(A) Experimental design where C57BL/6 mice were injected with 5×106 ID8 cancer cells. On days 7, 10, and 13 post-injection, mice were treated with αPD-1 or IgG control. On day 10 post-injection, mice were treated with 14 Gy CONV or 14 Gy FLASH irradiation. On day 27, mice were analyzed. (B) Macroscopic images of the tumor-bearing mice in each treatment arm. (C) Intraperitoneal tumor weight and volume of ascites fluid (n= 5 mice per group). *p<0.05, **p<0.01, ***p<0.001 compared by one-way ANOVA followed by Tukey’s multiple comparisons post-hoc test. Error bars represent standard deviation of the mean.