair, water, or biological material.

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IAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.2.1 Slide 1

6.2 QUANTITIES USED IN DESCRIBING PHOTON BEAMS6.2.1 Photon fluence and photon fluence rate

d N is the number of photons that enter an imaginary sphere ofcross-sectional area d A.

Unit of photon fluence is cm ! 2.

is defined as photon

Unit of photon fluence rate is cm ! 2 . s! 1.

! Photon fluence " =d Nd A

d " d t

! Photon fluence rate # = " = fluence per unit time.

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6.2 QUANTITIES USED IN DESCRIBING PHOTON BEAMS6.2.2 Energy fluence and energy fluence rate

.

is defined as the energy

! Energy fluence $ =d Ed A

d E is the amount of energy crossing a unit area d A. Unit of energy fluence $ is MeV %cm! 2

d $ d t

! Energy fluence rate & = $ = fluence $ per unit time.

Unit of energy fluence rate& is MeV %cm ! 2 %s! 1

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= $ ' ( ' () tr * ) tr *


6.2 QUANTITIES USED IN DESCRIBING PHOTON BEAMS6.2.3 Air kerma in air

! For a monoenergetic photon beam in air the air kerma inair ( K air )air at a given point away from the source is

( K air )air = "h+

, - .air

, - .air

( tr /- ) is the mass-energy transfer coefficient for air at photonenergy h+.

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6.3 PHOTON SOURCES FOR EXTERNAL BEAM THERAPY

! Photon sources with regard to type of photons : Gamma ray sources X-ray sources

! Photon sources with regard to photon energies :

Monoenergetic sources Heterogeneous sources

! Photon sources with regard to intensity distribution : Isotropic

Non-isotropic

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! For a given photon source, a plot of number of photons per energy interval versus photon energy is referred to as photon spectrum.

! All photons in a monoenergetic photon beam have thesame energy h+.

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! Photons in a heterogeneous x-ray beam form a distinctspectrum,

Photons are present in all energy intervals from 0 to amaximum value h+max which is equal to the monoenergetickinetic energy of electrons striking the target.

The two spikes in thespectrum representcharacteristic x rays ;the continuous spectrum

from 0 to h+max represents bremsstrahlung photons .

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! Gamma ray sources are usually isotropic and producemonoenergetic photon beams.

! X-ray targets are non-isotropic sources and produceheterogeneous photon spectra.

In the superficial and orthovoltage energy region the x-rayemission occurs predominantly at 90 o to the direction of theelectron beam striking the x-ray target.

In the megavoltage energy region the x-ray emission in thetarget occurs predominantly in the direction of the electron

beam striking the target (forward direction).

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6.4 INVERSE SQUARE LAW

! In external beam radio-therapy:

Photon sources are oftenassumed to be point

sources. Beams produced by

photon sources areassumed to bedivergent.

tan / = a / 2 f a

=b / 2 f b

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! Photon source S emits photons and produces a photon fluence " A at adistance f a and a photon

fluence " B at distance f b.! Number of photons N tot

crossing area A is equalto the number of photons

crossing area B.

N tot = " A A= " B B = const

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= 2 = 2f a

(K (f a))air

(K (f b))air = ' (



! We assume that N tot = const ,i.e., no photon interactions take

place in air. Therefore:

" A

" B=

B

A a

b2 f b2

X(f a)X(f b)

=

colaircolair

=00

Dmed

Dmed

) f b *, f a.

2

0! X , ( K aircol )air , and Dmed quantities allfollow the inverse square law.

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= 2 = 2f a

(K (f a))air

(K (f b))air = ' (



!

=

a

b2 f b2

X(f a)X(f b)

=

colaircolair

=00

Dmed

Dmed

) f b *, f a.

2

0! X , ( K aircol )air , and Dmed quantities allfollow the inverse square law.

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http://en.wikipedia.org/wiki/File:Inverse_square_law.svg

Flux isproportional to 1/

r^2


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6.5 PENETRATION OF PHOTON BEAMS INTO PATIENT

! A photon beam propagating through air or vacuum isgoverned by the inverse square law.

