PHITS Monte Carlo modeling for volumetric modulated arc therapy with the TrueBeam linear accelerator

Objective.Out-of-field dose (OFD) contributes to secondary cancer risk in radiation therapy. This study aimed to develop a comprehensive Monte Carlo (MC) framework for accurate OFD estimation during volumetric modulated arc therapy (VMAT).Approach.A 6 MV Varian TrueBeam linear accelerator with a Millennium 120-leaf multileaf collimator (120 MLC) was modeled using the Particle and Heavy Ion Transport code System (PHITS). Model validation included percentage depth dose (PDD), beam profiles, MLC transmission, and dosimetric leaf gap (DLG). A clinically representative VMAT brain plan was simulated in an Alderson Radiation Therapy (ART) phantom. OFDs at the thyroid and breasts were compared with GD-352M radiophotoluminescent glass dosimeter (RPLD) measurements using MC-derived energy correction factors.Main results.The simulated PDDs and beam profiles agreed closely with commissioning data, with the point-to-point differences of 0.6%-1.2% in high dose regions. The MC-derived MLC transmission and DLG values were consistent with local measurements and published data. The RPLDs demonstrated strong dose linearity (R²> 0.99) in out-of-field regions. In the ART phantom, the MC simulation predicted higher doses than the treatment planning system (TPS) for the target and organs at risk (OARs), with the largest differences in the hippocampus (+106.8%), optic chiasma (+69.4%), and lacrimal glands (+81.0%), predominantly in low dose regions. The gamma passing rates (GPR) (3%/2 mm) were 100% for the PTV but decreased near MLC leaf ends. The differences between the MC-calculated and RPLD-measured OFDs were 22.0% at the thyroid and 38.1% at the breasts, partly due to partial loss of electronic equilibrium at measurement positions.Significance.Integration of detailed MC modeling with energy-corrected RPLD measurements can help improve in-field and out-of-field dosimetry during VMAT by enhancing dose accuracy in radiosensitive and low dose regions, supporting improved risk assessment and mitigation of secondary cancer risks in advanced radiation therapy.