duce similar effects, as do many chemicals. The former presumably act 

 by increasing local reactivity and the latter by direct or indirect disrup- 

 tion of molecular integrity. 



THEORIES 



There are two major recognized hypotheses concerning mode of action 

 of ionizing radiations. These are commonly referred to as the "Target 

 theory" and the "Biochemical theory." The differences, however, rep- 

 resent differing viewpoints rather than fundamental theoretical ones. 



Target Theory 

 In its simplest form, this hypothesis views a quantum of emitted ra- 

 diation as a projectile, and the responding structure or element as a 

 target. On this basis the relationship between scored biological change 

 and radiation should be similar to the general relationship between phys- 

 ical targets and projectiles. For many kinds of biological effects such a 

 relationship does exist. Mathematical models based on this notion agree 

 well with observed responses such as cell lethality, chromosome break- 

 age, and mutation. The predictive and analytic values of this approach 

 are particularly well illustrated by the theoretical writings of D. E. Lea 

 (1955). The major weakness of the theory lies in the fact that it cannot 

 be extended reasonably to cover the effects of nonionizing radiation, 

 such as the mutagenic capacity of irradiated media for the induction of 

 bacterial mutants (Stone, et ah, 1947), or to the modifying effects of 

 many physical and chemical conditions. 



Biochemical Theory 

 Basically, this hypothesis views radiation-induced biological damage 

 as being the result of generation of highly reactive radicals through 

 absorption by suitable molecules of the incident radiation. Such radicals 

 then are presumed to have secondary effects on vital macromolecules. 

 In general, such an hypothesis would seem to be more compatible with 

 many modifying effects of chemicals as well as with indirect effects. 



CATEGORIES OF RADIATION EFFECTS 



Physiological 

 Although this category has become something of a catchall for effects 

 not assignable to disruption of some physical structure, certain com- 

 monly observed responses are well worth noting. It is quite clear that 



194 / CHAPTER 9 



