An alternative to absorbed dose, quality, and RBE at low exposures.

The microdosimetric distribution of event sizes, especially for small exposures and high-LET radiation, represents both a fractional involvement of the exposed cell population and variable amounts of energy transferred to the "hit" cells. To determine the fraction of cells that will respond quantally (be transformed) after receiving a hit of a given size, a hit size effectiveness function (HSEF) which appears to have a threshold has been derived from experimental data for pink mutations in Tradescantia. The value of the HSEF at each event size, multiplied by the fractional number of cells hit at that event size, and summed over all event sizes, yields a single value representing the fractional number of quantally responding cells and thus the population impairment for a given exposure. The HSEF can be obtained by unfolding (deconvoluting) several sets of biological and microdosimetric data obtained with radiation of overlapping event size distributions.