Results of FLASH Irradiation of Mice in vivo with High-Energy Protons

The FLASH effect of high-energy (660 MeV) proton irradiation using the Phasotron accelerator with the capacity of delivering dose rates of 80 Gy/s has been studied and compared to the effect after exposure to proton radiation at a conventional dose rate of 3 Gy/min. After FLASH and conventional dose-rate irradiation with doses of 1.0 and 1.5 Gy, the induction of cytogenetic damage to bone marrow cells and the state of lymphoid organs (thymus and spleen) were estimated; at doses of 7.0 and 8.0 Gy, the survival rate after total irradiation of mice in vivo was analyzed; and at doses of 40 and 60 Gy, the tumor growth rate was determined after irradiation ex vivo. It has been shown that irradiation of animals using the FLASH mode at a dose of 1.5 Gy protects the proliferative activity of the spleen and also leads to a decrease in cytogenetic injuries in bone marrow erythrocytes, based on the micronucleus test, as compared to the conventional irradiation at a dose of 1.5 Gy; thus, the FLASH effect has lower toxicity compared to conventional radiation. However, irradiation of mice, the FLASH effect which delivers high doses (7.0 and 8.0 Gy) of radiation, leads to earlier death of animals compared to those exposed to conventional radiation. Only after FLASH irradiation of a suspension of Ehrlich ascites carcinoma at a dose of 40 Gy, a tumor node with further growth was formed; no tumors were formed in all other groups.