Part VIII:Medical Exposures in Radiotherapy Lecture 5 : Determination of Dose to a Patient-I IAEA Post Graduate Educational Course on Radiation Protection

Part VIII:Medical Exposures in Part VIII:Medical Exposures in RadiotherapyRadiotherapy

Lecture 5 : Determination of Dose to a Patient-I

IAEA Post Graduate Educational Course on Radiation Protection and Safe Use of Radiation Sources

Module 3 Optimization of Protection for Medical Exposure

Part VIII.3.5 Determination of Dose to a Patient-I Slide 2

Scope- at the end of this lecture the Scope- at the end of this lecture the student should student should

• have understood the parameters such as have understood the parameters such as SSD, SAD, Isocentre, field size, penumbra SSD, SAD, Isocentre, field size, penumbra etcetc

• be able to appreciate the concept of be able to appreciate the concept of equivalent square and its useequivalent square and its use

• have understood the factors required for have understood the factors required for calculation of absorbed dosecalculation of absorbed dose


This lecture will cover…This lecture will cover…

• Definitions and explanations of parameters Definitions and explanations of parameters such SSD, Field Size, SAD, Isocentre, etcsuch SSD, Field Size, SAD, Isocentre, etc

• Concept of equivalent square and Concept of equivalent square and determination of equivalent square for determination of equivalent square for irregular fieldsirregular fields

• Definition and explanations of factors such as Definition and explanations of factors such as PDD, TAR, TMR used for calculating the PDD, TAR, TMR used for calculating the absorbed doseabsorbed dose


Some common terms in External Some common terms in External Beam Therapy (EBT)…Beam Therapy (EBT)…

• Source to Skin Distance (SSD)Source to Skin Distance (SSD)• Source to Axis Distance (SAD)Source to Axis Distance (SAD)• Isocentre Isocentre • Field SizeField Size• Equivalent squareEquivalent square• PenumbraPenumbra


What is SSD?What is SSD?

• SSD is the acronym of Source to Skin SSD is the acronym of Source to Skin distance. It is the distance between the source distance. It is the distance between the source and the patient skinand the patient skin

• Machines have Standard SSD at which output Machines have Standard SSD at which output and PDD are measuredand PDD are measured

• 50 cm for a Cs 137 Unit50 cm for a Cs 137 Unit• 80 – 100 cm for Co 60 unit80 – 100 cm for Co 60 unit• 100 cm for Linear accelerator100 cm for Linear accelerator


Source to Skin DistanceSource to Skin Distance

SSD=80cm


Why is SSD important?Why is SSD important?

• The dose calibration of the External beam The dose calibration of the External beam unit is at the SSD (or Isocentre – explained unit is at the SSD (or Isocentre – explained later)later)

• Any change in this will vary the dose by Any change in this will vary the dose by ‘inverse square’ factor‘inverse square’ factor


What is meant SAD?What is meant SAD?

• It is the distance from the source to Isocentre It is the distance from the source to Isocentre (axis of rotation)(axis of rotation)

• It is 80cm – 100 cm for cobalt unitsIt is 80cm – 100 cm for cobalt units• 100cm for Linear accelerator100cm for Linear accelerator


What is an isocentreWhat is an isocentre• The point in space where the axes of rotation The point in space where the axes of rotation

of the Gantry, couch, collimator and the of the Gantry, couch, collimator and the beam axis meetbeam axis meet

Isocentre of a treatment unitIsocentre of a treatment unit


What is the significance of Isocentre?What is the significance of Isocentre?

