274 



R. H. MOLE 



Figure 1 shows tliat mice aged 870 days wliicli survived the acute effects 

 of 650 to 850 rads of X-rays did not have their lives shortened at all as com- 

 pared with specific controls. Similarly 670-day and 470-day-old mice did not 

 have their lives shortened by similar doses of ~ 750 rads, although at slightly 

 higher doses in the upper range of acutely lethal exposures there was quite 

 definite life-shortenmg. Life-shortening was clearly not a linear function of 

 dose and the results do not suggest that radiation and chronological ageing 

 are additive in their efi^ects. 



Table I. Survival of female CBA mice of different ages exposed to 10 r 



y -radiation nightly continuously till death 



(Mole and Thomas, 1961) 



t Taken to equal the observed mean survival time of the specific controls for each age 

 group. The experiment is not quite complete: the results in brackets are provisional. 



These results are easier to understand if it is accepted that radiation 

 exposure initiates ultimately lethal processes which take time to develop to 

 the bulk or intensity which actually kills (cf. tumour development time, 

 Mole, 1962). If so, the 150 days which 870-day-old mice have still to live is 

 presumably too short for the full development of a killmg lesion. However 

 this line of argument leads to the conclusion that 280 days and 470 days are 

 also too short a period for the full development of kiUmg lesions in 670- and 

 470-day-old mice respectively. Alternatively the rate at which killing lesions 

 develop may be thought to depend on the magnitude of the dose. In either 

 case plain survival time or reduction in after-expectancy of life cannot be 

 expected to be any simple function of radiation dose. 



With continuous irradiation till death at 10 r nightly (Table I) both the 

 absolute amount of life-shortening and the proportion by which the life 

 expectancy was reduced were smaller the older the animals at the start of 

 their exposure over the range from 70 to 470 days. At still older ages life- 

 shortening was just detectable but was very small, in spite of the magnitude 

 of the accumulated doses. Again there was no additive or synergistic effect of 

 y-radiation and natural ageing. 



