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![Page 1: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/1.jpg)
Dose Distribution and Scatter Analysis
• Phantoms• Depth Dose Distribution• Percentage Depth Dose• Tissue-Air Ratio• Scatter-Air Ratio
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Phantoms
• Water phantom: closely approximates the radiation absorption and scattering properties of muscle and other soft tissues; universally available with reproducible
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• Basic dose distribution data are usually measured in a water phantom, which closely approximates the radiation absorption and scattering properties of muscle and other soft tissue
• Another reason for the choice of water as a phantom material is that it is universally available with reproducible radiation properties.
PHANTOMS
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• Solid dry phantoms– tissue or water equivalent, it must have the
same• effective atomic number
• number of electrons per gram
• mass density
– For megavoltage photon beams in the clinical range, the necessary condition for water equivalence
• same electron density (number of electrons per cubic centimeter)
PHANTOMS
Compton effect is the main interaction
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Solid dry phantoms
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Solid dry (Slab) phantoms
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Alderson Rando Phantom
• anthropomorphic phantom– Frequently used for cli
nical dosimetry– Incorporates materials
to simulate various body tissues, muscle, bone, lung, and air cavities
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RANDO phantomRANDO phantom
CT slice through lung
Head withTLD holes
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Depth Dose Distribution
• The absorbed dose in the patient varies with depth• The variation depends on depth, field size,
distance from source, beam energy and beam collimation
• Percentage depth dose, tissue-air ratios, tissue-phantom ratios and tissue-maximum ratios---measurements made in water phantoms using small ionization chambers
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Percentage Depth Dose
• Absorbed dose at any depth: d
• Absorbed dose at a fixed reference depth: d0
1000
d
d
D
DP collimator
surface
phantom
D d0
D d
d
d0
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PERCENTAGE DEPTH DOSE
• For orthovoltage (up to about 400 kVp) and lower-energy x-rays, the reference depth is usually the surface (do = 0).
• For higher energies, the reference depth is taken at the position of the peak absorbed dose (do = dm).
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Percentage Depth Dose
• For higher energies, the reference depth is at the peak absorbed dose ( d 0= d m)
• D max : maximum dose, the dose maximum, the given dose
100max P
DD d
100max
D
DP d
collimator
surface
phantom
D max
D d
d
dm
![Page 13: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/13.jpg)
Percentage Depth Dose
• (a)Dependence on beam quality and depth• (b)Effect of field size and shape• (c)Dependence on SSD
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Percentage Depth Dose(a)Dependence on beam quality and depth
• Kerma—(1) kinetic energy released per mass in the medium; (2) the energy transferred from photons to directly ionizing el
ectron; (3) maximum at the surface and decreases with depth due to d
ecreased in the photon energy fluence; (4) the production of electrons also decreases with depth
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Percentage Depth Dose(a)Dependence on beam quality and depth
• Absorbed dose: • (1) depends on the electron fluence;• (2) high-speed electrons are ejected from the surface and s
ubsequent layers;• (3) theses electrons deposit their energy a significant distan
ce away from their site of origin
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Fig. 9.3 central axis depth dose distribution for different quality photon beams
100max
D
DP d
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Percentage Depth Dose(b)Effect of field size and shape
• Geometrical field size: the projection, on a plane perpendicular to the beam axis, of the distal end of the collimator as seen from the front center of the source
• Dosimetric ( Physical ) field size: the distance intercepted by a given isodose curve (usually 50% isodose ) on a plane perpendicular to the beam axis
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PDD - Effect of Field Size and Shape
• Field size– Geometrical
– Dosimetrical or physical
SAD
FS
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• As the field size is increased, the contribution of the scattered radiation to the absorbed dose increases
• This increase in scattered dose is greater at larger depths than at the depth of D max , the percent depth dose increases with increasing field size
Percentage Depth Dose(b)Effect of field size and shape
100max
D
DP d
Dd
Dmax
Scatter dose
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Percentage Depth Dose(b)Effect of field size and shape
• Depends on beam quality• The scattering probability or cross-section
decreases with energy increase and the higher-energy photons are scattered more predominantly in the forward direction, the field size dependence of PDD is less pronounced for the higher-energy than for the lower-energy beams
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• PDD data for radiotherapy beams are usually tabulated for square fields
• In clinical practice require rectangular and irregularly shaped fields
• A system of equating square fields to different field shapes is required: equivalent square
• Quick calculation of the equivalent
Percentage Depth Dose(b)Effect of field size and shape
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square fieldB
A
c = 2 x A x B
A + B
rectangular field
c
c
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Percentage Depth Dose(b)Effect of field size and shape
• Quick calculation of the equivalent field parameters: for rectangular fields
• For square fields, since a = b, • the side of an equivalent square of a rectangular
field is
)(2 ba
ba
P
A
a
b
4
a
P
A
P
A4
P
A4
P
A4
![Page 25: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/25.jpg)
Percentage Depth Dose(3)--(b)Effect of field size and shape
• Equivalent circle has the same area as the equivalent square
P
Ar
4
a
b
P
A4
P
A4
r
![Page 26: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/26.jpg)
Percentage Depth Dose(c) dependence on SSD
• Photon fluence emitted by a point source of radiation varies inversely as a square of the distance from the source
• The actual dose rate at a point decreases with increase in distance from the source, the percent depth dose, which is a relative dose, increases with SSD
• Mayneord F factor
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PDD - Dependence on Source-Surface Distance
• Dose rate in free space from a point source varies inversely as the square of the distance. (IVSL)– scattering material in the beam may cause deviation
from the inverse square law.
