Clonogenic survival of human keratinocytes and rodent fibroblasts after irraditaion with 25 kV X-rays

Low energy X-rays (Eph < 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 value of X-rays in the range which is commonly used in mammography (10 - 30 keV). In the present study we have determined clonogenic survival after low-energy X-ray irradiation for 3 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 the human epithelial cell line. 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. In conclusion, 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.