Theories of the Biological Actions of Ionizing Radiations

Abstract

Almost exactly ten years ago, in the Twentieth Silvanus Thompson Memorial Lecture, Professor J. A. Crowther summed up the theories of the mode of action of ionizing radiations then current in the following words:— “Explanations of the biological action of X rays fall into two main categories, which for convenience of reference we may label the ‘poison’ hypothesis and the ‘target’ hypothesis. The poison hypothesis postulates that the radiation produces, by photochemical action on the substance of the nucleus, a certain concentration of some unspecified poisonous material which, by diffusion to the structures concerned, results in the changes which are actually observed. The target hypothesis focuses attention on the discontinuous nature of X-ray absorption, and assumes that action takes place whenever one of the biological structures is ‘hit’ by the radiation… My own suggestion… is that a ‘hit’ is registered when a pair of ions is produced anywhere within the sensitive particle” (Crowther, 1938). In the light of the data available, Crowther rejected the “poison” hypothesis, since only the “target” hypothesis could be reconciled with all the experimental facts. During the intervening ten years the subject of radiobiology has made rapid progress and a considerable body of experimental evidence has been accumulated. In the hands notably of the late Dr. D. E. Lea and his collaborators, the target “hypothesis” has become the target “theory”, elaborated very clearly in his book, Actions of Radiations on Living Cells (Cambridge University Press, 1946).