Synchroton-produced ultrasoft X rays: equivalent cell survival at the isoattenuating energies 273 eV and 860 eV

In this paper we report on survival of Chinese hamster V79 and mouse C3H 10T1/2 cells after irradiation with synchrotron-produced 273 eV and 860 eV ultrasoft X rays. These two energies, which are available by multilayer monochromatization of the synchrotron output spectrum, exhibit equal attenuation within living cells. Such an isoattenuating energy pair allows the direct examination of how biological effectiveness varies with the energy of the ultrasoft X rays. In comparing survival results, we find similar biological effectiveness of these two energies for both the C3H 10T1/2 and the V79 cells. These results are not consistent with previous findings of increasing RBE with decreasing ultrasoft X-ray energies. In addition, after correcting for mean nuclear dose based on measurements of cell thickness obtained with confocal microscopy, we find no significant differences in survival between the two ultrasoft X-ray energies and 250 kVp X rays. These results suggest that RBE does not increase with decreasing energy of ultrasoft X rays between 860 eV and 273 eV. The possible impact of our results on past results for ultrasoft X rays is discussed.