. Cellular basis and aetiology of late somatic effects of ionization radiation. Radiation -- Physiological effect. 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 res
. Cellular basis and aetiology of late somatic effects of ionization radiation. Radiation -- Physiological effect. 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 --â Total of 1100 r ^â - 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
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