! A photon beam propagating through a phantom or patientis affected not only by the inverse square law but also bythe attenuation and scattering of the photon beam insidethe phantom or patient.

! The three effects make the dose deposition in a phantomor patient a complicated process and its determination acomplex task.

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! For a successful outcome of patient radiation treatment itis imperative that the dose distribution in the target volumeand surrounding tissues is known precisely and accurately .

! This is usually achieved through the use of severalempirical functions that link the dose at any arbitrary pointinside the patient to the known dose at the beamcalibration (or reference) point in a phantom.

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! Dosimetric functions are usually measured with suitableradiation detectors in tissue equivalent phantoms.

! Dose or dose rate at the reference point is determined for,or in, water phantoms for a specific set of referenceconditions , such as:

Depth in phantom z Field size A Source-surface distance (SSD).

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! Typical dose distribution for an external photon beamfollows a known general pattern:

The beam enters the patienton the surface where it deliversa certain surface dose D s.

Beneath the surface the dosefirst rises rapidly, reaches amaximum value at a depth z max ,and then decreases almostexponentially until it reaches a

value D ex at the patients exit point.

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6.5 PENETRATION OF PHOTON BEAMS INTO PATIENT6.5.1 Surface dose

! Surface dose:

For megavoltage x-ray beams thesurface dose is generally muchlower (skin sparing effect ) than themaximum dose at z max .

For superficial and orthovoltage beams z max = 0 and the surfacedose equals the maximum dose.

The surface dose is measured with parallel-plate ionization chambers

for both chamber polarities, with theaverage reading between the two polarities taken as the correctsurface dose value.

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6.5 PENETRATION OF PHOTON BEAMS INTO PATIENT6.5.2 Buildup region

! Buildup dose region :

The region between the surface ( z = 0) and depth z = z max inmegavoltage photon beams is called the dose buildup region.

The dose buildup results fromthe relatively long range ofsecondary charged particlesthat first are released in the

patient by photon interactionsand then deposit their kineticenergy in the patient throughCoulomb interactions.

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6.5 PENETRATION OF PHOTON BEAMS INTO PATIENT6.5.3 Depth of dose maximum

! Depth of dose maximum z max depends upon: Photon beam energy (main effect) Field size (secondary effect)

! For a given field size: z max increases with photon

beam energy. For 5x5 cm 2 fields, the

nominal values of z max are:

Energy 100 kV p 350 kV p Co-60 4 MV 6 MV 10 MV 18 MV

z max (cm) 0 0 0.5 1.0 1.5 2.5 3.525

Why would field size affect depth ofdose?


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6.5 PENETRATION OF PHOTON BEAMS INTO PATIENT6.5.3 Depth of dose maximum z max

! At a given beam energy: For fields smaller than 5 ! 5 cm 2,

z max increases with increasingfield size because of in-phantomscatter.

For field 5 ! 5 cm 2, z max reachesits nominal value.

For fields larger than 5 ! 5 cm 2, z max decreases with increasingfield size because of collimator

and flattening filter scatter.

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6.5 PENETRATION OF PHOTON BEAMS INTO PATIENT6.5.3 Exit dose

! The dose delivered to the patient at the beam exit point iscalled the exit dose .

! Close to the beam exit point the dose distribution curvesslightly downwards from the dose curve obtained for ainfinitely thick phantom as aresult of missing scattercontribution for points beyondthe dose exit point.

! The effect is small and

generally ignored.

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6.6 RADIATION TREATMENT PARAMETERS

! The main parameters in external beam dose delivery with photon beams are:

Depth of treatment zFields size ASource-skin distance (SSD) in SSD setupsSource-axis distance (SAD) in SAD setupsPhoton beam energy h+

Number of beams used in dose delivery to the patientTreatment time for orthovoltage and teletherapy machines

Number of monitor units (MUs) for linacs

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6.6 RADIATION TREATMENT PARAMETERS

! Several functions are in use for linking the dose at areference point in a water phantom to the dose at arbitrary

points inside the patient.

