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Modeling scatter‐to‐primary dose ratio for megavoltage photon beams
Author(s) -
Wang Ken KangHsin,
Zhu Timothy C.
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3480480
Subject(s) - photon , monte carlo method , physics , dosimetry , attenuation , photon energy , linear particle accelerator , optics , beam (structure) , energy (signal processing) , computational physics , nuclear medicine , statistics , mathematics , medicine , quantum mechanics
Purpose: A three‐parameter semiempirical model for scatter‐to‐primary dose ratio (SPR) is proposed to fit PDD (or TPR) and S pbeam data. The SPR formula proposed in this study is more accurate than the previously published formula utilizing two parameters, especially for lower energy megavoltage photon beams, because the effect of backscattered photons is now taken into account. Methods: Monte Carlo (MC) calculated SPR for photon energy spectrum between C60 o and 24 MV are used to evaluate the accuracy of the models. Based on fitting the MC data, the dependence of the SPR parameters( a 0 , w 0 , d 0 )with the attenuation coefficients of the photon beams is obtained and they were incorporated into the authors’ optimization routine. The ability of the optimization routine to fit measured clinic data is examined for photon energies ranging fromC60 o to 25 MV for all major cobalt and linear accelerator manufacturers. Results: The authors’ model successfully fits the measured photon beam data for field size ( E / 3 – 40 cm ) , where E is photon energy in MV and for clinically usable depths,d maxto 20 cm forC60 o ,d maxto 30 cm for 4 MV, andd maxto 40 cm for 6 MV and higher photon energies. The maximum error among these fits is better than 2% for photon energies aboveC60 o . Conclusions: The new SPR formula, along with the optimization routine, can serve as an efficient tool for performing quality control of x‐ray beam data that conforms to AAPM Radiation Therapy Committee TG40 and Therapy Physics Committee TG142 reports on beam data requirement.