• Isocentre is the centre of Couch rotation, Isocentre is the centre of Couch rotation, Gantry rotation and radiation beam central Gantry rotation and radiation beam central axisaxis

• This ensures accurate beam direction if the This ensures accurate beam direction if the tumour centre is positioned at the isocentre of tumour centre is positioned at the isocentre of the unitthe unit


IISSOOCCEENNT T R R IICC

TTRREEAATTMMEENNTT

Click to demonstrate isocentric treatment


Field SizeField Size

• Field size is the width and length of the radiation Field size is the width and length of the radiation beam at SSD or SADbeam at SSD or SAD

• Field size at any depth is usually defined by the 50% Field size at any depth is usually defined by the 50% width of the profile at that depthwidth of the profile at that depth


PenumbraPenumbra• Penumbra is the unsharp edge of the radiation Penumbra is the unsharp edge of the radiation

beam created mainly by the finite source size beam created mainly by the finite source size • Penumbra could be classified as Penumbra could be classified as

– Geometric Penumbra which depends onGeometric Penumbra which depends on• Source SizeSource Size• Source to Diaphragm distanceSource to Diaphragm distance• Source to skin distanceSource to skin distance

– Radiological Penumbra which is Radiological Penumbra which is • Geometric penumbra + ScatterGeometric penumbra + Scatter


The Geometric PenumbraThe Geometric Penumbra

P =S x (SSD+d-SDD)

SDD

SDD Source to Diaphragm distance

SSD Source to skin distance

P =S x f+d-fd

fd

CE - penumbra at surface

CE =S x f-fd

fdSAD Field size at surface


Physical Penumbra / Radiological Physical Penumbra / Radiological penumbrapenumbra

• Radiation beam penumbra Radiation beam penumbra

– Lateral scatter increases the radiation Lateral scatter increases the radiation beam penumbrabeam penumbra

– Lower energy, more side scatter and Lower energy, more side scatter and hence larger penumbrahence larger penumbra

– Density of the scattering medium affects Density of the scattering medium affects the penumbrathe penumbra

• Issues : larger the penumbra, greater the dose to Issues : larger the penumbra, greater the dose to normal tissuesnormal tissues


Scattered radiationScattered radiation

Scatter


Physical / Radiological penumbraPhysical / Radiological penumbra

In Air In Phantom


Radiological PenumbraRadiological Penumbra

Radiological penumbra is defined as the 80%-20% width of the dose profile


Factors for determination of Factors for determination of absorbed doseabsorbed dose

• Percentage depth dose (PDD)Percentage depth dose (PDD)• Tissue Air Ratio (TAR)Tissue Air Ratio (TAR)• Tissue Maximum Ratio (TMR)Tissue Maximum Ratio (TMR)• Tissue Phantom Ratio (TPR)Tissue Phantom Ratio (TPR)• Scatter Air Ratio (SAR)Scatter Air Ratio (SAR)• Scatter Maximum Ratio (SMR)Scatter Maximum Ratio (SMR)


Percentage Depth Dose (PDD)Percentage Depth Dose (PDD)

PDD (S,Q,f,d) =D

Dmax

x 100

Field Size (s)=AB


Percentage Depth Dose Percentage Depth Dose 6060CoCo

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35

Depth in cm

% d

ep

th d

os

e

Co60


Depth Dose Depth Dose Points to RememberPoints to Remember

• In case of depth dose measurements the field In case of depth dose measurements the field size is defined at surface of phantomsize is defined at surface of phantom

• Increases with SSD, Beam quality (energy), Increases with SSD, Beam quality (energy), Field size.Field size.

• Decreases with depthDecreases with depth

• Value is normalized to 100 % at depth of dValue is normalized to 100 % at depth of dmax max

for all field sizesfor all field sizes


Depth dose - point to ponder Depth dose - point to ponder

• Depth dose increases with SSD - Why?Depth dose increases with SSD - Why?

• Ans: Larger the SSD, smaller is the decrease Ans: Larger the SSD, smaller is the decrease in dose due to inverse square lawin dose due to inverse square law


Convert PDD from One SSD to Convert PDD from One SSD to another...another...

