Background and Aims

There has been increasing evidence of the protective effect of ultra-high dose rate (FLASH) irradiation on different normal tissues recently. Here we intend to verify the FLASH effect of proton irradiation of the intestine, brain and skin.

Methods

For the partial abdominal irradiation, 6-8-week-old C57BL/6j mice and Rag1^-/-/C57 mice were exposed to FLASH (120-130 Gy/s) or conventional dose rate (CDR, ~0.4 Gy/s) proton irradiation of 16 Gy or 16.2 Gy.

For the brain irradiation, 10-12 weeks C57BL/6j mice received a single dose of 10 Gy whole brain irradiation. BrdU was injected to label the proliferating neural stem/progenitor cells in the hippocampus.

To study radiation-induced skin injury, Indian ink was injected intracutaneously into the skin of FVB/N mice the day before irradiation. The distance between the two ink dots was measured with a vernier caliper.

Results

Long-term observation (278 days) of the C57BL/6j mice after 16.2 Gy abdominal irradiation showed no significant survival difference between the FLASH and CDR groups (Figure A). The survival of Rag1^-/-/C57 mice in the 16 Gy FLASH group was lower than that of the 16 Gy CDR group (Figure B). No significant difference was observed in the number of the BrdU labeled cells within the subgranular zone of the hippocampus. Skin contraction after 25 and 27 Gy was significantly greater in mice receiving conventional irradiation compared to FLASH groups (Figure C).

Conclusions

Proton FLASH irradiation protection was observed in skin tissue. However, no significant FLASH sparing effect was observed for intestine and brain.

Background and Aims

The objective of this study was to assess the use of optically stimulated luminescence detectors (OSLDs) to support radiobiological experiments for ultra-high dose rates (FLASH) proton beams.

Methods

Two experimental setups were tested to accommodate either biological samples or multiple mm²-sized Al₂O₃:C OSLDs. The OSLDs were read out using a protocol with a reference irradiation under known conditions to account for material differences. The experiments were conducted in a single pencil beam at the PSI Gantry 1 at a wide dose rate range of (1-3800) Gy/s. A third experiment assessed the spot reconstruction at 9000 Gy/s.

Results

The OSLDs were demonstrated to be dose rate independent with a negligible signal fading. The OSLD evaluated doses were on average (n=66) within 1 % of the nominal dose for (3 – 33) Gy for dose-rates (1 – 1000) Gy/s. The discrepancy between the OSLDs and the nominal dose was higher for the (3800-9000) Gy/s dose rates due to averaging effects of the narrow pencil beam over the OSLD surface, where a correction was demonstrated. An OSLD dose measurement was overall found to be reproducible within 1 %. The use of an OSLD grid enabled an estimation of the beam spot size and position in agreement (deviation < 2%) with radiochromic film measurements.

Conclusions

The results demonstrate that the almost point-like OSLDs are applicable for accurate proton dosimetry in ultra-high dose rates and suitable to support radiobiological experiments in water and air.

Background and Aims

To present alternatives, which rely on a Faraday Cup (FC) as reference, to the use of ionization chambers (ICs) for dosimetry in mm-small proton beams at ultra-high dose rates (UHDRs). Indeed, significant ion recombination combined with the volume averaging effect severely challenge the use of ICs in UHDR small-field dosimetry. Three distinct applications of a FC are presented: i) Prediction of the delivered dose; ii) Response characterization of field detectors up to UHDR; iii) On-line verification of delivered dose to biological samples.

Methods

250MeV transmission pencil beams can be delivered to small biological samples and detectors at currents up to ~700nA (~9000Gy/s on beam axis). For i) FC, beam width and integral depth-dose measurements are used to model the delivered dose. For ii) and iii) the FC is positioned downstream from the detectors or samples to be examined, which are then exposed to a wide range of dose rates. Following detectors have been studied: PTW IC 7862, PTW microDiamond 60019, EBT3 Gafchromic films, scintillating screens.

Results

EBT3 films and scintillating screens are dose rate independent, as well as microDiamond detectors (within +/-0.7%) over the range considered. The PTW IC 7862, though reproducible, exhibits a drop in response larger than 30% at ~9000Gy/s. Reproducibility of delivered dose for the proposed setup better than 1%.

Conclusions

FC are versatile dosimetry instruments that can be employed for dose prediction, field detector characterization and on-line dose verification for pre-clinical experiments at UHDR. microDiamond detectors showed promising results for their suitability for UHDR experiments for proton beams.