Elaboration and experimental validation of a Monte Carlo source model for linac 6 MV photon beams with and without Flattening Filter

The concept of virtual source model has been developed to provide an efficient and precise particle generator mainly involved in standard photon beam treatment planing and Monte Carlo dosimetry simulations. In this work, we propose a source model for 6 MV linac photon beam for both irradiation modes, with and without flattening filter. Two sources are considered to represent primary and a scattered radiations. Energy spectra taking into account the correlation between energy and diffusion angle have been used. Model parameters were tuned through a dose distribution iterative optimisation approach. The convergence condition relies on γindex evaluation. The simulations were performed in two steps, phase space files were generated using Geant4 toolkit, then energy deposit in water were computed using DPM code. Monte Carlo dose distributions of the model were compared to experimental data of two linac systems, an Elekta Synergy® with flattening filter and a Varian TrueBeam® with and without flattening filter. Good results were obtained in terms of γindex 2%/2 mm criterion, dose difference and distance to agreement (DTA) analysis for field size range from 3 × 3 cm2 to 30 × 30 cm2.