Monte Carlo simulation of 6 MV flattening-filter-free beams in TrueBeam accelerator

Abstract

Objective To find the best model parameters through Monte Carlo simulation of 6 MV flattening-filter-free (FFF) beams in TrueBeam accelerator, and establish the foundation for the further study of the clinical dosimetry on 6 MV FFF X-rays. Methods Using the BEAMnrc and DOSXYZnrc codes, the percentage depth dose (PDD) and the off-axis ratio (OAR) curves of field ranges from 4 cm×4 cm to 40 cm×40 cm were simulated for 6 MV FFF X-ray by adjusting the incident beam energy, radial intensity distribution and angular spread, respectively. The simulation results and measured data were compared, where the optimal Monte Carlo model input parameters were acquired. Results The simulation was most comparable to the measurement when the incident electron energy, full width at half maximum (FWHM) and the spread angle were set as 6.1 MeV, 0.75 mm and 0.9°, respectively. The deviation of 1 mm (position)/1% (local dose) could be met by the PDD of all tested field sizes and by the OAR when the fields sizes were no larger than 30 cm ×30 cm. The OAR of 40 cm×40 cm field sizes fulfilled criteria of 1 mm (position)/1.5% (local dose). Conclusions Monte Carlo simulation agrees well with the measurement and the proposed model parameters, which can be used for further clinical dosimetry studies of 6 MV FFF X-rays. Key words: TrueBeam accelerator; Flattening-filter-free; Monte Carlo; Percentage depth dose; Off-axis ratio