IntroductionSeldom possible to measure dose distributiondirectly on patients.Data are derived from Phantoms.Calculations done to predict the dose distributionin actual patient.We discuss various quantities and conceptsrelated to dose distribution calculation. 2
PhantomsWater phantoms commonly used.Measurements using ionizing chambers.TLDs, Diodes, Films are occasionally used. 3
Depth Dose DistributionThe absorbed dose in the patient varies withdepth.The variation depends on depth, field size,distance from source, beam energy.Percentage depth dose(PDD),Tissue-air ratios(TAR),are the measurements made in water phantomsusing small ionization chambers. 4
Percentage Depth Dose Absorbed dose at any depth: d Absorbed dose at a fixed reference depth: d0 Dd collimato P= ×100 r Dd 0 surface d0 D d0 d Dd phantom 5For orthovoltage, d0 is the surface, for higher energy it is the dmax.
Percentage Depth DoseDependence on beam quality and depthEffect of field size and shapeDependence on SSD 6
Percentage Depth Dose Dependence on beam quality and depthAbsorbed dose:Depends on the electron fluenceHigh-speed electrons are ejected from the surface andsubsequent layers.These electrons deposit their energy a significant distanceaway from their site of origin 7
Fig. central axis between the surface and dmax is called different quality photon8bea Region depth dose distribution for the Dose Buildup Region. Skin Sparing Effect is due to the same reason.
Percentage Depth Dose Effect of field size and shapeAs the field size is increased, thecontribution of the scattered radiation to theabsorbed dose increases.This increase in scattered dose is greaterat larger depths than at the depth of D max ,the percent depth dose increases withincreasing field size. 9
Percentage Depth Dose Effect of field size and shapeDepends on beam qualityThe scattering probability decreases withenergy increase and the higher-energyphotons are scattered more in the forwarddirection.The field size dependence of PDD is lesspronounced for the higher-energy than forthe lower-energy beams. 10
Percentage Depth Dose Effect of field size and shapePDD data for radiotherapy beams areusually tabulated for square fields.In clinical practice require rectangular andirregularly shaped fields.A system of equating square fields todifferent field shapes is required:equivalent square. 12
Equivalent Squares of Rectangles: Hospital Physicists association 15
Square FieldThumb rule by Stering et.al. rectangular fieldA rectangular field is equivalent to a square field if they have the same area/perimeter. c A B c AxB c=2x A+B 16
Percentage Depth Dose Effect of field size and shapeEquivalent circle has the same area as theequivalent square 4 A r= × √π P A a 4× P r b 4× A P 17
Percentage Depth Dose Dependence on SSDPhoton fluence emitted by a point source ofradiation varies inversely as a square ofthe distance from the source.The actual dose rate at a point decreaseswith increase in distance from the source,the percent depth dose, which is a relativedose, increases with SSD. 18 Mayneord Factor
Tissue-Air ratioFor rotational therapy.The ratio of the dose ( D d ) at a given point in thephantom to the dose in free space ( D f s )TAR depends on depth d and field size rd at thedepth: Dd TAR(d,r d )= D fs No phantom phantom d rd rd Dd D fs 20
Tissue-Air ratio Effect of DistanceIndependent of the distance from the sourceThe TAR represents modification of the dose at apoint owing only to attenuation and scattering ofthe beam in the phantom compared with the doseat the same point in the free air. Dd TAR(d,r d )= D fs 21
Tissue-Air ratioVariation with energy, depth, and field sizeFor the megavoltage beams, the TAR builds upto a maximum at the d m and then decreases withdepth.As the field size is increased, the scatteredcomponent of the dose increases and thevariation of TAR with depth becomes morecomplex. 22
Tissue-Air ratio Variation with energy, depth, and field size: BSFBackscatter factor (BSF) depends only on thebeam quality and field size D max BSF=TAR ( d m ,r dm ) = D fsAbove 8 MV, the scatter at the depth of Dmaxbecomes negligibly small and the BSFapproaches its minimum value of unity 23
Variation of backscatter factors with beam quality 24
The meaning of Backscatter factorFor example, BSF for a 10x10 cm field for 60Co is1.036 means that D max will be 3.6% higher thanthe dose in free space D max =1 . 036 D fsThis increase in dose is the result of radiationscatter reaching the point of D max from theoverlying and underlying tissues 25
Calculation of Dose in Rotational TherapyIsocentric irradiation whre source moves around the axis.The average TAR is determined at the isocenter.The radii are drawn from the point selected at regular angles.TAR for each depth is given to us.Average TAR gives us the dose at the isocenter. 26
Tissue-Air ratiocalculation of dose in rotation therapy d=16.6 27
Scatter-Air Ratio(SAR)Calculating scattered dose in the mediumThe ratio of the scattered dose at a given point inthe phantom to the dose in free space at thesame pointTAR(d,0): the primary component of the beam No Phantom phantom d rd rd Dd D fs SAR ( d,r d )=TAR ( d,r d )−TAR ( d, 0 ) 28
Scatter-Air Ratio: Dose calculation in irregular fields: Clarkson’s Methodscattered component of the depth dose can be calculated separatel TAR=TAR ( 0 )+ SAR TAR= Average tissue-air ratio SAR= Average scatter-air ratio TAR ( 0 ) = tissue-air ratio for 0 x 0 field 29 SAR for various radii is provided.