Physical aspects of radiation sensitivity

Abstract

A characteristic feature of ionizing radiation is the high energy concentration that occurs in the tracks of charged particles regardless of the level of absorbed dose. It is this feature that accounts for cellular radiation sensitivity at low doses. A survey is first given over the energies required to inactivate various microorganisms, and these energies are then related to the average number of DNA single strand and double strand breaks that are produced at the mean inactivation doses. It is pointed out that the production of DNA double strand breaks is always a single particle effect except at very high doses or in aqueous solution. However, a consideration of sigmoid dose effect curves and of the LET dependence of various biological effects indicates that a synergism of energy transfers or of radiation induced sublesions occurs over much larger distances. Individual double strand breaks cannot, therefore, be the lesions responsible for cellular radiation effects. However, they may be the sublesions that combine to produce the observed effects. The microdosimetric analysis that permits an estimation of the interaction distances of sublesions and the earlier analysis by Lea 16 are described in their essentials. A more recent analysis, based on an explicit description of the spatial correlation of energy transfers in charged particle tracks, is also discussed. This analysis utilizes the so-called “proximity functions.” The use of these functions is exemplified by the application to a recent experiment of Rossi et al. 19 where cells are exposed to pairs of deuterons with variable lateral separation.