Dose Distribution and Scatter Analysis. Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio. Phantoms.
Compton effect is the main interaction
(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
Fig. 9.3 central axis depth dose distribution for different quality photon beams
DdPercentage Depth Dose(b)Effect of field size and shape
A x B
c = 2 x
A + B
bPercentage Depth Dose(b)Effect of field size and shape
bPercentage Depth Dose(3)--(b)Effect of field size and shape
scattering material in the beam may cause deviation from the inverse square law.
PDD increases with SSD
dPDD - Dependence on Source-Surface Distance
(c) dependence on SSD
Fig. 9.5 Plot of relative dose rate as inverse square law function of distance from a point source. Reference distance = 80 cm
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).
D f s
( c ) relationship between TAR and PDD
( c ) relationship between TAR and PDD-- Conversion of PDD from one SSD to another : The TAR method
( d ) calculation of dose in rotation therapy
Average scatter-air ratio
TAR ( 0 ) = tissue-air ratio for 0 x 0 fieldScatter-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