Influence of different dose rates on cell recovery and RBE at different spatial positions during protracted conformal radiotherapy

The effect of simultaneous induction and repair of sub-lethal lesions during protracted irradiation is potentially important for conformal irradiation techniques and may influence the relative biological effectiveness (RBE) of different radiation qualities. The importance of repair for 50 kV X-rays from a miniaturised X-ray source producing an essentially isotropic dose distribution for intra-operative radiotherapy (IORT) was verified for inactivation of V79 cells in different distances from the source. The experimental data were evaluated in terms of the linear-quadratic model and the generalised Lea-Catcheside time factor. Furthermore, the shape of survival curves for 14 MeV [d(0.25) + T] neutrons at different dose rates implicated a role of repair for fast neutrons. Micro-dosimetric measurements have previously demonstrated variations in radiation quality between different positions in a therapy phantom for conformal moving-beam therapy with this radiation. Experimental data on the effectiveness for inactivation of V79 cells irradiated at such positions are presented and the influence of repair is analysed. (authors)