What's in a Proton FLASH Beam? Characterizing Ultra-High Dose Rate Protons Using a Commercial Plastic Scintillator

While biological studies of the FLASH effect in proton beams have mainly been performed in the plateau region at maximum beam energy and current, this type of delivery has limited clinical applications. Naturally, it is anticipated that plans to treat patients clinically with FLASH-radiotherapy (FLASH-RT) will capitalize on the Bragg peak. However, as the proton spot widens with depth, the time required to deliver the entire dose to any single point increases. This decreases the dose rate, making the ultra-high dose rates required to trigger the FLASH effect harder to achieve over large areas. Importantly, the dose rate is difficult to measure directly. Time and dose linearity of a fast-resolving commercial plastic scintillation detector were characterized against an ionization chamber. The percent depth dose of a 250 MeV proton beam scanned across a small area (3.5 × 3.5 cm2) was measured at depths of 3-40 cm in solid water. The plastic scintillation detector was used to evaluate the instantaneous and voxel-averaged dose rates as a function of depth for conventional (2 nA nozzle current) and ultra-high dose rate (100 nA) beams. The response of the plastic scintillation detector was shown to be linear with time (±2.5 ms) and absorbed dose (±2%). The scintillator and ionization chamber measurements agreed well as a function of depth (and therefore energy) within 2% for depths <34 cm. Beyond 34 cm, expected quenching effects were observed in the plastic scintillation detector. The voxel-averaged dose rate varied from 52.7 Gy/s at the entrance to 29.3 Gy/s at mid-depth, to 70.4 Gy/s near the Bragg peak, while the maximum instantaneous dose rate decreased from 472 Gy/s near the entrance to 236 Gy/s at the Bragg peak. The plastic scintillation detector has proven useful for investigators to evaluate the complex relationship between dose rate and pencil-beam scanning ultra-high dose rate beam characteristics. There is a loss of dose rate near the Bragg peak due to spot widening, which may acutely impact our ability to exploit the FLASH effect for sparing normal tissues upstream of the intended treatment area. A thorough preclinical investigation of whether the FLASH effect is maintained near the Bragg peak is necessary before this technique can begin translation to the clinic.