Is Ultrahigh Dose Rate Critical for the Effectiveness of Microbeam Radiation Therapy in a Broad-Beam Combined Treatment?

Purpose: The superior therapeutic index of preclinical synchrotron microbeam radiation therapy (MRT) over conventional broad-beam (BB) radiation therapy (RT) has been clearly established. Published data demonstrate that a combination of 1 MRT session with 2 consecutive daily BB-RT sessions effectively controls the primary tumor and triggers an antitumor immune response. Here, we aimed to establish whether an ultrahigh dose rate of MRT, available exclusively on the synchrotron, is essential for effective treatment of murine triple-negative mammary carcinoma. Methods and Materials: The in vitro response of mammary tumor cells was investigated using a clonogenic survival assay at different MRT dose rates (∼1000, 100, or 10 Gy/s). The in vivo tumor responses at the same dose rates, with an MRT/BB/BB treatment schedule, were evaluated by measuring tumor volume and the induced immune response. Animal survival and normal skin reactions were also assessed. Results: Clonogenic survival decreased with decreasing MRT dose rate, with most cell sparing at the highest dose rate (1000 Gy/s), indicating that lower dose rates could be more effective against tumors. However, no significant differences in in vivo tumor growth delay were observed in response to dose-rate variations. Decreased infiltration of irradiated tumors with leucocytes and their subpopulations was observed at the lower dose rates, including decreased expression of immune checkpoints PD-1/PD-L1 and prognostic marker CD103, which could impact the overall tumor response in vivo. The higher dose rates were associated with increased antitumor immunity. Early euthanasia of mice, as dictated by ethical endpoints, prevented long-term observation of differences in animal survival. Skin toxicity in the vicinity of irradiated tumors was mild and developed later at the highest MRT dose rate. Overall, MRT at 10 to 1000 Gy/s was tolerated similarly, despite dramatic changes in the dose rate. Conclusions: These findings are promising for the clinical translation of MRT in combination with conventional RT, using emerging compact x-ray MRT sources with dose rates lower than those delivered by synchrotron sources.