Double-strand break induction and repair in V79-4 hamster cells: The role of core ionisations, as probed by ultrasoft X-rays

Purpose To compare the induction of double-strand breaks (DSB) in cells irradiated by 250 and 350 CV ultrasoft X-rays and assess the residual yield of breaks 2 hours post irradiation in order to unravel the correlation between the sharp increase in cell-killing efficiency of ultrasoft X-rays above versus below the carbon-K threshold (284 eV) and the induction of core events in DNA atoms Materials and methods. V79-4 hamster cells were irradiated with synchrotron ultrasoft X-rays at isoattenuating energies of 250 eV and 350 eV DSB were quantified using Pulse field gel electrophoresis Results A significant increase in DSB induction was observed for 350 CV ultrasoft X-rays above the carbon-K threshold, compared to 250 eV below the threshold, per unit dose to the cell. The DSB induced by the 350 eV ultrasoft X-rays were less repaired 2 h after irradiation Conclusion The increased DSB induction at 350 eV is attributed 10 the increase In the relative proportion of photon interactions in DNA resulting in significant dose inhomogeneity across the cell with a local increase in dose to DNA It results from all increase in carbon-K shell interactions and the short range of the electrons produced Core ionisations in DNA, through core-hole relaxation in conjunction with localised effects of spatially correlated low-energy photo- and Auger-electrons lead to an increase in number and the complexity of DSB.