Validation of an EGSnrc Monte Carlo Model of a 10 MV Photon Beam for Varian Clinac iX Linac, Including Flattening and Flattening-Free Modes

Modelling of linear accelerators using the Monte Carlo method is critical for precise radiotherapy planning. In addition, detailed and accurate dose estimation to the organ at risk can be assessed and optimized. In this study, EGSnrc Monte Carlo code was utilized to model, tune, and validate a 10 MV photon head model of a Varian Clinac iX linear accelerator for different field sizes, including flattening and flattening-free modes. Gamma analysis was utilized to compare the model with measured data to determine the best parameters for the incident electron on the target. The main results revealed that, for both flattening and flattening-free modes, the incident electron’s optimal energy is 9.5 MeV, with a 0.1 cm circular full width half maximum (FWHM) and a 0.07° angular divergence. The model is suitable for field sizes extending from 1 × 1 to 30 × 30 cm2. The comparison of large field sizes, which includes both 20 × 20 and 30 × 30 cm2, reflects the accuracy of the geometrical model of the flattening filter. Altering the FWHM has a notable effect on the profile, particularly in the penumbral region, although adjusting the angular divergence has little effect. The dose rate for the flattening filter-free beam compared to the flattening filter beam increased by a factor of four. The validated model demonstrates excellent agreement with measured data. Thus, it can provide accurate dose calculations and can be used in future studies to test treatment accuracy and patient safety, especially for advanced radiotherapy techniques.