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G. W. CASARETT 



reduction in acute lethal dose, and that the LD50 dose for irradiation de- 

 creases with increasing age (Hursh et al, 1958). Limited experimental 

 experience with measurement of irreparable injury by decrement in radiation 

 LD50 at various times after brief total-body irradiation suggests the possibility 

 that the irreparable radiation injury may remain more or less constant, in 

 comparison with non-irradiated animals of the same age (Hursh et al, 1958), 

 at least until the point in time when age-dependent diseases increase con- 

 siderably in incidence prior to this occurrence in the non-irradiated population 

 (Fig. 2). After this time it seems probable that there may be greater differences 

 in LD50 between the two groups when compared at the same chronological 

 ages, but perhaps not when compared at the median death times. 



Cumulative irreversible injury 



Death 



Terminal diseases 



Accelerated ageing 

 (repeated doses) 



Precocious ageing 

 (single dose) 



Ageing injury 



Time 

 Fig. 2. Cumulative irreversible injury. 



If these observations prove to be true, and if it is true also that the 

 irreparable injury of irradiation is equivalent to ageing injury in an actuarial 

 sense, then in the absence of any effect of the irradiation on the incidence and 

 distribution of age-independent causes of death it may be expected that a 

 brief total-body life-shortening radiation exposure would result in a survival 

 curve which is similar in shape or slope to the control curve, but displaced to 

 an earlier time period in a manner compatible with the concept of precocious 

 ageing. 



This often seems to be the case in single-dose experiments. Sometimes, 

 however, the arithmetic survival curves of briefly irradiated groups become 

 flattened and must be considered compatible with either the concept of 

 accelerated ageing or of increased incidence of age-independent mortality or 

 a combination of precocious ageing and increase in age-independent mortality. 



On the other hand, protracted irradiation, e.g. daily or weekly irradiation 



