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

PURPOSE: 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 (c-t-c) 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 solidwater depth (2.066 cm WET) with Lynx PT scintillator. Profile comparisons between different spot trajectories and between experiments and TPS were performed with 2D gamma analyses at 2%/2mm 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 PBS and voxel-wise maximum dose-rate calculations for targeted spot trajectories across the central nine apertures were above 60 Gy/s and 300 Gy/s respectively. SIGNIFICANCE AND CONCLUSION: 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.