Hypofractionated Radiotherapy With Prolonged Interfraction Interval: Impact On Tumor Control And Immune Activation

PURPOSE: Radiotherapy combined with immunotherapy is a novel promising strategy for comprehensive cancer treatment.Daily fractionated radiotherapy may pose constant stress on immune activation.The aim of the current study is to explore whether prolonged inter-fraction interval with non-ablative dose of fractionated radiotherapy(Hypo-Slow Radiotherapy,HSRT)may enable better immune priming of tumor than daily radiation.METHODS: A mouse MC38 colon cancer xenograft model was used for radiation.Different radiation regimens were designed to explore the impact of fraction interval and fraction size on tumor control,immune mobilization,and synergistic effect with anti-PD-1 immunotherapy.Immune activation was measured as peripheral blood and intratumor CD4+,CD8+ T cell analysis by flow cytometry,and plot curves and statistical graphs.After radiotherapy coupled with immunotherapy,tumor growth and immune cell changes are assessed again.RESULTS: In a fixed dose experiment,6×5Gy QOD produced identical tumor control as 6×5Gy QD whereas 6×5Gy BIW resulted in relapse of tumor.However,both QOD and BIW regimens showed ability to activate immune response whereas QD regimen did not.In a fixed biological equivalent dose(BED)experiment,different regiments with increased fraction size but prolonged fraction interval was designed.Compared with 12×3Gy QD regimen,HSRT regimens,including 6×5Gy QOD,4×7Gy BIW,and 2×11Gy QW,had identical tumor control.Importantly,all HSRT regiments showed significant mobilization of host immunity whereas 12×3Gy QD did not.Both peripheral and intratumor CD4+ and CD8+ cell increase with increased fraction interval and fraction size.Finally,all HSRT regimens combined with anti-PD-1 immunotherapy showed a higher-level intratumor CD8+ T cells and better tumor growth delay than any single treatment but 12×3Gy QD regimen did not.CONCLUSION: The fraction dose and frequency of radiotherapy is significantly related to the synergistic effect with immunotherapy.In this study,by using mouse model,we demonstrated that conventional daily fraction radiotherapy is not beneficial for host immune response against tumor.Preliminary results of the current thesis suggested that prolonged inter-fraction interval with increased fraction size may be an optional strategy to balance the tumor control and immune activation.Thesis will provide basic preclinical research data for future clinical radiotherapy co-immunotherapy and provide ideas and references for the implementation of relevant clinical trials in the future.