RBE of 20 kV and 70 kV X-rays determined for survival of V 79 cells.

The survival curves of V 79 cells were compared for various types of sparsely ionizing radiation: 20 kV, 70 kV and 42 MV X-rays, 60Co gamma-rays and 40 MeV electrons. A carefully calibrated thin window chamber was used for accurate dosimetry. The survival curves for cells exposed to 20 kV X-rays and 70 kV X-rays differed from that for 60Co gamma-rays only by a dose-scaling factor t(RBE). RBE with respect to 60Co gamma-rays could then be considered as dose-independent: 1.19 +/- 0.06 for 70 kV X-rays and 1.40 +/- 0.07 for 20 kV X-rays. RBE of 42 MV X-rays and 40 MeV electrons was not significantly different from 1.