Some of these functions can be used in the whole energy range ofinterest in radiotherapy from superficial through orthovoltage andcobalt-60 to megavoltage

Others are only applicable at energies of cobalt-60 and below. Or are used at cobalt-60 energy and above.

! Cobalt-60 serves as a transition point linking various

dosimetry techniques.

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6.6 RADIATION TREATMENT PARAMETERS6.6.1 Radiation beam field size

! Four general groups of field shape are used in radiotherapy Square (produced with collimators installed in therapy machine) Rectangular (produced with collimators installed in therapy machine) Circular (produced with special collimators attached to treatment

machine)

Irregular (produced with custom made shielding blocks or withmultileaf collimators)

! For any arbitrary radiation field and equivalent square fieldor equivalent circular field may be found. The equivalentfield will be characterized with similar beam parameters andfunctions as the arbitrary radiation field.

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aeq = 1r eq


6.6 RADIATION TREATMENT PARAMETERS6.6.1 Radiation beam field size

! Equivalent square for rectangular field:

An arbitrary rectangular field with sidesa and b will be approximately equal to a

a eq =

r eq =

2aba+ b

a

1

square field with side a eq when both fieldshave the same area/perimeter ratio

(Days rule). ab aeq2

= 2( a+ b ) 4aeq

! Equivalent circle for square field: An arbitrary square field with side a

will be equivalent to a circular field withradius r eq when both fields have thesame area.

2 2

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0

6.6 RADIATION TREATMENT PARAMETERS6.6.2 Collimator factor

! Exposure in air X air , air kerma in air ( K air )air and dose to smallmass of medium in air Dmed contain two components: Primary component is the major component.

It originates in the source, comes directly from the source, and doesnot depend on field size.

Scatter component is a minor, yet non-negligible, component.It represents the scatter from the collimator, air and flattening filter(in linacs) and depends on the field size A.

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For this course our focus is on the knownquantity of (K air )air .


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0

6.6 RADIATION TREATMENT PARAMETERS6.6.2 Collimator factor

! X air,( K air )air , and Dmed depend upon: Field size A

Parameter called the collimator factor (CF)or

collimator scatter factor S correlative exposure factor (REF).

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6.6 RADIATION TREATMENT PARAMETERS6.6.3 Peak scatter factor

! PSF gives the factor by which the radiation dose at point P inair is increased by scattered radiation when point P is in the

phantom at depth z max .! PSF depends upon:

Field size A(the larger is the fieldsize,the larger is PSF).

Photon energy h+ (except at very low

photon energies,PSF decreaseswith increasingenergy).

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6.6 RADIATION TREATMENT PARAMETERS6.6.3 Peak scatter factor

! PSF( A,h+) =0

DP( z max , A, f ,h+) DP( A,h+)

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Point in air

Point in phantom


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6.6 RADIATION TREATMENT PARAMETERS6.6.4 Relative dose factor

! For a given photon beam with energy h+ at a given SSD, thedose at point P (at depth z max) depends on field size A; thelarger is the field size the larger is the dose .

! The ratio of the dose at point P for field size A to the dose at point P for field size 10 ! 10 cm 2 is called the relative dosefactor RDF or total scatter factor S c,p :

D P ( z max , A, f , h + ) D P (z max ,10,f,h + )

RDF(A,h + ) = S c,p(A,h+ ) =

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! For A < 10 ! 10 cm


6.6 RADIATION TREATMENT PARAMETERS6.6.4 Relative dose factor

D P( z max , A, f ,h+) D P( z max ,10, f ,h+)

! Relative dose factor:RDF( A,h+) = S c,p( A,h+) =

2

RDF( A ,h+ ) < 1

! For A = 10 ! 10 cm 2

RDF( A,h+) = 1

! For A > 10 ! 10 cm 2

RDF( A ,h+ ) > 1

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= 100


6.7 CENTRAL AXIS DEPTH DOSES IN WATER: SSD SETUP6.7.1 Percentage depth dose

! Central axis dose distributions inside the patient are usuallynormalized to Dmax = 100 % at the depth of dose maximum

z max and then referred to as percentage depth dose (PDD)distributions.

