Monitor Unit Calculations Part 2. Calculation of machine setting. Collimator setting

Transcription

1 Monitor Unit Calculations Part 2 George Starkschall, Ph.D. Department of Radiation Physics U.T. M.D. Anderson Cancer Center Calculation of machine setting reference dose machine setting =, reference dose output reference dose output = calibration dose output correction for collimator setting correction for distance from source. correction for beam modifiers correction for attenuation and scatter Collimator setting Multiply calibration dose output by output factor (field size factor) Dose at d max with collimator set to specified value divided by dose at d max for calibration value If calibration done at another depth d, use ratio of doses at d 1

2 Collimator setting Factors affecting output If f calibration dose output measured in air only factor causing change in output factor is collimator scatter (S c ) If calibration dose output measured in phantom additional factor due to phantom scatter (S p ) Phantom scatter dependent on field size and not collimator setting Collimator setting Note difference between collimator setting and field size Collimator setting field size of unblocked field at source to isocenter distance Field size is actual (or effective) size of treatment field at specified distance from source Field size may be larger than or smaller than collimator setting Collimator setting If f field size significantly difference from collimator setting, phantom scatter can be accounted for by multiplying by ratio of peakscatter factors PSF PSF field coll. 2

3 Collimator setting For high-energy (>10 MV or so) linac beams, peakscatter factor essentially independent of field size, so value of 1.0 is often used without introducing serious error Collimator setting Khan approach Khan prefers to separate collimator scatter explicitly from phantom scatter and write the collimator setting correction as S c S p S c : collimator scatter factor S p : phantom scatter factor Summary MDACC approach Khan approach PSF ( ) ( fs) OF cs Sc ( cs) S p ( fs) PSF( cs) 3

4 Where to find data Data typically available in machine data book (or files) Data normally presented for square fields For rectangular fields, need to determine equivalent square The square field that has the same dosimetric properties as the rectangular field Purely an empirical quantity Equivalent square Table look up Good rule of thumb s = 4 a/p = 2 lw/(l+w) The two are equivalent for all but extremely elongated fields Calculating an output factor Determine equivalent square Look up value in table If not explicitly expressed, estimate interpolation, e.g., output factor for 9 9 should be half the difference between 8 8 and Express values to 3 decimal places, e.g.,

5 Distance from source Corrects for differences between distance of calibration point from source and distance of reference point from source Calibration point may be at SAD+d max, while reference point may be at SAD Distance from source Example: Calibrated dose output at SAD + d max from source and reference point at isocenter Distance correction factor is SAD + d SAD max 2. Distance from source Example: Calibrated dose output at isocenter and reference point at isocenter Distance correction factor is

6 Beam modifiers Accounts for effects of trays, wedges, compensators, anything else that may be placed in beam between radiation source and patient Beam modifiers Beam modifier factors frequently taken to be constant In some cases, field size and/or depth dependent Beam modifiers Care when incorporating wedge factors, especially if extracting information from treatment planning system For some systems, reference dose is dose to reference point for unwedged field reference dose output must be for unwedged field, and no wedge factor included 6

7 Beam modifier data Beam modifier data typically found in machine data book (or files) Data generally acquired as part of beam commissioning process Effect of patient Must account for attenuation of beam in patient Most common factor is tissue-maximum ratio (TMR) We will present several other factors that may be easier to understand than TMR first Effect of patient If calibration c dose output specified in air -- all calculations up to this point give dose rate to reference point in air To get dose rate in tissue, multiply by tissue- air ratio (TAR) 7

8 Effect of patient If calibration c dose output specified in tissue at depth all calculations up to this point give dose rate to reference point at depth To get dose rate in tissue, multiply by either tissue-phantom ratio (TPR), or, if specified depth is d max, tissue-maximum ratio (TMR) Let Tissue-air ratio (TAR) D n dose delivered to small mass of tissue at some depth in water phantom D air dose delivered to same mass of tissue in air at same distance from the source, then D TAR = D n air Tissue-air ratio (TAR) D TAR = D n air 8

9 Tissue-air ratio (TAR) Example: Calculate treatment time required to deliver 150 cgy to a point at 6 cm depth using 8 cm 8 cm 60 Co beam with in-air dose rate of 135 cgy/min at 80 cm SAD. Tissue-air ratio (TAR) Dose rate in air 135 cgy/min TAR for 8 cm 8 cm 60 Co beam at depth of 6 cm found from TAR table to be Dose rate in tissue: 135 cgy/min = cgy/min Tissue-air ratio (TAR) To o deliver dose of 150 cgy requires treatment time of 150 cgy = 1.31min cgy/min 9

10 Tissue-air ratio (TAR) TAR relates dose to a point to dose in air at the same point Used when patient set up isocentrically Used when calibration dose output expressed in air Typically only for low-energy photon beams TAR properties Even though TAR is no longer frequently used, we will examine some properties of the TAR, as analogous behavior is exhibited by the more commonly used TMR TAR properties Effect of depth: TAR decreases with increasing depth beam attenuation TAR evaluated at depth of central axis maximum (d max ) backscatter factor (BSF) For depths <d max, TAR increases with depth 10

11 Effect of beam quality: At low energies BSF increases with increasing energy increased penetration of scattered photons TAR properties Effect of beam quality: At high energies BSF decreases with increasing energy greater amount of forward scatter TAR properties Effect of beam quality: BSF reaches maximum value around HVL mm Cu where it may be as high as 1.5 TAR properties 11

12 TAR properties Effect of field size: TAR increases with increasing field size increased amount of scatter reaching calculation point TAR properties Effect of source distance: TAR independent of source distance both D n and D air measured at same distance from source TAR data Find TAR tables: For 60 Co: BJR, supplement 25 For specific linacs: in physics data notebook Important to use correct table for appropriate machine 12

13 Tissue-air ratio (TAR) In n order to calculate dose rate at distance other than the SAD use inverse square relationship to go from one source distance to another Important to note that inverse square relationship only holds in air Tissue-air ratio (TAR) To relate dose rate in tissue at one distance to that at another: Calculate dose rate in air by dividing by appropriate TAR Use inverse square law to go from one distance to the other Calculate dose rate in tissue at new distance by multiplying in-air dose rate by appropriate TAR 13