• PDD increases with SSD– IVSL
dmd d
dm
SSD
SSD’
![Page 28: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/28.jpg)
Fig. 9.5 Plot of relative dose rate as inverse square law function of distance from a point source. Reference distance = 80 cm
Percentage Depth Dose
(c) dependence on SSD
F1+dm
F2+dm
F1+d F2+d
100max
D
DP d
![Page 29: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/29.jpg)
sddm Ke
df
dffrdP m ..100),,( )(
2
1
11
sddm Ke
df
dffrdP m ..100),,( )(
2
2
22
2
2
1
2
1
2
1
2
),,(
),,(
df
df
df
df
frdP
frdP
m
m
d
dm
f1
r
d
dm
r
f2
![Page 30: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/30.jpg)
f2
d
dm
d
dm
f1
r
r
PDD increases with SSD the Mayneord F Factor ( without considering changes in scattering ) 2
2
1
2
1
2
df
df
df
dfF
m
m
![Page 31: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/31.jpg)
PDD - Dependence on Source-Surface Distance
• PDD increases with SSD
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Example The PDD for a 15×15 field size, 10-cm depth,
and 80-cm SSD is 58.4-Gy (C0-60 Beam).Find the PDD for the same field size and
depth for a 100-cm SSD
Assuming dm=0.5-cm for (C0-60 Gamma Rays).
F=1.043P= 58.4*1.043=60.9
![Page 33: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/33.jpg)
Percentage Depth Dose(c) dependence on SSD
• Under extreme conditions such as lower energy, large field (the proportion of scattered radiation is relatively greater), large depth, and large SSD, the Mayneord F factor is significant errors
• In general, the Mayneord F factor overestimates the increase in PDD with increase in SSD
![Page 34: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/34.jpg)
PDD - Dependence on Source-Surface Distance
• PDD increases with SSD – the Mayneord F Factor
• works reasonably well for small fields since the scattering is minimal under these conditions.
• However, the method can give rise to significant errors under extreme conditions such as lower energy, large field, large depth, and large SSD change.
![Page 35: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/35.jpg)
Tissue-Air ratio
• The ratio of the dose ( D d ) at a given point in the phantom to the dose in free space ( D f s )
• TAR depends on depth d and field size rd at the depth:
fs
dd D
DrdTAR ),(
d
Dd
rd
D f s
rd
phantom Equilibrium mass
(BSF)
![Page 36: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/36.jpg)
Tissue-Air ratio( a ) Effect of Distance
• Independent of the distance from the source• The TAR represents modification of the dose at a
point owing only to attenuation and scattering of the beam in the phantom compared with the dose at the same point in the miniphantom ( or equilibrium phantom ) placed in free air
![Page 37: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/37.jpg)
Tissue-Air ratio( b ) Variation with energy, depth, and field size
• For the megavoltage beams, the TAR builds up to a maximum at the d m and then decreases with depth
• As the field size is increased, the scattered component of the dose increases and the variation of TAR with depth becomes more complex
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Tissue-Air ratio( b ) Variation with energy, depth, and field size: BSF
• Backscatter factor (BSF) depends only on the beam quality and field size
• Above 8 MV, the scatter at the depth of Dmax becomes negligibly small and the BSF approaches its minimum value of unity
fs
dmm D
DrdTARBSF max,
![Page 39: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/39.jpg)
Fig. 9.8 Variation of backscatter factors with beam quality
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The meaning of Backscatter factor
• For example, BSF for a 10x10 cm field for 60Co is 1.036 means that D max will be 3.6% higher than the dose in free space
• This increase in dose is the result of radiation scatter reaching the point of D max from the overlying and underlying tissues
036.1max fsD
D
![Page 41: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/41.jpg)
Tissue-Air ratio
( c ) relationship between TAR and PDD
100)(
1),(),,(
2
df
df
rBSFrdTARfrdP m
d
![Page 42: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/42.jpg)
Tissue-Air ratio
( c ) relationship between TAR and PDD-- Conversion of PDD from one SSD to another : The TAR method
Burns’s equation:
F
rBSF
FrBSFf
F
rdPfrdP
)(
/,,),,( 12
![Page 43: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/43.jpg)
Tissue-Air ratio
( d ) calculation of dose in rotation therapy
d=16.6
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Scatter-Air Ratio(SAR)• Calculating scattered dose in the medium• The ratio of the scattered dose at a given point in
the phantom to the dose in free space at the same point
• TAR(d,0): the primary component of the beam
)0,(),(),( dTARrdTARrdSAR dd
d
Dd
rd
D f s
rd
phantom Equilibrium mass
![Page 45: Dose Distribution and Scatter Analysis Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio.](https://reader035.fdocuments.us/reader035/viewer/2022081506/56649dc65503460f94abb3d8/html5/thumbnails/45.jpg)
Scatter-Air Ratio--Dose calculation in irregular fields: Clarkson’s Method
Based on the principle that the scattered component of the depth dose can be calculated separately from the primary component
SARTARTAR )0(
TAR
SAR
Average tissue-air ratio
Average scatter-air ratio
TAR ( 0 ) = tissue-air ratio for 0 x 0 field