Determination of the RBE of low energy X-rays
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 Radiation Physics department, the RBE of low-energy X-rays has been determined for different cell lines. As a soft X-ray source, the an X-ray tube (W anode, operated at 25 kV with 0,3 mm Al filter or 10 kV, without filter) was used, whereas a conventional x-ray tube (W anode, 200 kV, 0,5 mm cu filter) was used as a reference source at the radiation biology laboratory of the Medical faculty of TU Dresden.
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 (MN) induction for the human mammary epithelial cell line MCF-12A. The RBE for clonogenic survival was found to be significantly higher than 1 for surviving fractions in the range 0.005 < S < 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.
Survival curves (left) and MN-induction (right) in the cells MCF-12A after irradation with 200 kV and 25 kV X-rays