CONCErX AND CRITERIA OF RADIOLOGIC AGEING 



191 



tioii, this can result in change in survival curves and even in increased after- 

 survival. On the other hand, an activating effect on pre-existing chronic or 

 latent infectious disease can alter survival curves in other ways. The most 

 that can properly be said of a survival or mortality curve, is that it is com- 

 patible or incompatible with a supposed process (Comfort, 1959). 



Survival curves 



Time 

 Fig. 1. Survival curves. 



It is well-established experimentally in mammals that exposure of the whole- 

 body or parts of the body to ionizing radiation in substantial but sublethal 

 doses can shorten life-span. Numerous mathematical analyses and interpre- 

 tations of such data have been performed in relation to the ageing problem 

 (Blair, 1956; NAS-NRC, 1961; Brues and Sacher, 1952; Mole, 1957; Furth 

 et al., 1959; Neary et al., 1957; Sacher, 1955). In the case of partial-body 

 exposure, the life-shortening effect is variable in degree, depending on the 

 kind and amount of tissue irradiated as well as on the dose (NAS-NRC, 1961). 



The observation of radiation life-shortening, the reduced radiation LDgo 

 of irradiated groups as compared with their non-irradiated controls of the 

 same chronologic age, and the residual tissue damage and delayed effects of 

 irradiation, aU indicate that some of the radiation injury or its consequent 

 damage is irreparable. 



According to Blair's theory of radiological life-shortening (Blair, 1956), 

 the injury of ageing and the irreparable injury by irradiation are additive in 

 producing or contributing to radiation lethality, and the irreparable 

 component of the injury is equivalent to premature ageing (at least in an 

 actuarial sense), in that it ultunately deprives the animal of part of its expected 

 life-span. 



Limited experimental observations in this laboratory suggest that irre- 

 parable injury is detectable, after an interval of maximal tissue recovery as a 



