MODIFYING FACTORS 347 



We have just seen that at present the absolute shapes and slopes of 

 dose-effect curves can yield information about mechanism only when we 

 are dealing with radiobiological actions in which /i is a constant and both 

 m/r and n/s are very small integers. On the other hand, we have seen 

 that in all actions the dose-effect relations must be determined by these 

 three quantities, even though one or more of them may vary from cell 

 to cell. Accordingly, we can often obtain useful information from rel- 

 ative slopes and shapes of dose-effect curves when they can be changed 

 by various modifying factors. 



Quantitative Changes Produced by Modifying Factors 



Referring to Figs. 1 and 2, let us see how a modifying factor could 

 change the dose-effect relations. It is conceivable that it might produce 

 a qualitative change in the nature of the decisive process, even though 

 the nature of the end effect might not be changed in any distinguishable 

 way. However, such a drastic modification of mechanism would seem 

 to be quite unlikely. Any changes in dose-effect relations are much more 

 likely to stem from quantitative changes bearing on decisive processes 

 and the decisive state. Let us inquire how such quantitative changes 

 might affect the shape or slope of a dose-effect curve. Since the curve is 

 determined by m/r, n/s, and h, the modifying factor clearly must operate 

 through changes in one or more of these quantities. Both m and n ob- 

 viously depend only on the natures of the biological object and of the 

 radiobiological end effect. On the other hand, r and s can depend not 

 only on these biological properties but also on the nature of the irradia- 

 tion. Accordingly m/r and n/s can vary with the nature either of the 

 biological system or of the phj^sical agent. The same may be said of h, 

 since it must be influenced by the morphological and chemical composi- 

 tion of the cell and since, as pointed out when h was defined, it can de- 

 pend on the values of r and s. Therefore, in summary, any of the param- 

 eters m/r, n/s, and h can be changed by any factor which, in any relevant 

 way, changes the morphological or chemical constitution of the cell or 

 affects the nature of the individual energy transfers or their distribution 

 in space and time. In the following paragraphs, let us bear in mind that 

 changes in m/r and n/s are reflected in alterations of the shape of the 

 dose-effect curve and that changes in h affect the slope. 



It is often observed that different species of cells, or even different 

 strains of the same species, yield different dose-effect curves, even though 

 the experimental conditions and the end effect studied are identical so 

 far as we can determine. Since even closely related but morphologically 

 or physiologically distinguishable types of cells must differ in chemical 



