A gamma analysis approach to determine optimal parameters for Monte Carlo simulation of 6MV Varian ClinaciX photon beam

Abstract

Monte Carlo (MC) method is the gold standard dose calculation algorithm. Determination of the electron beam parameters for MC simulation is often estimated using trial and error methods. However, this can be tedious and time-consuming. This paper aims to validate MC simulated data using 1D gamma analysis for 6MV photon beam to obtain the optimal parameters. BEAMnrc codes were used to generate phase space files for conventional field sizes 10 × 10 cm 2 , 6 × 6 cm 2 , 4 × 4 cm 2 and small field sizes 2 × 2 cm 2 , 1 × 1 cm 2 , 0.5 × 0.5 cm 2 . For conventional field sizes, simulations were benchmarked against Golden Beam Data (GBD). Simulations for small fields were benchmarked against measurements obtained using EDGE Detector and PTW Diode SRS detector in a Sun Nuclear 3D scanner. Dose profiles in water were calculated using DOSXYZnrc codes. Initial reference parameters were approximated using average percentage dose differences of different mean electron energy and electron beam radial distribution (Full Width at Half Maximum, FWHM). Subsequently, the optimal parameters were validated by 1D gamma analysis using varying gamma criteria from γ 0.3%/0.3mm to γ 2.0%/2.0mm for depth dose and lateral dose profiles. Comparisons were performed along the central region at depth dose 1.6 cm . Optimal parameters were found to be unique for small field sizes. As field size decreases, smaller FWHM were required to match measured data. By using 95% passing rate, a generic set of optimal electron beam parameters in a MC model for all field sizes could be accurately determined. Our findings provide MC users a set of optimal parameters with sufficient accuracy for MC simulation work.

Sensitivity analysis of a 6 MeV Photon Beam Monte Carlo model

“…The PDD showed more sensitivity to E than to FWHM. As other studies found, the effect of change in FWHM on PDD is insignificant,[ 26 27 ] since by increasing FWHM of incident electron beam only the width of bremsstrahlung photons beam increases and its energy remains constant. Therefore, the PDD, which is mainly affected by photon energy, is not affected significantly by FWHM [ Figure 5b ].…”