! PDD is thus defined as follows:

D Q D Q D P D P

PDD( z , A, f ,h+) = 100

DQ and DQ are the dose and dose rate, respectively, at arbitrary point Q at depth z on the beam central axis.

DP and DP are the dose and dose rate, respectively, at reference point P at depth z max on the beam central axis.

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6 C A A S OS S A SS S


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= 100



! Percentage depth dose depends on four parameters:

D Q D Q D P DP

PDD( z , A, f ,h+) = 100

Depth in phantom zField size A on patients surfaceSource-surface distance f = SSD

Photon beam energy h+

! PDD ranges in value from

0 at z 2 3 To 100 at z = zmax

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! The dose at point Q in the patient consists of twocomponents: primary component and scatter component.

The primary component is expressed as:

2

DP , f + z .

eff is the effective linear attenuation coefficient for the primary beamin the phantom material (for example, eff for a cobalt-60 beam inwater is 0.0657 cm -1).

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! The dose at point Q in the patient consists of twocomponents: primary component and scatter component.

The scatter component at point Q reflects the relative contributionof the scattered radiation to the dose at point Q. It depends in acomplicated fashion on various parameters such as depth, field

size and source-skin distance. Contrary to the primary component in which the photon

contribution to the dose at point Q arrives directly from the source,the scatter dose is delivered by photons produced throughCompton scattering in the patient, machine collimator, flattening

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! For a constant A, f , and h+, PDD( z , A, f ,h+) first increasesfrom the surface to z = z max (buildup region), and then

decreases with z .

! Example :

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6.8 CENTRAL AXIS DEPTH DOSES IN WATER: SAD SETUP

! SAD setups are used in treatment of deep seated tumourswith multiple beams or with rotational beams.

! In comparison with constant SSD setup that relies on PDDdistributions, the SAD setup is more practical and relies onother dose functions such as:

Tissue-air ratio (TAR)

Tissue-phantom ration (TPR)

Tissue-maximum ratio (TMR)

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6.8 CENTRAL AXIS DEPTH DOSES IN WATER: SAD SETUP6.8.1 Tissue-air ratio

! Tissue-air ratio (TAR) wasintroduced by Johns tosimplify dose calculations inrotational radiotherapy butis now also used for

treatment with multiplestationary beams.

! The SSD varies from one beam to another; however,the source-axis distanceSAD remains constant .

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6.8 CENTRAL AXIS DEPTH DOSES IN WATER: SAD SETUP6.8.1 Tissue-air ratio

! In contrast to PDD (z,A,f,h+ ) which depends on four parameters, TAR depends on three beam parameters: Depth of isocentre z Field size at isocentre AQ Beam energy h+

! The field size AQ is defined at point Q which is normally placed into the isocentre of the treatment machine.

TARs are most reliably measured with ionization chambers ;however, the measurements are much more cumbersomethan those of PDD because in TAR measurement thesource-chamber distance must be kept constant.

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6 8 CENTRAL AXIS DEPTH DOSES IN WATER SAD SETUP


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, f + z max .' (


6.8 CENTRAL AXIS DEPTH DOSES IN WATER: SAD SETUP6.8.2 Relationship between TAR and PDD

!

2

PDD( z , A, f ,h+)) f + z * 100

TAR( z , AQ,h+) = PSF( A,h+)

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*Peak Scatter Fraction relates measurement in air to phantom(tissue)


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6.9 OFF-AXIS RATIOS AND BEAM PROFILES

! Dose distributions along the beam central axis are usedin conjunction with off-axis beam profiles to deliver anaccurate dose description inside the patient.

! The off-axis data are usually given with beam profiles

measured perpendicularly to the beam central axis at agiven depth in a phantom.

