RBE of 10 kV X-rays determined for the Human Mammary Epithelial Cell Line MCF-12A

Presently, an extended discussion about the dependence of the relative biological effectiveness (RBE) on photon energy takes place. The increasing amount of in vitro data in the low-energy region indicates this is a complex dependence which is influenced by the endpoint and cell line studied. In the present investigation, the RBE of 10 kV rays (W anode) was determined relative to 200 kV X-rays (W anode, 0.5 mm Cu filter) for cell survival in the dose range 1 - 10 Gy and induction of micronuclei in the range 0.5 - 3.6 Gy for the human mammary epithelial cells MCF-12A. The RBE for cell survival was found to increase with decreasing dose, being 1.21 ± 0.03 at the 10 % survival rate. Considerably higher values were obtained for the micronucleus induction, where RBEM obtained from the ratio of the linear coefficients of the dose-effect curves was equal to 2.6 ± 0.4 for the fraction of binucleated cells (BNC) with micronuclei (MN) and 4.1 ± 1.0 for the number of MN per BNC. These values, together with our previous data, support a monotonic increase in RBE with decreasing photon energy down to the mean energy of 7.3 keV used in the present study.