Feasibility of proton ultra-high-dose-rate spatially fractionated radiotherapy system for preclinical experiments on a compact proton synchrocyclotron

Abstract Objective. This study aims to investigate the feasibility of ultra-high-dose-rate (UHDR) proton FLASH spatially-fractionated-radiation-therapy (FLASH-SFRT) irradiations on a compact proton synchrocyclotron for preclinical experiments. Approach. A specialized FLASH-SFRT brass collimator measuring 11 × 11 × 8 cm 3 was designed with 71 milled circular apertures measuring 0.5 cm diameter, 1.7 cm center-to-center distance. These apertures were designed to be divergently matched to a clinical proton synchrocyclotron (IBA Proteus®ONE) at a collimator-to-isocenter distance of 9 cm. Dose calculations were performed with three different spot trajectories: 10 × 10 cm 2 uniformly-weighted map at 0.5 cm spot spacing and targeted spot maps consisting of 0.5 × 0.5 and 1.0 × 1.0 cm 2 fields centered over each aperture at similar spot spacings. 2D profiles were measured at 2.0 cm solid–water depth (2.066 cm water-equivalent-thickness) with Lynx PT scintillator. Profile comparisons between different spot trajectories and between experiments and treatment planning system (TPS) were performed with 2D gamma analyses at 2%/2 mm criteria. Absolute dose measurements were performed at the central peak and neighboring valley with Razor Nano and microDiamond detectors with cross-calibrations against an ADCL-calibrated PPC05 plane-parallel ion chamber. Main results. Lynx scintillator response at UHDRs were within ±1% with conventional dose-rate (CONV) response. Experimental 2D profiles across all spot trajectories were well-matched with gamma pass rates above 98.0%. Comparisons between experiments and TPS had gamma pass rates above 95.6%. Gamma analyses showed slight deviations at off-axis-distances beyond 4.5 cm. Absolute UHDR doses measurements were within 1.32% ± 1.99% and −1.43% ± 3.15% across the peak and valley respectively. Proton pencil-beam-scanning and voxel-wise maximum dose-rate calculations for targeted spot trajectories across the central nine apertures were above 60 Gy s −1 and 300 Gy s −1 respectively. Significance. In this work, we demonstrated that FLASH-SFRT irradiations can be performed on a clinical proton synchrocyclotron with a brass collimator. Proton UHDRs can be achieved with targeted spot trajectories over each aperture.