The relative biological effectiveness of radiations of different quality

This report is a review of the literature relevant to the selection of relative biological effectiveness (RBE) values for use in arriving at values of the quality factor (Q). Emphasis is placed on responses to small (< 0.2 Gy) absorbed doses, or on larger absorbed doses delivered at low rates, to insure relevance to the predominant dose and time pattern of exposure important in connection with radiation protection. The data available are, in general, insufficient in quantity and quality to permit accurate estimation of the RBE at the low dose and some rates relevant to radiation protection. However for a wide spectrum of biological endpoints, RBE varies as a function of radiation quality, dose, dose rate, and endpoint. Although exceptions exist, for most endpoints examined, ranging from cell inactivation to tumor induction in laboratory mammals, RBE values appear to increase as the dose decreases, and as the dose rate decreases. The maximized values of RBE determined in these regions are designated RBEM. In a wide variety of systems, the RBEM for fast (fission) neutrons, with low doses and dose rates, appears to be of the order of 20 or more compared to moderately filtered 250 kVp x rays and 40 or more compared to higher energy gamma rays. These values, which are much larger than those observed with large dosed delivered at high dose rates, are due mainly, but not entirely, to a decrease in the slope of the curve for the low-LET reference radiation at low dose. 380 refs., 58 figs., 28 tabs