Monte Carlo investigation of spectral modifications induced by a W/Al selective filter in 6 MV flattening filter-free photon beams

This study investigated the impact of a selective tungsten/aluminum energy filter [SF (W/Al) ] on the spectral characteristics of 6 MV Flattening Filter-Free (FFF) photon beams. The analysis included the energy spectrum and fluence, pinpointing the photon and electron energy ranges that contribute most to energy deposition on the water phantom surface. Monte Carlo (MC) simulations were performed using GATE for a 10 × 10 cm 2 field size at a source-to-surface distance (SSD) of 100 cm. The simulation model was validated against Varian phase-space file (PSFs) data using the gamma index criterion of 2%/2 mm. Excellent agreement was obtained, with gamma pass rates reaching 99% for percentage depth-dose (PDD) curves and 98% for lateral dose profiles. The spectral analysis demonstrated that FFF beams substantially increased the contribution of low-energy particles to surface dose deposition compared with flattened (FF) beams. Relative changes of 139.68 ± 2.18% for photons and 60.35 ± 2.71% for electrons, relative to the FF beam, were observed within the energy ranges of 0.49–0.525 MeV and 0.28–0.315 MeV, respectively. After introducing the SF (W/Al) , the corresponding relative changes decreased to 114.76 ± 1.82% for photons and 38.24 ± 1.76% for electrons within the same energy ranges, corresponding to relative reductions of 17.84% and 36.64%, respectively. These findings demonstrate that using a dedicated filter to control low-energy fluence selectively is an effective strategy to reduce surface dose and improve the clinical safety of FFF photon beams while preserving their therapeutic properties.