Figure 4

Numbers of DSBs per nucleus for a variety of photon irradiations. The number of DSBs was obtained by γ-H2AX foci formation assay. Error bar indicates the standard deviation in the number of DSBs per nucleus. The number of cell analyzed for each condition is 143–228 (by microscopy) or 2416–68673 (by flow cytometry). Significant differences are observed (by the Tukey-Kramer test) for the cases of lower energy X-rays (60, 100, 250 kVp) and in-field irradiation with 6MV-linac X-rays (**p < 0.01). In general, the numbers of foci for diagnostic kilo-voltage X-rays are greater than those of therapeutic mega-voltage X-rays.