130 RADIATION BIOLOGY 



minute amounts of some relatively simple chemicals, such as vitamins and 

 hormones. Therefore it stands to reason that the chance of an eventual 

 observable "event" is particularly high whenever an activation happens 

 to occur within, or near to, the key elements of an organism. For 

 example, a charged particle which traverses the nucleus of a cell seems 

 a priori much more likely to produce a genetic effect than a particle which 

 crosses the cytoplasm only and misses the nucleus. 



This concept has been elaborated upon in a variety of more or less 

 detailed working models of the action of radiation. The core of these 

 theoretical argumentations lies in the idea that the statistical fluctuations 

 in the occurrence of biological effects reflect primarily the statistical 

 accidents in the location of the primary physical effects. The statistical 

 fluctuations of radiobiological effects may be visualized as similar to the 

 fluctuations in the casualties among a troop of men exposed to a barrage 

 of rifle or mortar fire. The severe casualties correspond to those indi- 

 viduals which happen to have been hit, and hit in some particularly 

 sensitive spot. 



Because of this analogy the whole hue of argumentation goes under the 

 name of "hit theory" or "target theory" of radiobiological actions 

 {Treffertheorie in German). The argumentation appears particularly 

 suggestive in the circumstances considered thus far, i.e., when the dose- 

 effect curves are exponential, but it can be extended to include more 

 complicated statistical relations. The target theory has given rise to 

 considerable controversy. D. E. Lea's (1946) book on the action of 

 radiations on the living cell constitutes a major effort to describe the 

 main results of radiation biology within the frame of the target theory 

 (see also Timofeeff-Ressovsky and Zimmer, 1947; Buzzati-Traverso and 

 CavalH, 1948). Most of the material discussed in that book is reviewed 

 again in the following chapters. 



In a restrictive formulation of the target theory it has been assumed 

 that a definite biological effect, e.g., a specific mutation or even the death 

 of a cell, results whenever an ionization occurs within some well-defined 

 submicroscopic structure such as a gene. According to this hypothesis, 

 the structure itseff constitutes a "sensitive volume," whose magnitude 

 coincides with the "effective volume" for production of the effect by an 

 ionization. Timofeeff-Ressovsky, Zimmer, and Delbruck (1935) sug- 

 gested that an ionization within a group of about 1000 atoms may produce 

 a specific mutation from "eosin" to "white" eye color in a fruit fly sperm. 

 The model affords an attractive opportunity for determining the size of 

 key submicroscopic structures by means of radiobiological studies. Con- 

 firmation of the model and determination of further details concerning 

 the submicroscopic structure was expected from comparative studies of 

 the effectiveness of different radiations (see Sect. 5-5). Thus the study 



