Generating antitumor immunity by targeted radiation therapy: Role of dose and fractionation

Accumulating evidence supports the role of radiation therapy in the induction of antitumor immunity. With recent advancements in stereotactic radiation therapy, there is increasing appreciation that, when combined with immune checkpoint blockade, the type of radiation dose and fractionation regimen selected may both influence local tumor control and also affect the generation of immune responses that are important for systemic control. Although a broad range of radiation dose and fractionation schema have been tested in both the preclinical and clinical settings, recent preclinical evidence suggests the existence of a dose per fraction threshold beyond which radiation becomes less effective in generating tumor immune responses. Such a threshold seems to be tumor dependent, probably reflecting different genetic mutations of cancer. In this review we discuss the key preclinical and clinical evidence relating to radiation dose and fractionation considerations. Future clinical trials should focus on identifying optimal radiation dose and fractionation schedules, which may depend on the clinical context.