RBE of soft X-rays for chromosomal damage determined for two human mammary epithelial cell lines

There is an ongoing discussion on the relative biological effectiveness (RBE) of low-energy X-rays below about 50 keV. Considering their wide application in diagnostic radiology and particularly in mammography, RBE has to be determined for an appropriate biological object. Further, the dependence on the photon energy distribution of the radiation sources has to be considered.
We have determined the effectiveness of low-energy X-rays by X-ray tube irradiation for two human mammary epithelial cell lines, MCF-12A and 184A1. For the soft X-ray irradiation, a tungsten-anode X-ray tube operated at either 10 kV (no filtration) or 25 kV (0.3 mm Al filter) was used. The reference radiation source was a 200 kV X-ray tube filtered with 0.5 mm Cu. The dose rate for all irradiations was 0.3 -1.9 Gy/min, and doses in the range 0.5 - 5 Gy were applied. The cytokinesis-block micronucleus (MN) test was used to assess chromosomal damage in the cell line MCF-12A. The fraction of binucleated cells (BNC) with MN, the number of MN per BNC and the number of MN per BNC with MN were determined for all radiation qualities. Additionally, chromosomal aberrations were studied by conventional Giemsa-staining in both cell lines. Dicentrics, rings and excess acentric fragments were assessed. In addition, first results will be presented on stable chromosome aberrations studied by whole-chromosome FISH painting of chromosomes 1, 8 and 17 for the cell line 184A1.
All data has been fitted to the linear-quadratic model. The soft X-rays were observed to be more effective in inducing radiation damage, RBE increasing with decreasing photon energy. Whereas the RBE for 25 kV X-rays determined by the MN test resulted in a value of 1.1 - 1.4, values of up to 3 were obtained for the 10 kV X-rays for the cell line MCF-12A. The chromosomal aberration data for both cell lines resulted in an RBE value of up to 2 for both soft X-ray radiation qualities.
The data obtained in the present work confirm the observations of other authors for the photon energy dependence of RBE in this energy range. However, the study of the detailed RBE dependence on photon energy is possible only at a monochromatic X-ray source. The progress in the installation of a quasi-monochromatic channelling radiation source at the ELBE accelerator at Forschungszentrum Rossendorf will be presented.