Dosimetric Impact of Dose Rate Fall-Off in FLASH Proton Pencil Beam Scanning Treatment

Abstract

The lifetime of hydroxyl radicals, which cause radiobiological damage, is estimated to be a few microseconds. This time is much shorter than the actual spot-to-spot scanning time in proton pencil beam scanning (PBS) delivery (> 10 milliseconds). This mismatch in timescales becomes problematic in the case of FLASH treatment using PBS. Given that the FLASH effect is a spatially local and temporally finite phenomenon, it will be more accurate to evaluate it based on the spatial dose rate in Gy/sec/ cm3. Thus, delivering high doses at FLASH dose rates using PBS could be harmful to healthy tissues irradiated by the non-FLASH dose rate. In this study, we evaluate the extent of the healthy tissues that can be exposed to non-FLASH dose as well as the quantification of the received non-FLASH doses.