RBE of 25 kV X-rays for the Survival and Induction of Micronuclei in the Human Mammary Epithelial Cell Line MCF-12A

The broad application of low energy X-rays below about 50 keV in radiation therapy and diagnostics and especially in mammography substantiates the precise determination of their relative biological effectiveness (RBE). A quality factor of 1 is stated for photons of all energies in the ICRP Recommendations. However, the RBE of low-energy X-rays compared to high-energy photons was found to be dependent on photon energy, cell line and endpoints studied, hence varying from less than 1 up to about 4. In the present study, the human mammary epithelial cell line MCF-12A has been chosen due to the implementation of the results in the estimation of risk from mammography procedures. The RBE of 25 kV X-rays (W anode, 0.3 mm Al filter) relative to 200 kV X-rays (W anode, 0.5 mm Cu filter) was determined for clonogenic survival and micronuclei induction. The RBE for clonogenic survival was found to be significantly higher than 1 for surviving fractions in the range between 0.005 and 0.2. The RBE decreased with increasing survival, with an RBE0.1 at 10 % survival of 1.13 ± 0.03. The effectiveness of soft X-rays for micronuclei induction was found to be 1.40 ± 0.07 for the fraction of binuclear cells (BNC) with micronuclei (MN) and 1.44 ± 0.17 for the number of MN per BNC. In contrast, the RBE determined from the number of MN per MN-bearing BNC was found to be 1.08 ± 0.32. This indicates that the effectiveness of 25 kV X-rays results from an increase in the number of damaged cells, which, however, do not have higher number of micronuclei per cell.