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SU‐FF‐T‐296: Parameterizing the Phantom Scatter Components for Polyenergetic Photon Beams
Author(s) -
Finlay J,
Zhu T
Publication year - 2005
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 - 10.1118/1.1998025
Subject(s) - imaging phantom , monte carlo method , photon , physics , optics , field size , offset (computer science) , computational physics , field (mathematics) , range (aeronautics) , materials science , mathematics , statistics , pure mathematics , computer science , composite material , programming language
Purpose: To develop an improved analytical model of the phantom scatter‐to‐primary ratio ( SPR ) of megavoltage photon beams with improved accuracy for a wider range of field sizes and depths than the existing analytical model [1] by accounting for backscattered photons. Method and Materials: EGS4 Monte‐Carlo simulations are performed to calculate the scatter and primary doses from parallel photon beams for60 Co spectrum and the Mohan spectra simulating photon beams with nominal energies of 4, 6, 10, 15, and 24 MV. The depths varied from d max to 30 cm. The field size varied between 3 and 40 cm. SPR has previously been modeled by the equationSPR = ( a 0s d ) / ( w 0s + d ) [1]. We model SPR bySPR = ( a 0s ( d + d 0) ) / ( w 0s + ( d + d 0) ) , where s is the field size at depth, d is the depth in the phantom, and a 0 , w 0 and d 0 are free parameters. The addition of the depth offset d 0 accounts for the dependence of SPR on field size at shallow depths. We fit the SPR data derived from Monte Carlo simulation to determine a 0 , w 0 and d 0 for each of the simulated nominal energies. Results: The phantom scatter‐to‐primary ratio increases with increasing depth and field size, up to 161% and 34% for60 Co and 24 MV, respectively, for s = 40 cm and d = 30 cm. The maximum (standard) error for the new and standard analytical models are 3.5% (1.1%) and 2.2 % (1.1%), respectively, for 6 MV. At shallow depths ( d max ), the maximum (standard) error of the fitting are 6.4% (3.9%) and 4.2 % (2.4%), respectively, for60 Co , and decreases with increasing nominal energy. Conclusion: The addition of the offset parameter d 0 improves the fitting of the data, significantly reducing the error for clinical energies.