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Chapter 9 Dose Distribution and Scatter Analysis. Phantoms Depth Dose Distribution Percentage Depth Dose Tissue-Air Ratio Scatter-Air Ratio. Phantoms(1).
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Chapter 9 Dose Distribution and Scatter Analysis • Phantoms • Depth Dose Distribution • Percentage Depth Dose • Tissue-Air Ratio • Scatter-Air Ratio
Phantoms(1) • Water phantom: closely approximates the radiation absorption and scattering properties of muscle and other soft tissues; universally available with reproducible • Solid dry phantoms: same effective atomic number, number of electrons per gram and mass density
Phantoms(2) • Alderson Rando Phantom: incorporates materials to simulate various body tissues---muscle, bone, lung, and air cavities • Solid water: epoxy resin--based solid substitute for water
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
collimator surface d0 D d0 d D d phantom Percentage Depth Dose(1) • Absorbed dose at any depth: d • Absorbed dose at a fixed reference depth: d0
collimator surface dm D max d D d phantom Percentage Depth Dose(2) • For higher energies, the reference depth is at the peak absorbed dose ( d 0= d m) • D max : maximumdose, the dose maximum,the given dose
Percentage Depth Dose(3) • (a)Dependence on beam quality and depth • (b)Effect of field size and shape • (c)Dependence on SSD
Percentage Depth Dose(3)--(a)Dependence on beam quality and depth • Kerma--(1) kinetic energy released 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
Percentage Depth Dose(3)--(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
Fig. 9.3 central axis depth dose distribution for different quality photon beams
Percentage Depth Dose(3)--(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
Scatter dose Dmax Dd Percentage Depth Dose(3)--(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
Percentage Depth Dose(3)--(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
Percentage Depth Dose(3)--(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
a b Percentage Depth Dose(3)--(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 Percentage Depth Dose(3)--(b)Effect of field size and shape • Equivalent circle has the same area as the equivalent square r
Percentage Depth Dose(3) -- (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
f1 r dm d dm d f2 r Mayneord F factor(for small fields since the scattering is minimal)
Percentage Depth Dose(3) --(c) dependence on SSD F1+dm F2+dm F1+d F2+d F1+d F2+d 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(3) --(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
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
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
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
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
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
Tissue-Air ratio-- ( c ) relationship between TAR and PDD-- Conversion of PDD from one SSD to another : The TAR method Burns’s equation:
Tissue-Air ratio--( d ) calculation of dose in rotation therapy d=16.6
Equilibrium mass phantom d rd rd Dd D f s Scatter-Air Ratio • 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
Average tissue-air ratio Average scatter-air ratio TAR ( 0 ) = tissue-air ratio for 0 x 0 field 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