Determination of RBE of X-rays by a novel quasi-monochromatic X-ray source

Cell survival for NIH/3T3 mouse fibroblasts, V79 hamster fibroblasts and neonatal human keratinocytes (HEKn) was measured for soft X-rays (25 kV) and reference X-rays (200 kV). From the linear-quadratic model fit of the cell survival curves, RBE values close to 1.0 were calculated at 10% survival level. Further endpoint studied was the induction of micronuclei in 3T3 cells, HEKn cells and primary human fibroblasts (HFIB). For all cell lines, RBE values of 1.1. 1.2 were found. However, the precise determination of the energy dependence of RBE has to be performed by means of irradiation with a monochromatic source. One of the applications of the superconducting electron linear accelerator ELBE, at present under construction at Forschungszentrum Rossendorf, will be to provide such X-rays. The first cell irradiation will be performed with X-rays in the range 10. 50 keV generated by channeling radiation from relativistic electrons in a diamond crystal. The properties of the electron beam (picosecond structure) as well as the intrinsic energy and intensity distribution of the channeling radiation require consideration of precise dose delivery and monitoring, the results of which will be also presented. (orig.)