Clonogenic survival of human keratinocytes and rodent fibroblasts after irradiation with 25 kV x-rays.

Low energy x-rays (E(ph) </=50 keV) are widely used in diagnostic radiology and radiotherapy. However, data on their relative biological effectiveness (RBE) are scarce. Of particular importance for risk estimation are the RBE values of x-rays in the range which is commonly used in mammography (10-30 keV). We have determined clonogenic survival after low-energy x-ray irradiation for three cell lines: primary human epidermal keratinocytes (HEKn), mouse fibroblasts (NIH/3T3) and Chinese hamster fibroblasts (V79). Experiments were performed with a 25 kV x-ray tube and compared to 200 kV x-rays as a reference. Compared to the effect of 200 kV x-rays, irradiation with 25 kV x-rays resulted in a decreased survival rate in the murine fibroblasts, but not in the human epithelial cell line. The RBE value was calculated for 10% surviving fraction. For HEKn cells, RBE was 1.33+/-0.27, for NIH/3T3 cells 1.25+/-0.07 and for V79 cells 1.10+/-0.09, respectively. No consistently increased RBE was observed in the various cell lines. Nevertheless, a potential of increased cytogenetic changes has to be considered for risk estimation of low-energy x-rays.