RADIATION-INDUCED LIFE-SPAN SHORTENING IN MICE 



279 



an irradiation of 1,100 r of X-rays given in four doses (3 x 300 r -j- 1 x 200 

 r) at 3-weekly intervals since the magnitude of the life-span shortening of 

 this treatment (see Fig. 1) was well-known to us (Alexander and Connell, 

 1960). More than 90% of the animals were alive 30 days after the 

 last irradiation. Under these irradiation conditions malignant disease con- 

 stitutes only a small fraction of the deaths. The post-mortem results showed 



End of 

 treatment 



o 



E 



25 



50 



75 



100 



" Control unirradiated 



--— Total of 1100 r ^■- 



50 mice in each experiment 



J 



100 



600 



700 800 



Fig. 



200 300 400 500 



Age (days) 



1. Mortality curves for CBA male mice after 1,100 r of X-rays 



that only 30% of the animals had tumours, or leukaemia, other than the 

 benign hepatomas (see Table I on p. 261 of our other paper in this symposium). 

 This experiment therefore represented a typical example of Life-span shorten- 

 ing which is so often called radiation-accelerated ageing. 



EXPERIMENTAL 



Male CBA mice aged between 11 to 14 months at the time of the first 

 irradiation were used. The animals were exposed at a dose-rate of 55 r/min 

 to 300 r -I- 300 r -F 300 r -I- 200 r of 250 kV X-rays at 3-weeldy intervals. In 

 all respects the experimental conditions were exactly the same as those 

 reported earlier (Alexander and Connell, 1960). After the last irradiation the 

 animals were divided into groups and designated to be killed at a specified 

 date. Batches were killed at 3, 6 and 9 months and thereafter at monthly 

 intervals. It was intended that there should be ten mice alive in each batch 

 at the time killing was due. Consequently, allowance had to be made for 

 intercurrent deaths. From our earlier data on the effect of radiation on the 

 sur\dv^al curve the number of animals that had to be allocated to each 

 particular batch could be calculated. 



