Chromosome aberrations and cell inactivation induced in mammalian cells by ultrasoft Xrays: correlation with the core ionizations in DNA

Purpose: To study the frequency of chromosome aberrations induced by soft X-rays. To see if the core ionization of DNA atoms is involved in this end-point as much as it appears to be in cell killing. Materials and methods: V79 hamster cells were irradiated by synchrotron radiation photons isoattenuated in the cell (250, 350, 810 eV). The morphological chromosome aberrations detected in the first post-irradiation cell division (dicentrics and centric rings) were studied by Giemsa staining. Results: The chromosome aberrations at 350 eV were, respectively, 2.6+-0.8 and 2.1+-0.8 times more numerous than at 250 and 810 eV for the same average dose absorbed by the nucleus. These relative effectivenesses are comparable with the ones already measured for cell killing. Moreover, they roughly vary such as the relative numbers of core ionizations (including in the phosphorus Lshell) produced in DNA and its bound water (water being involved only at 810 eV through the oxygen atoms). In particular, they reproduce the characteristic twofold enhancement at 350 eV, above the carbon K threshold. Conclusions: Correlations suggest that the core ionization process is likely a common and essential mechanism initiating both chromosome aberration and cell killing end-points at these photon energies.