Recovery from sublethal damage and potentially lethal damage Proton beam irradiation vs. X-ray irradiation

In order to clarify the biological response of tumor cells to proton beam irradiation, sublethal damage recovery (SLDR) and potentially lethal damage recovery (PLDR) induced after proton beam irradiation at the center of a 10 cm spread-out Bragg peak (SOBP) were compared with those seen after XaEuroray irradiation. Cell survival was determined by a colony assay using EMT6 and human salivary gland tumor (HSG) cells. First, two doses of 4 Gy/GyE (Gray equivalents, GyE) were given at an interfraction interval of 0-6 h. Second, five fractions of 1.6 Gy/GyE were administered at interfraction intervals of 0-5 min. Third, a delayed-plating assay involving cells in plateau-phase cultures was conducted. The cells were plated in plastic dishes immediately or 2-24 h after being irradiated with 8 Gy/GyE of XaEurorays or proton beams. Furthermore, we investigated the degree of protection from the effects of XaEurorays or proton beams afforded by the radical scavenger dimethyl sulfoxide to estimate the contribution of the indirect effect of radiation. In both the first and second experiments, SLDR was more suppressed after proton beam irradiation than after XaEuroray irradiation. In the third experiment, there was no difference in PLDR between the proton beam and XaEuroray irradiation conditions. The degree of protection tended to be higher after XaEuroray irradiation than after proton beam irradiation. Compared with that seen after XaEuroray irradiation, SLDR might take place to a lesser extent after proton beam irradiation at the center of a 10 cm SOBP, while the extent of PLDR does not differ significantly between these two conditions.