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 International Commission on Radiological Protection 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 one up to about four. 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 in the dose range 1-10 Gy and micronuclei (MN) induction in the range 0.5-3.5 Gy. The RBE for clonogenic survival was found to be significantly higher than 1 for surviving fractions in the range 0.005