Purpose: To investigate the feasibility of real-time proton FLASH relative dose rate monitoring via prompt gamma ray (PG) timing. PG event measurements are used for secondary relative verification of absolute dose rate delivery and for visualization into the temporal dose rate characteristics of proton FLASH deliveries up to 138 Gy/s. PG counts are processed to provide insight into an optimized PG detection system performance.
Methods: A PG detection and measurement system consisting of a NaI(Tl) scintillator and nuclear electronic pulse processing system are used along with transmission ionization chamber absolute dose measurements. Varying dose rates were delivered to measure PG counts and count rates to investigate PG count and proton FLASH dose rate linearity, time structures, and delivery stability.
Results: PG counts and count rates are found to increase linearly with delivered dose and dose rate. Upon beam termination, PG counts exhibit an exponential decay due to scintillation crystal afterglow with a half-life of 2.495 ± 1.35 ms, which corresponds to the theoretical minimum response time in which a real-time dose rate monitoring system would be able to terminate beam delivery upon recognition of ≥7.0% dose rate variation per single delivery. With no detector collimation, FLASH dose rates cause detection system paralyzation, leading to PG count saturation; after detector dead time and paralyzation correction calculations, true unsaturated PG count rates for dose rates up to 138 Gy/s were determined. Variations in absolute dose rate measurements provide a >6.59% threshold variation for which PG counts could provide substantial dose rate variation detectability.
Conclusion: PG counting systems can be designed to perform real-time relative proton dose rate monitoring for <36 Gy/s deliveries, and with corrections accounting for PG count saturation, real-time relative monitoring may be feasible for FLASH dose rates; allowing for verification of critical research and clinical setting delivery parameters.
TH- External Beam- Particle/high LET therapy: Proton therapy – experimental dosimetry