Hypofractionated Radiotherapy with Prolonged Inter-Fraction Interval (Hypo-Slow RT) to Facilitate Immune Priming of Tumor

Abstract

Daily fractionated radiotherapy may pose constant stress for immune activation. A mouse model was built to explore whether hypofractionated radiotherapy with prolonged inter-fraction interval (Hypo-Slow Radiotherapy, HSRT) may enable better immune-priming of tumor than daily radiotherapy. A subcutaneous MC38 murine colon cancer mouse model was irradiated at different radiation regimens. The impact of inter-fraction interval and dose per fraction on tumor control, immune mobilization, and synergistic effect with anti-PD-1 immunotherapy was explored. Immune activation was assessed by analyzing CD4+ and CD8+ T cells from peripheral blood, and intratumoral CD4+ and CD8+ T cells by flow cytometry; both were sampled three days after the completion of scheduled irradiation. In a fixed dose per fraction experiment, the 6×5Gy QOD and 6×5Gy QD irradiation schemes resulted in identical tumor control, while the 6×5Gy BIW scheme led to tumor progression; moreover, both QOD and BIW regimens showed ability to activate immune response whereas QD regimen did not. In a fixed biological equivalent dose (BED) experiment, the comparison of different regimens with increased dose per fraction and prolonged inter-fraction interval showed that 12×3Gy QD regimen, HSRT regimens, including 6×5Gy QOD, 4×7Gy BIW, and 2×11Gy QW, led to identical tumor control. Importantly, all HSRT regimens showed significant mobilization of host immunity whereas 12×3Gy QD did not. Both peripheral and intratumor CD4+ and CD8+ cell increase with increased inter-fraction interval and dose per fraction. Finally, all HSRT regimens combined with anti-PD-1 immunotherapy showed enhanced tumor growth delay than any single treatment while 12×3Gy QD regimen did not. This pre-clinical model demonstrated that conventional daily fractionated radiotherapy is not beneficial for host immune activation against tumor. Preliminary results suggested that prolonged inter-fraction interval with increased dose per fraction may be an optional strategy to balance the tumor control and immune activation.