Theories of the Biological Actions of Ionizing Radiations

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Abstract
J. A. Crowther''s consideration of 2 main categories of explanation of biologic action of X-rays is reviewed, namely, the "poison" and "target" hypotheses. Crowther believed a "hit" was registered when a pair of ions was produced anywhere within the sensitized particle. He rejected the "poison" hypothesis. The late D. E. Lea made the "target hypothesis" the "target theory" (1946). The author sets forth several reasons for re-consideration of the subject after re-examination of the chemical effects of radiation. Three mechanisms are discussed in detail. These chiefly concern electron transfer from the (OH)- to H+ ions and breakdown of an (H2O)+ ion followed by several alternative reactions and resulting in free H and OH radicals. The free radical hypothesis gives the chemical explanation of the so-called "protection effect," i.e., when 2 solutes, each of which is radiosensitive alone, are irradiated together in aqueous soln., one is preferentially destroyed and "protects" the other. It moreover gives a simple and consistent explanation of many known radiochemical reactions in vitro. It revives the "poison" hypothesis since release of free hydroxyl radicals must inevitably cause serious disturbance of cell metabolism. Reasons are given to stress that the target theory and free radical theory are not mutually exclusive. Emphasis is laid on the difference in mechanism between ionization occurring in a chromosome and in intracellular fluid.

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