! The depths of measurement are typically at:

Depths z = z max and z = 10 cm for verification of machinecompliance with machine specifications.

Other depths required by the particular treatment planningsystem used in the department.

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! Example of beam profiles measured for two field sizes(10 ! 10 cm 2 and 30 ! 30 cm 2) of a 10 MV x-ray beam at

various depths in water.

! The central axis profile

values are scaled bythe appropriate PDDvalue for the two fields.

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! Combining a central axis dose distribution with off-axisdata results in a volume dose matrix that provides 2-D

and 3-D information on the dose distribution in the patient.

! The off-axis ratio OAR is usually defined as the ratio ofdose at an off-axis point to the dose on the central beamaxis at the same depth in a phantom.

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! Megavoltage beam profiles consist of three regions: Central region represents the central portion of the profile

extending from the central axis to within 1 cm to 1.5 cm of thegeometric field edges of the beam.

Penumbra is the region close to geometric field edges where thedose changes rapidly and depends on field defining collimators,the finite size of the focal spot (source size) and the lateralelectronic disequilibrium.

Umbra is the region outside of the radiation field, far removedfrom the field edges. The dose in this region is low and resultsfrom radiation transmitted through the collimator and headshielding.

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! For each of the three beam profile regions there arespecific requirements to optimize the clinical photon beam:

The dose profile in the central region should meet flatness andsymmetry specifications.

The dose profile in the penumbral region should have a rapid falloffwith increasing distance from the central axis (narrow penumbra)to optimize beam sharpness at the target edge.

The dose profile in the umbral region should be close to zero doseto minimize the dose delivered to tissues outside the target

volume.

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! Ideal dose profile: Central region:

constant dose fromtarget centre to edgeof target.

Penumbra: zero width.

Umbra: zero dose.

! Actual dose profile: Central region: profile flat in 80 % of central portion of the field.

Penumbra is typically defined as the distance between 80 % and 20 %dose on the beam profile normalized to 100 % at the central axis.

Umbra is typically less than 1 % of the dose on the central axis.

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! Geometric or nominal field size is:

Indicated by the optical light field of the treatment machine.

Usually defined as the separation between the 50% dose level points on the beam profile measured at the depth of dosemaximum z max .

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! In the central region, the off-axis points of the beam profileare affected:

For cobalt-60 beams , by the inverse squarelaw dose fall-off andthe increased phantomthickness as the off-axisdistance increases.

For linacs , by the energyof electrons strikingthe target, by the atomic

number of the target,and the atomic numberand shape of the flatteningfilter.

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! The total penumbra is referred to as the physical penumbraand consists of three components:

Geometric penumbraresults from the finitesource size.

Scatter penumbraresults from in-patient photon scatteroriginating inthe open field.

Transmission penumbraresults from beamtransmitted throughthe collimation device.

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6.9 OFF-AXIS RATIOS AND BEAM PROFILES6.9.1 Beam flatness

! Beam flatness F is assessed by finding the maximum Dmax and minimum Dmin dose point values on the beam

profile within the central 80 % of the beam width.

! Beam flatness F is defined as:

F = 100 4 Dmax ! Dmin Dmax + Dmin

! Standard linac specifications require that F 5 3 %whenmeasured in a water phantom at a depth z = 10 cm withSSD = 100 cm for the largest field size available(typically 40 ! 40 cm 2).

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! Compliance with the flatness specifications at a depth z = 10 cm in water results in:

Over-flattening at z max , manifesting itself in the form of hornsin the profile.

Under-flattening at depths exceeding z = 10 cm. Thisunderflattening becomes progressively worse as the depth zincreases beyond z = 10 cm.

! The over-flattening and under-flattening of the beam profiles is caused by the lower beam effective

energies in off-axis directions compared with thecentral axis direction.

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! Typical profiles measured in water with a 40 ! 40 cm 2field at SSD = 100 cm. The data for depths z = 10 cmand z = z max are used for verification of compliance withstandard machine specifications.

