LEUKAEMOGENIC EFFECT 31 



infection originating in the skin ulcers mentioned above. No predilection of one 

 of the experimental groups for this skin disease was observed, nor was there 

 an apparent relation between this condition and the development of thymic 

 lymphomas. 



The number of animals dying, or killed when moribund, with thymic 

 lymphomas is shown in Figs. 1 and 2 as a function of time after initiation of 

 the experiment. The difference between the number of 18 thymomas in the 

 ^°Co-y-irradiated groups I-III observed within 250 days versus none in all 

 the other groups including controls is statistically significant (P < 0-01, 

 Chi-square test). The incidence of thymic lymphomas per cage (groups of 8 

 animals) ranged from zero to 4. One ^°Co-y-irradiated mouse developed a 

 mammary gland carcinoma within 8 months after exposure. 



DISCUSSION AND CONCLUSIONS 



From these data it can be concluded that under the conditions of the 

 experiment the incidence of thymic lymphomas in mice within 250 days after 

 fractionated whole-body ^°Co-y-irradiation (3 x 160 r at 7 days intervals) was 

 significantly higher than after three subcutaneous injections at weekly 

 intervals of ^HTDR (3 x 10 ^ac/g body weight) or ^HCR (3 x 10 ^c/g body 

 weight) or both. Up to 250 days after initiation of the experiment no difference 

 in the development of thymic lymphomas could be observed between the 

 untreated controls and the animals given tritiated pyrimidine nucleosides 

 as mentioned above. Whether ^HTDR, ^HCE, or both, given to female 

 57CBL/6J mice in three weekly doses of 10 /itc/g body weight at the age of 6 to 

 8 weeks promotes the incidence of this particular neoplastic disease, can only 

 be evaluated when the animals have been observed over their entire life-span. 

 The most important period in this respect will be around 400 days after first 

 exposure, the average latent period reported before thymic lymphomas 

 appear in low-dose irradiated female C57BL mice (Kaplan and Brown, 

 1952). No comment of course is as yet possible as to an eventual influence of 

 these tritiated compounds on life-span. 



If it is true that in CAF mice a single injection of ^HTDR (specific activity 

 360 mc/m mole in a dose of 1 /xc/g body weight does have a tumorigenic effect 

 (Lisco et at., 1961), then our results give rise to some questions. The lowest 

 dose of ionizing radiation given to mice as a sliort whole-body irradiation 

 reported to have given a tumorigenic effect, other than the production of 

 ovarian tumours, is of the order of 200 r (Kaplan and Brown, 1952; Furth et al., 

 1959). To produce increased numbers of tumours by continuous whole-body 

 y-irradiation, excejst for the induction of ovarian tumours, daily doses in the 

 order of 0-11 r were necessary (Lorenz et al., 1955). If a single dose of 1 /xc 



