Flattening filter-free (FFF) photon beams offer significant dosimetric advantages over conventional flattened (FF) beams for modern radiotherapy; however, their inherently elevated surface dose remains a clinical concern that has not been adequately addressed through validated mitigation strategies. This study presents a systematic Monte Carlo (MC) investigation of 6-MV FF and FFF photon beams from a Varian TrueBeam linear accelerator, employing dual independent simulation platforms (GAMOS (v6.2) and GATE (v9.1)) benchmarked against ionization chamber measurements in water. Five clinically relevant field sizes (3 × 3 to 30 × 30 cm 2 ) were evaluated for percentage depth dose (PDD), lateral beam profiles, photon energy spectra, out-of-field dose (OFD), head scatter correction factor (HSCF), and total scatter correction factor (TSCF). FFF beams delivered 2.3-fold higher dose rates (3.81 versus 1.60 Gy/min), reduced OFD by approximately 25% at 3.5 cm beyond the field edge, and exhibited 8.5% narrower penumbra widths compared with FF beams. Field-size-dependent scatter factor analysis revealed that removal of the flattening filter eliminates approximately 60% of head scatter, with HSCF values converging toward unity for FFF beams (1.008–1.024) versus substantially elevated values for FF beams (1.015–1.062), quantitatively explaining the observed reductions in peripheral dose and improved dose conformity. Conversely, FFF beams manifested 15% higher surface doses and 13% shallower depth of maximum dose ( d max ), attributable to the softer photon energy spectrum in the absence of beam hardening. To mitigate this limitation, a novel helium gas substitution strategy (replacing the air column between secondary collimator jaws and the phantom surface) was investigated, demonstrating efficacy in reducing the FFF-to-FF surface dose ratio from 1.16 to 1.07 and recovering 50% of the d max difference, while preserving penumbra sharpness and out-of-field dose characteristics. Cross-validation between GAMOS and GATE yielded mean inter-platform differences below 1.2%, with gamma index pass rates exceeding 98% (2%/2 mm criteria) against experimental data. These findings provide a validated dual-platform computational framework for FFF beam commissioning and introduce helium substitution as a proof-of-principle strategy for surface dose normalization, supporting evidence-based beam modality selection in stereotactic radiosurgery, stereotactic body radiotherapy, and hypofractionated treatment paradigms.
Alshehri et al. (Thu,) studied this question.