F = 100 4 Dmax ! Dmin Dmax + Dmin

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6.9 OFF-AXIS RATIOS AND BEAM PROFILES6.9.2 Beam symmetry

! Beam symmetry S is usually determined at z max toachieve maximum sensitivity.! Typical symmetry specifications for a 40 ! 40 cm 2 field:

Any two dose points on a beam profile, equidistant from thecentral axis point, should be within 2 % of each other.

Areas under the z max beam profile on each side (left and right) ofthe central axis extending to the 50 % dose level (normalized to100 % at the central axis point) are determined.

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6.9.2 Beam symmetry

!

S is calculated from

and should be less than 2 %.

! Practical options for determination of areas under the profile curve with a hard copy of the profile are:

using a planimeter

OR counting squares on graph paper.

= 100 4 area left ! area right

area left + area rightS

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6.10 ISODOSE DISTRIBUTIONS IN WATER PHANTOMS

! Planar and volumetric dose distributions are usuallydisplayed with isodose curves and isodose surfaces ,

which connect points of equal dose in a volume ofinterest.

! The isodose curves and surfaces are usually drawn atregular intervals of absorbed dose and are expressed asa percentage of the dose at a specific reference point.

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! An isodose chart for a given single beam consists of afamily of isodose curves usually drawn at regular

increments of PDD.

! Two normalization conventions are in use:

For SSD set-ups, all isodose values are normalized to 100 % at point P on the central beam axis (point of dose maximum).

For SAD set-ups, the isodose values are normalized to 100 % atthe isocentre.

! The isodose charts for an SSD set-up are thus plots ofPDD values; isodose charts for an SAD set-up are plotsof either TAR or TMR (outside of scope for this course).

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IAEARadiation Oncology Physics: A Handbook for Teachers and Students - 6.10 Slide 4


! For SSD set-ups, all isodosevalues are normalized to 100 %at point P on the central beamaxis (point of dose maximum atdepth z max ).

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! For SAD set-ups, the isodosevalues are normalized to 100 %at the isocentre.

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! Parameters that affect the single beam isodosedistribution are:

Beam quality

Source size

Beam collimation

Field size

Source-skin distance

Source-collimator distance

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! Treatment with single photon beams is seldom usedexcept for superficial tumours treated with superficial

or orthovoltage x rays.

! Deep-seated tumours are usually treated with a

combination of two or more megavoltage photon beams.

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! Isodose distributions for various photon radiation beams:orthovoltage x rays, cobalt-60 gamma rays, 4 MV x rays, 10 MV x rays

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! Isodose charts are measured with:

Ionization chambersSolid state detectors such as diodesStandard radiographic filmRadiochromic film

! In addition to direct measurements, isodose charts mayalso be generated by calculations using various algorithmsfor treatment planning, most commonly with commerciallyavailable treatment planning systems (TPSs).

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! Phantom measurements are normally characterized by:

Flat phantom surface

Perpendicular beam incidence

Homogeneous, unit density phantom

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! Clinical situations are usually more complex: The patients surface may be curved or of irregular shape,

requiring corrections for contour irregularities .

The beam may be obliquely incident on patients surface requiring

corrections for oblique beam incidence . Some tissues such as lung and bone have densities that differ

significantly from that of water, requiring corrections for tissueheterogeneities (also called inhomogeneities ).

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! Isodose distributions in patients are determined by one oftwo radically different approaches:

Correction-based algorithms use depth dose data measured inwater phantoms with a flat surface and normal incidence inconjunction with various methods to correct for irregular patient

contours, oblique beam incidence, and different tissue densities. Model-based algorithms obviate the correction problem by

modeling the dose distributions from first principles andaccounting for all geometrical and physical characteristics of the

particular patient and treatment.

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6 11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS


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IAEARadiation Oncology Physics: A Handbook for Teachers and Students - 6.11.1 Slide 1

6.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.1 Corrections for irregular contours and beam obliquity

! A radiation beam striking an irregular or sloping patientsurface produces an isodose distribution that differs from

the standard distributions obtained with normal beamincidence on a flat phantom surface.

