Discordance in Acute Gastrointestinal Toxicity between Synchrotron-Based Proton and Linac-based Electron Ultra-High Dose Rate Irradiation

Purpose: Proton FLASH has been investigated using cyclotron and synchrocyclotron beamlines but not synchrotron beamlines. We evaluated the impact of dose rate [ultra-high vs conventional (CONV)] and beam configuration [shoot-through (S-T) vs spread-out Bragg peak (SOBP)] on acute radiation-induced gastrointestinal toxicity (RIGIT) in mice. We also compared RIGIT between synchrotron-based protons and linac-based electrons with matched mean dose rates. Methods and Materials: We administered abdominal irradiation (12-14 Gy single fraction) to female C57BL/6J mice with an 87-MeV synchrotron-based proton beamline (2-cm-diameter field size as a lateral beam). Dose rates were 0.2 Gy/s (S-T pCONV), 0.3 Gy/s (SOBP pCONV), 150 Gy/s (S-T pFLASH), and 230 Gy/s (SOBP pFLASH). RIGIT was assessed by the jejunal regenerating crypt assay and survival. We also compared responses to proton (pFLASH and pCONV) with responses to electron CONV (eCONV, 0.4 Gy/s) and electron-beam FLASH (188-205 Gy/s). Results: The number of regenerating jejunal crypts at each matched dose was lowest for pFLASH (similar between S-T and SOBP), greater and similar between pCONV (S-T and SOBP) and eCONV, and greatest for electron-beam FLASH. Correspondingly, mice that received pFLASH SOBP had the lowest survival rates (50% at 50 days), followed by pFLASH S-T (80%), and pCONV SOBP (90%), but 100% of mice receiving pCONV S-T survived (log-rank P = .047 for the 4 groups). Conclusions: Our findings are consistent with an increase in RIGIT after synchrotron-based pFLASH versus pCONV. This negative proton-specific FLASH effect versus linac-based electron irradiation underscores the importance of understanding the physical and biological factors that will allow safe and effective clinical translation.