A-B, U2OS and BT549 cells irradiated with 6MV photons or 76.8 MeV protons delivered at the entrance (ENT) or the Bragg peak (BP) of the proton beam profile and clonogenic survival was assessed. Data are presented as the mean ± SEM from three independent experiments. C, The proton entrance and proton Bragg peak RBE2Gy for the experiments in A and B was quantified. D-E, U2OS cells were treated with 2 Gy using 76.8 MeV protons delivered at ENT or the BP and the formation and resolution of γ-H2AX (D-E) were assessed using immunofluorescence. Representative images (D) and quantification (E) are displayed, (mean ± SEM) from three independent experiments. P values were calculated using ANOVA; *p <0.05, ***p<0.001.