! Two approaches are used to deal with this problem:

The flat phantom / normal incidence isodose distribution iscorrected numerically to obtain the actual dose distribution in the

patient.

To achieve flat phantom / normal incidence distributions in a

patient the physical effect can be compensated for through theuse of wedges, bolus materials or special compensators.

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6.11 SINGLE FIELD ISODOSE DISTRIBUTIONS IN PATIENTS6.11.1 Corrections for irregular contours and beam obliquity

! Methods for correcting the standard flat surface / normalincidence isodose distributions for contour irregularities

and oblique beam incidence are:

Effective SSD method

TAR or TMR method

Isodose shift method

! These methods are applicable for:

Megavoltage x rays with angles of incidence up to 45 o.

Orthovoltage beams with angles of incidence up to 30 o.

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6.11.2 Missing tissue compensation

! Wedge filters are used to even out the isodose surfacesfor photon beams striking relatively flat patient surfaces

under an oblique beam incidence.

! Two types of wedge filter are in use:

Physical wedge is made of lead, brass, or steel. When placed in aradiation beam, the wedge causes a progressive decrease in theintensity across the beam and a tilt of isodose curves undernormal beam incidence.

Dynamic wedge provides the wedge effect on isodose curvesthrough a closing motion of a collimator block during irradiation.

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! Two parameters are ofimportance for wedges:

Wedge transmission factor isdefined as the ratio of doses at

z max in a water phantom on the

beam central axis (point P) withand without the wedge.

Wedge angle is defined as theangle through which an isodosecurve at a given depth in water(usually 10 cm) is tilted at thecentral beam axis under thecondition of normal beamincidence.

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! Bolus is tissue equivalent material placed directly ontothe patients skin surface:

To even out irregular patient contour. To provide a flat surface for normal beam incidence.

! In principle, the use of bolus is straightforward and practical; however, it suffers a serious drawback: formegavoltage photon beams it results in the loss of theskin sparing effect in the skin covered with the bolus(i.e., skin sparing effect occurs in the bolus rather than inthe patient).

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! Compensators are used to produce the same effect asthe bolus yet preserve the skin sparing effect ofmegavoltage photon beams.

! Compensator is a custom-made device that mimics theshape of the bolus but is placed in the radiation beam atsome 15 cm 20 cm from the skin surface to preservethe skin sparing properties of the radiation beam.

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! Typical compensator materials are: Lead Special low melting point alloys such as Cerrobend (Lipowitzs

metal). Water equivalent materials such as wax.

! Since compensators are placed at some distance fromthe skin surface, their shape must be adjusted for:

Beam divergence Linear attenuation coefficient of the compensator material. Reduction in scatter at various depths in patient.

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6.11.4 Model based algorithms

! Model based algorithms for computation of dose distributionin a patient are divided into three categories:

Primary dose plus first order Compton scatter method is rudimentaryas it assumes a parallel beam of monoenergetic photons and ignoresinhomogeneities and scattering above the first order.

Convolution-superposition method accounts for the indirect nature ofdose deposition from photon interactions, separating the primaryinteractions from the transport of scattered photons and charged

particles produced through primary photon interactions.

Monte Carlo method uses well established probability distributionsgoverning the individual interactions of photons and secondarycharged particles and their transport through the patient.

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! Monte Carlo simulation can be used directly to compute photon dose distributions for a given patient andtreatment geometry.

! The current limitation of direct Monte Carlo calculations isthe time required to calculate the large number of

histories needed to reduce stochastic or randomuncertainties to acceptable levels.

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! Advances in computer technology will, within a few years,reduce Monte Carlo calculation times to acceptable levelsand make Monte Carlo methods the standard approachto radiotherapy treatment planning.

! The electron densities for various tissues of individual

patients are obtained with CT scanners or CT simulatorsand form an essential component of any Monte Carlo based dose distribution calculation.

Documents

Lecture 7 Chapter 06 Photon Beams