Our study aimed to establish whether there are differences in the repair of potentially lethal damage (PLD) resulting from exposure to 65 MeV proton beams and (137)Cesium (Cs-137) gamma-rays. Irradiation with proton beams was carried out at the plateau and Bra,og peak in unmodulated beams, and at the spread-out Bragg peak (SOBP) for modulated beams. After irradiation, Chinese hamster ovary (CHO) cells in plateau phase were either plated immediately or kept in their own spent medium at 37 degrees C for various times up to 8 hours before plating to dishes or microplates. Survival was assessed with two methods: clonogenic and metabolic activity (MTT) assay. Obvious repair was observed in response to both kinds of irradiation beams, including different irradiation positions in proton beams. The clonogenic assay showed the PLD recovery ratios of 2.05, 1.79, 1.66 and 1.32 at two hours after 5 Gy irradiation for Cs-137 gamma-rays, plateau, SOBP and Bragg peak of proton beams, respectively. Recovery ratios for the SOBP and Bragg peak were significantly lower than that for gamma-rays (p<0.05 or p<0.01). There were no significant differences in the recovery ratios assessed by different methods (clonogenic and MTT assay), except for the Bragg peak. PLD repairs for proton beams and Cs-137 gamma-rays were not found at lower dose irradiation (3 Gy). Our results indicate that PLD repair in response to 65 MeV proton beams (SOBP and Bragg peak) is less and occurs later than for Cs-137 gamma-rays. The MTT assay was found to be a useful method in radiobiological research of proton beams.

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