• PDDPDD(s,f(s,f22,Q,d),Q,d) = PDD = PDD(s,f(s,f11,q,d) ,q,d) FFss22

• WhereWhere

PDDPDD(s,f(s,f22,Q,d),Q,d) = PDD = PDD(s,f1,q,d)(s,f1,q,d)

PSF [S/F]

PSF [S]

f1 +d f2

f1 f2 +d

f1 +d f2+ dm

f1 +dm f2+ d

(f1 +d)2 (f2+ dm)2

(f1 +dm)2 (f2+ d)2 PSF [S]

PSF [S/F]

Fs =F =


Tissue Air RatioTissue Air Ratio

Field Size(S)=AB

TAR(s,d,Q)= Dd /Dair


Variation of TAR with depthVariation of TAR with depthTAR Vs Depth

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35

Depth in cm

TA

R

TAR


Variation of TAR with field sizeVariation of TAR with field sizeTAR Vs Field size

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60

Side of the square field (cm)

TA

R


TARs - Points to RememberTARs - Points to Remember• Field Size is defined at SADField Size is defined at SAD

• TAR is the ratio of phantom dose to dose “in air” at DTAR is the ratio of phantom dose to dose “in air” at Dmaxmax

• Independent of SSDIndependent of SSD

• Depends on beam quality and depthDepends on beam quality and depth

• Value is 1.00 at depth of d Value is 1.00 at depth of d maxmax for FS --> 0 for FS --> 0

• Position of the detector not altered between Position of the detector not altered between measurementsmeasurements

• Increase the depth by adding more water above the pointIncrease the depth by adding more water above the point


TAR - Point to ponderTAR - Point to ponder

• TAR does not depend on SSD - Why?TAR does not depend on SSD - Why?

• Ans: The distance between the Source to the Ans: The distance between the Source to the detector is not altered between measurements detector is not altered between measurements and hence no ‘inverse square law’ influenceand hence no ‘inverse square law’ influence


Scatter air ratioScatter air ratio

SAR (S,Q,d)= TAR(S,Q,d) - TAR(S,Q,0)

Field Size(S)=AB


SAR’s Points to RememberSAR’s Points to Remember

• Increases with beam quality and Field size, Increases with beam quality and Field size, decreases with depthdecreases with depth

• Obtained by extrapolation (TARObtained by extrapolation (TAR00) and subtraction) and subtraction

• SAR value is the phantom scatter dose per unit dose SAR value is the phantom scatter dose per unit dose “in air”“in air”

• Sector SAR used to predict dose for complex field Sector SAR used to predict dose for complex field shapes (e.g. IRREG)shapes (e.g. IRREG)

– inside open area, behind shields, outside of inside open area, behind shields, outside of fieldfield


Peak scatter factorPeak scatter factor

PSF (S,Q) = Dmax/ Dair

Field Size(S)=AB


PSF or BSF PSF or BSF Points to RememberPoints to Remember

• BSF(Back scatter factor) is an older term than BSF(Back scatter factor) is an older term than PSF, used for lower energiesPSF, used for lower energies

• PSF or BSF is actually a TAR value for a PSF or BSF is actually a TAR value for a depth of ddepth of dmaxmax

• Useful for conversion of dose in air to dose in Useful for conversion of dose in air to dose in tissue and vise versatissue and vise versa


Tissue Phantom RatioTissue Phantom Ratio

TPR(S,Q,d) = Dd / Dref

Field Size(S)=AB


Tissue Maximum RatioTissue Maximum Ratio

TMR(S,Q,d) = Dd / Dmax

Field Size(S)=AB


TMR Vs DepthTMR Vs DepthTMR Vs Depth

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35

Depth in cm

TM

R


Variation of TMR with Field SizeVariation of TMR with Field SizeTMR Vs Field Size

0.72

0.74

0.76

0.78

0.8

0.82

0.84

0.86

0 10 20 30 40 50

Side of squre field (cm)

