SU-FF-T-296: Parameterizing the Phantom Scatter Components for Polyenergetic Photon Beams

Abstract

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 backscatteredphotons.Method and Materials: EGS4 Monte‐Carlo simulations are performed to calculate the scatter and primary doses from parallel photon beams for 60 Co spectrum and the Mohan spectra simulating photon beams with nominal energies of 4, 6, 10, 15, and 24 MV. The depths varied from dmax to 30 cm. The field size varied between 3 and 40 cm. SPR has previously been modeled by the equation SPR=(a 0 s d)/(w 0 s + d) [1]. We model SPR by SPR=(a 0 s (d+d 0 ))/(w 0 s + (d+d 0 )) , where s is the field size at depth, d is the depth in the phantom, and a0, w0 and d0 are free parameters. The addition of the depth offset d0 accounts for the dependence of SPR on field size at shallow depths. We fit the SPR data derived from Monte Carlo simulation to determine a0 , w0 and d0 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% for 60 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 (dmax ), the maximum (standard) error of the fitting are 6.4% (3.9%) and 4.2 % (2.4%), respectively, for 60 Co , and decreases with increasing nominal energy. Conclusion: The addition of the offset parameter d0 improves the fitting of the data, significantly reducing the error for clinical energies.