1. Dose Distribution and Scatter Analysis
Phantoms
Depth Dose Distribution
Percentage Depth Dose
Tissue-Air Ratio
Scatter-Air Ratio

2. Phantoms
Water phantom: closely approximates the radiation absorption and scattering properties of muscle and other soft tissues; universally available with reproducible

3. PHANTOMS
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.

4. Compton effect is the main interaction
PHANTOMS
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)
Compton effect is the main interaction

5. Solid dry phantoms

6. Solid dry (Slab) phantoms

7. Alderson Rando Phantom
anthropomorphic phantom
Frequently used for clinical dosimetry
Incorporates materials to simulate various body tissues, muscle, bone, lung, and air cavities

8. RANDO phantom
CT slice
through lung
Head with
TLD holes

9. 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

10. Percentage Depth Dose Absorbed dose at any depth: d
Absorbed dose at a fixed reference depth: d0
collimator
surface
phantom
D d0
D d d d0

11. 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).

12. 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
collimator
surface
phantom
D max
D d d dm

13. Percentage Depth Dose (a)Dependence on beam quality and depth
(b)Effect of field size and shape
(c)Dependence on SSD

14. 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 electron;
(3) maximum at the surface and decreases with depth due to decreased in the photon energy fluence;
(4) the production of electrons also decreases with depth

15. 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 subsequent layers;
(3) theses electrons deposit their energy a significant distance away from their site of origin

16. Fig. 9.3 central axis depth dose distribution for different quality photon beams

17. 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

18. PDD - Effect of Field Size and Shape
Geometrical
Dosimetrical or physical
SAD FS

19. Percentage Depth Dose (b)Effect of field size and shape
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 Dd
Dmax
Scatter dose

20. 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

21. Percentage Depth Dose (b)Effect of field size and shape
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

22. rectangular field c A
square field B c
A x B
c = 2 x
A + B

24. 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 a b

25. Percentage Depth Dose(3)--(b)Effect of field size and shape
Equivalent circle has the same area as the equivalent square a b r

26. 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

27. 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 dm d
SSD
SSD’

28. Percentage Depth Dose (c) dependence on SSD
F1+dm
F2+dm
F1+d
F2+d
Fig Plot of relative dose rate as inverse square law function of distance from a point source. Reference distance = 80 cm

29. d dm f1 r d dm r f2

30. the Mayneord F Factor ( without considering changes in scattering )dm f1 r f2 r
PDD increases with SSD
the Mayneord F Factor ( without considering changes in scattering )

31. PDD - Dependence on Source-Surface Distance
PDD increases with SSD

32. 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=1.043
P= 58.4*1.043=60.9

33. 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

34. 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.

35. 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:
(BSF)
Equilibrium mass
phantom d rd rd Dd
D f s

36. 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

37. 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

38. 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

39. Fig. 9.8 Variation of backscatter factors with beam quality

40. The meaning of Backscatter factor
For example, BSF for a 10x10 cm field for 60Co is 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

41. ( c ) relationship between TAR and PDD
Tissue-Air ratio
( c ) relationship between TAR and PDD

42. Tissue-Air ratio
( c ) relationship between TAR and PDD-- Conversion of PDD from one SSD to another : The TAR method
Burns’s equation:

43. ( d ) calculation of dose in rotation therapy
Tissue-Air ratio
( d ) calculation of dose in rotation therapy
d=16.6

44. 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 d Dd rd
D f s
phantom
Equilibrium mass

45. 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
Average tissue-air ratio
Average scatter-air ratio
TAR ( 0 ) = tissue-air ratio for 0 x 0 field