TM

R


TPRs and TMRsTPRs and TMRsPoints to RememberPoints to Remember

• Field size defined at SAD Field size defined at SAD

• Increases with beam quality and field sizeIncreases with beam quality and field size

• Cousins of TAR, but with calibration “in Cousins of TAR, but with calibration “in phantom”phantom”

• TPR (or TMR) value is 1.00 at dTPR (or TMR) value is 1.00 at drefref for all field for all field sizessizes

• TMR is a special case of TPR where TMR is a special case of TPR where reference depth is dreference depth is dmax max depth i.e d depth i.e d ref ref = d= d max max


Relative dose factor (Output factor)Relative dose factor (Output factor)

RDF(S) = D(dref,S) / D(dref, S ref)

Scatter from reference field Additional Scatter from larger field


RDF Vs Field SizeRDF Vs Field Size

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

0 10 20 30 40

Side of Squre Field

RD

F


Relative Dose factor - points to Relative Dose factor - points to rememberremember

• The beam output depends on field sizeThe beam output depends on field size• The output increases with increase in field The output increases with increase in field

size due to size due to – increased collimator scatter; characterised increased collimator scatter; characterised

by collimator scatter factor Sby collimator scatter factor Scc

– Increase in scatter within the phantom; Increase in scatter within the phantom; characterised by phantom scatter factor Scharacterised by phantom scatter factor Spp


Relative Dose Factor Relative Dose Factor Points to RememberPoints to Remember

• The field size is varied, relative to a reference The field size is varied, relative to a reference field size (usually a 10 x 10 cmfield size (usually a 10 x 10 cm22))

• RDF measured “in phantom” onlyRDF measured “in phantom” only

• Increases with filed size due to increased Increases with filed size due to increased scatterscatter

• Used to convert dose from reference field size Used to convert dose from reference field size to a specific field sizeto a specific field size


Square Field to Rectangular fields..Square Field to Rectangular fields..

10cm

10cm D

Measured data for this But may treat with this

20cm

5cm

Square field Rectangular field


Equivalent square fieldEquivalent square field


Equivalent square TableEquivalent square Table


Wedge FactorWedge Factor

Wedge factor Wf= = Dw/Dref

Dose at reference point without wedge

Dose at reference point with wedge


Shielding tray factorShielding tray factor

Shielding Tray factor Ws= = Ds/Dref

Dose at reference point with tray

Dose at reference point without tray


Summary – you learnt about …Summary – you learnt about …

• The parameters such as SSD, SAD, Field The parameters such as SSD, SAD, Field Size, Penumbra, Equivalent Square and their Size, Penumbra, Equivalent Square and their importanceimportance

• Method of obtaining equivalent square for Method of obtaining equivalent square for irregular fieldsirregular fields

• Factors that are required to determine the Factors that are required to determine the absorbed dose in patientabsorbed dose in patient


Try these questions Try these questions

• Define Isocentre and explain the advantages Define Isocentre and explain the advantages of isocentric treatmentof isocentric treatment

• Explain the factors that affect the radiation Explain the factors that affect the radiation penumbrapenumbra

• What is scatter air ratio? And explain how this What is scatter air ratio? And explain how this factor is useful factor is useful

• Calculate the correction factor to be applied Calculate the correction factor to be applied to PDD for an increase in SSD from 80cm to to PDD for an increase in SSD from 80cm to 110 cm110 cm


ReferencesReferences

• The Physics of RadiologyThe Physics of Radiology– H.E Johns & J R CunninghamH.E Johns & J R Cunningham

• The Physics of Radio Therapy The Physics of Radio Therapy – Faiz M KhanFaiz M Khan

• The Modern Technology of Radiation The Modern Technology of Radiation Oncology, edited by J. Van DykOncology, edited by J. Van Dyk

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Part VIII:Medical Exposures in Radiotherapy Lecture 5 : Determination of Dose to a Patient-I IAEA Post Graduate Educational Course on Radiation Protection