Determination of RBE of 10 kV and 25 kV X-rays

X-rays in the range ~10 – 50 keV are widely applied in the diagnostic radiology (particularly mammography) and radiotherapy (e. g. brachytherapy). However, the published data about their relative biological effectiveness (RBE) allow up to now no definitive conclusions about the action on different biological systems. Moreover, the biological effects depend on the spectral distribution of the photon source. In order to study this dependence, the RBE of 10 kV and 25 kV X-rays has been determined relative to 200 kV X-rays by X-ray tube irradiation.
The studies were carried out on the mouse fibroblasts NIH/3T3 and the human mammary epithelial cells MCF-12A. For the soft X-rays irradiation, a tungsten-anode X-ray tube operated at 10 kV (no filtration) or at 25 kV (0.3 mm Al filtration) was used. The reference irradiation was performed with a 200 kV X-ray tube with 0.5 mm Cu filter. The dose rate for all irradiations was in the range 0.3 – 1.9 Gy/min. The spectral dose distribution of all radiation qualities was also determined. Cell survival was studied after irradiation with 0.5 – 10 Gy by the clonogenic assay. Chromosomal damage was assessed by the cytokinesis-block micronucleus (MN) test in the dose range 0.2 – 5 Gy. In addition, first results for the RBE determination by chromosomal aberrations induction in MCF-12A, will be presented.
The cell survival data were fitted to the linear-quadratic model, resulting in an RBE value of 1.1 – 1.3 at the 10% survival level, depending on the used radiation quality and cell line. For both cell lines, an increase of RBE was found with decreasing dose after 10 kV X-rays, whereas a decrease of RBE of 25 kV rays was observed. The MN test results for the fraction of binucleated cells (BNC) with MN and the number of MN per BNC were fitted to a quadratic dependence, resulting in an RBE of 1.1 – 1.4.
The data obtained in the present work are in good agreement with observations of other authors as well as with theoretical predictions for this photon energy range. However, the detailed RBE dependence on photon energy can be determined only at a monochromatic X-ray source. Currently, the installation of an intensive, tunable, quasi-monochromatic source for cell irradiation in the energy range 10 – 100 keV is under progress at the ELBE accelerator at Forschungszentrum Rossendorf. The experimental verification of the theoretical calculation of the spectral distribution and intensity of this novel photon source will be presented.