J. F. LOUTIT 



two and a half years. It is against this low background that the leukaemias 

 experimentally induced l)y radiation are scored. 



Mice aged from eight to ten weeks have been irradiated with X-rays 

 either in single doses or by courses. The irradiation is applied to the whole 

 body; for it has long been known that if part of the mouse is shielded, 

 particularly a part containing active bone marrow which in the mouse is 

 pretty well universally distributed throughout the skeleton, the shielding 

 exerts a protective effect against the induction of leukaemia. The mice were 

 irradiated to predetermined total doses up to 1500 r. Figure 1 in Mole's 

 paper shows cumulative incidence of thymic leukaemia in the groups of 

 female CBA mice that ultimately received 1500, 1000, 750 and 500 r. The 

 overall exposure time was from two to thirty weeks. There is a striking 

 'threshold effect'. Thymic leukaemia was only induced by a total of 750 r 

 and more. Figure 2 which gives the data for non-thymic leukaemia shows 

 a completely different effect. In each group the incidence of non-thymic 

 leukaemia increased and doses of 500 r apparently had a greater effect later in 

 life than the higher doses. However the numbers at risk were smaller in the 

 groups given the larger dose. Mole admits that for this and other reasons 

 much more analysis is needed to see whether the best basis for classification 

 is clinical picture, morbid anatomy or cytological character. 



If one takes from Mole's work, some of it unpublished, the data for the 

 one dose of 750 r, this given in a single exposure causes about a five per cent 

 mortality due to the radiation syndrome. Survivors which live for another 

 18 months or so all show the signs of residual radiation effects — the greying 

 of hair, failure to attain normal weight and formation of cataract — but the 

 death rate from leukaemia is only very slightly raised. However, as one 

 may see from Figure 2, if the dose of 750 r is given in fractions over a few 

 weeks the total incidence of leukaemia may be 50 per cent or thereabouts. 

 This result was obtained in spite of the fact that the sequelae of greying of 

 the hair, diminished weight and formation of cataract were not prominent. 

 Such results as these are not at first sight in accord with the hypothesis for 

 linear relationship between total dose and incidence of leukaemia. 



Mole and his colleagues have also been irradiating CBA mice chronically, 

 either with fast neutrons in one of the Harwell piles or with y-rays from 

 ^"Co. Some of these results have been published by Neary et al.^, but much 

 still remains unpublished. Anyway, the incidence of leukaemia in these 

 mice chronically irradiated either by fast neutrons or by y-rays, is again 

 remarkably small, even though the integrated dose which they have received 

 is substantial. This is further evidence against a directly proportional 

 relationship between dose and effect. 



Results of this sort have led Mole to set up an experiment whereby mice 

 are irradiated with ^"Co y-rays at differing dose rates. This experiment 

 has not yet been running for a sufficient length of time for anything like a 

 comprehensive picture to have been obtained. Nevertheless the early results 

 were so striking that it was felt justifiable to report them at the recent 

 conference on the Peaceful Uses of Atomic Energy at Geneva^*'. The daily 

 dose, the total dose and the overall exposure period were the same for each 

 of these groups of mice. Mice of two strains were used and the results at 

 10 months after the start of the irradiation are given in Table 1. 



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