Geometry-optimized electron beam scattering foils enabling dose uniformity and dose rate enhancement for FLASH radiotherapy studies.

The development of FLASH radiotherapy (FLASH-RT) is limited by the availability of ultra-high dose rate (UHDR) irradiation platform. This study aims to optimize electron scattering foils (SFs) for a compact 6 MeV linear accelerator (linac) operating at a short source-to-surface distance (SSD), enabling lateral uniform dose delivery with UHDR for FLASH-RT studies.Based on a custom-built linac, optimized aluminum SFs were designed using the Nelder-Mead simplex algorithm coupled with Geant4 Monte-Carlo simulations to achieve lateral dose uniformity in 10 mm of water/PMMA below the surface at a reduced SSD. Two geometric optimization strategies, namely stacked-layer structure and ring structure, were utilized for different field sizes. Dose distributions were quantified using radiochromic EBT-3 films, while the operation parameters-electron-gun anode voltage, pulse width, and pulse frequency-were modulated to explore dose rate dependencies.Utilizing an optimized stacked-layer SF and a 4 cm diameter resin collimator, uniform integrated lateral dose profiles in the first 10 mm of PMMA (flatness <5%) were measured for a 3.5 cm diameter field at 11 cm SSD. By modulating the operation parameters including anode voltage, pulse width and frequency, dose-per-pulse was continuously adjustable from 0.09 to 8.37 Gy, yielding instantaneous dose rates of 4.25 × 10to2.09×106Gy sand the mean dose rates spanning from10-2to103 Gys-1. Simulations further demonstrated that a flatness <5% was achievable for fields up to 10 cm diameter at the same SSD when using the ring structure SFs combined with large-diameter collimators.The system's capability to operate across conventional and UHDR regimes within a single framework supports comparisons of conventional radiotherapy and FLASH-RT effects with minimized systematic errors. This work offers insights into SF design methodology and facilitates incremental refinements of UHDR irradiation parameters, findings applicable to develop compact FLASH platforms based on other electron beam systems.