PRINCIPLES OF RADIOLOGICAL PHYSICS 131 



of radiation action appeared as a sharp tool for the investigation of key 

 biological structures. 



Extensive studies of dose-effect curves under various conditions, stimu- 

 lated by the target theory, have produced results generally consistent 

 with the theory. However, these results are not of a crucial nature and 

 do not lend substantial support to the rather arbitrary working models of 

 the target theory. Thus, from this standpoint, the theoretical specula- 

 tions do not appear particularly fruitful. 



Moreover, indirect evidence unfavorable to the target theory has been 

 accumulating. If the randomness of biological effects depends primarily 

 on the random location of the primary actions, the correlation between 

 these random events should be fixed mainly by the geometrical character- 

 istics of the key structures of the organism. This is to say, that the 

 slope of the "dose-effect" curves should be essentially independent of 

 environmental factors such as the temperature or the biochemical con- 

 ditions of the organisms. 



To be sure, the demonstrated influence of any one environmental factor 

 may be interpreted from the standpoint of the target theory by a suitable 

 adaptation of the w^orking model. Nevertheless, the accumulation of 

 evidence which requires a special interpretation detracts from the 

 attractiveness of the initial working hypothesis. Detailed discussion of 

 the pertinent evidence is beyond the scope of this chapter. It is sufficient 

 to recall the demonstrated great influence of oxygen concentration on the 

 rate of action of radiation (see Bacq, 1951; HoUaender et al., 1951). 



It may also be added that closer analysis of the phenomena which dis- 

 play the most strikingly simple dose-effect curves has often brought to 

 light unexplained details (see Caspari and Stern, 1948; Fano, 1947). 

 Here, again, any single difficulty could probably be explained away but 

 the collective import of the evidence seems considerable. 



The statistical analysis of exponential dose-effect curves and the 

 expression of the rates of action as "effective volumes" or "effective 

 areas" are derived from the original development of the target theory 

 (Blau and Altenburger, 1923). Thence also is derived the name "single- 

 hit curves" for the exponential dose-effect curves. ("Single-hit" means 

 that the observed biological event is derived from a single physical 

 process, for example, from the passage of an ionizing particle.) 



The exponential curves and the "effective volume" were discussed in 

 Sect. 5-2c without reference to the concepts of the target theory, because 

 they involve only a deduction from experimental results (within the limits 

 of accuracy of the observations) and afford a convenient manner of 

 expressing those results. The introduction of more or less specific 

 hypothetical working models and the development of deductions based on 

 the models constitute quite different procedures of theoretical analysis. 



