Abstract Purpose: Monte Carlo (MC) simulation is widely regarded as the gold standard for radiation transport modelling in medical linear accelerators because of its high dosimetric accuracy. However, detailed modelling of treatment head components and the large number of particle histories required for low statistical uncertainty often result in long computation times. This study aimed to develop and validate a 6 MV Varian TrueBeam™ photon beam model in the TOPAS MC toolkit using a two-stage phase-space generation approach to improve computational efficiency while maintaining dosimetric accuracy. Methods: IAEA-compliant phase-space files provided by the manufacturer, each containing 9 × 10⁸ particle histories and positioned 26.7 cm below the source plane, were used as the primary beam source. A simplified model of the movable X and Y secondary collimator jaws was implemented to generate secondary phase-space files for field sizes ranging from 3 × 3 cm² to 40 × 40 cm². Secondary phase-space files scored at 94 cm from the source plane were used for dose calculations in a virtual water phantom. Validation was performed using percent depth dose (PDD) curves, lateral dose profiles, and output factors compared with Varian TrueBeam™ gold beam data. Results: The two-stage phase-space approach retained 0.42%–88.83% of primary particles depending on field size, corresponding to estimated runtime reductions of approximately 15%–95%. Simulated PDDs agreed within 3% of reference data for depths between 0.2 cm and 30 cm, with accurate reproduction of dmax. More than 98% of profile points satisfied the 3%/2 mm gamma criterion. Output factors showed an average difference of 0.12% from reference data. Conclusion: A validated 6 MV Varian TrueBeam™ MC beam model was successfully developed in TOPAS using a two-stage phase-space approach. The method substantially reduced computational requirements while maintaining accurate dosimetric agreement, demonstrating its suitability for further MC-based dose calculation studies.
Murcia et al. (Thu,) studied this question.