Dosimetric and biologic intercomparison between electron and proton FLASH beams

The FLASH effect has been validated in different preclinical experiments with electrons (eFLASH) and protons (pFLASH) operating at an average dose rate above 40 Gy/s. However, no systematic intercomparison of the FLASH effect produced by eFLASHvs. pFLASH has yet been performed and constitutes the aim of the present study.

The electron eRT6/Oriatron/CHUV/5.5 MeV and proton Gantry1/PSI/170 MeV were used to deliver conventional (0.1 Gy/s eCONV and pCONV) and FLASH (≥110 Gy/s eFLASH and pFLASH) dose rates. Protons were delivered in transmission. Dosimetric and biologic intercomparisons were performed using previously validated dosimetric approaches and experimental murine models.

The difference between the average absorbed dose measured at Gantry 1 with PSI reference dosimeters and with CHUV/IRA dosimeters was -1.9 % (0.1 Gy/s) and + 2.5 % (110 Gy/s). The neurocognitive capacity of eFLASH and pFLASH irradiated mice was indistinguishable from the control, while both eCONV and pCONV irradiated cohorts showed cognitive decrements. Complete tumor response was obtained after an ablative dose of 20 Gy delivered with the two beams at CONV and FLASH dose rates. Tumor rejection upon rechallenge indicates that anti-tumor immunity was activated independently of the beam-type and the dose-rate.

Despite major differences in the temporal microstructure of proton and electron beams, this study shows that dosimetric standards can be established. Normal brain protection and tumor control were produced by the two beams. More specifically, normal brain protection was achieved when a single dose of 10 Gy was delivered in 90 ms or less, suggesting that the most important physical parameter driving the FLASH sparing effect might be the mean dose rate. In addition, a systemic anti-tumor immunological memory response was observed in mice exposed to high ablative dose of electron and proton delivered at CONV and FLASH dose rate.