DNA damage induced by heavy ions and its repair

When natural logarithms of the surviving fractions are plotted as a function of radiation dose, a curve with a shoulder at relatively high survival level is obtained for γ-rays. Curves were practically linear for HMV-I cells and HA-1 cells irradiated with charged particle beams. HMV-I and HA-1 cells were derived from human melanoma and Chinese hamster, respectively. The amount of DNA double strand breaks (dsb) caused by γ-rays increases quadratically with an increasing dose in both HMV-I and HA-1 cells, when the dsb is detected using neutral filter elution. The survival fraction for HA-1 cells is slightly higher than that for HMV-1 cells at the same dose, and the amount of dsb for HA-1 cells is considerably greater than that for HMV-I cells. These results suggest that the radiosensitivity to γ-rays in different cell lines does not correspond to the number of dsb. The amount of non-repairable dsb increases quadratically with increasing dose of γ-rays and almost linearly with increasing dose of N-ions (LET=530 keV/μm) and α-particles (LET=36 keV/μm for HA-1 cells, LET=77 keV/μm for HMV-I cells). The dose response curves of non-repairable dsb for these two radiations seemed to be a mirror image of the dose-survival curves for these radiations. The number of nonrepairable dsb at the same survival level was much the same. These results show a close relationship between the induction of non-repairable dsb and cell killing. (author)