Purpose: Flash irradiation requires delivering a spatial high dose-rate that is consistently above the Flash dose-rate threshold (>40Gy/sec). This requirement is not fully met for Gaussian beam distributions such as proton pencil beam scanning as the lateral fall-off of dose (or dose-rate) profile can be below the Flash threshold. In this study, we evaluate the use of proton beam collimation to reduce the non-Flash dose-rate of proton pencil beam.
Methods: The profile of a proton pencil beam captured using a plastic scintillator with and without 8 mm pinhole collimation using a lead aperture. The measurements were performed using 245 MeV proton beam at 5, 15 and 37 cm depth in water, similarly, for 150 MeV beam at of 5 and 15 cm depths. The data analysis focused on (i) Comparison of the spatial Intensity distribution of the collimated and non-collimated beams, (ii) Determining the adequate collimation ratio, defined as the ratio of beam spot-size to diameter of the pinhole aperture, and (iii) Determining the proportion of non-flash dose delivered by the collimated beam.
Results: The collimation of 245 MeV (with the spot size of σ=3.62 mm) reduces the non-Flash dose by to 6% , 9% and 15% of the delivered dose at the respective depths of 5, 15 and 37 cm in comparison to 26% from non-collimated beam. The collimation of 150 MeV beam (σ=4.47mm) is satisfactory as it limits the non-Flash dose to less than 4.3% for all depths comparing to 25% for non-collimated beam
Conclusion: The collimation of Gaussian proton beam can reduce the Flash beam dose-rate fall-off as long as the collimation factor is higher than 0.5.
Not Applicable / None Entered.