Direct and Indirect Action of Radiation

Abstract

In the previous chapters, an attempt has been made to draw conclusions as to the nature of the radiation lesions by applying formal physical and mathematical procedures to the interpretation of dose-response curves, and by investigating the LET-dependence of radiation sensitivity. It was assumed that such a thing as a well defined target really exists. Basically, this assumption seems to be justified, since the formal definition of target used in the hit theory was so general that difficulties are only encountered when attempts are made to identify the target with sensitive biological structures. However, as this is the main theme of the target theory, it is necessary to consider the extent to which realistic targets can be obtained from dose-response curves. The concept of a target does not make any allowance for damage from the “outside”, which is the rule rather than the exception. If the concept is to be retained, then the fraction of the energy contributed from outside to the target must be determined. This leads to the concepts of direct and indirect effect as outlined in Chapter 1.3. Such a classification is meaningful only at the molecular level, where the chances of distinguishing between these two effects are greatest. If the absorption of radiation occurs in the molecule in which the lesion appears, then this is the direct action of radiation,while with indirect action the absorption of the radiation energy and the response to this energy occur in different molecules. This definition is considerably more rigorous than the one used in the past, in which the irradiation of dry systems was considered as direct, while the indirect effect was considered to occur predominantly in the presence of water.