1166 RADIATION BIOLOGY 



by irradiation are predominant. Furth and Furth (1936) mention also 

 an increase of myeloid leukemias, but this lacks confirmation. The type 

 of leukemia induced by irradiation may be influenced by the state of 

 the hemopoietic organs at the time of irradiations. This is suggested by 

 the observations of Hueper (1934). 



The factor of age was studied by Kaplan (1948). The maximum 

 incidence of leukemia occurred in mice of strain A when irradiated at 

 one month (29.0 per cent), with a sharp decrease at two months (2.8 per 

 cent) and later (at 3 months 10.9 per cent, at 4 months 6.2 per cent, at 

 6 months per cent). In mice irradiated at two weeks of age the 

 leukemia incidence was 14.3 per cent. The variability of values is prob- 

 ably due to the small number of animals used. These observations were 

 confirmed by him with C57 black mice. 



The influence of age and dose rate on leukemia induction was investi- 

 gated by Brues et al. (1949). Throughout most of the life, the suscepti- 

 bility to irradiation-induced lymphoma approximately doubled at inter- 

 vals of 80 days. After 700 days of age no further increase in morbidity 

 rate was observed. Using different patterns of X-radiation treatment, it 

 was found that 400 r total-body X radiation is more effective if divided 

 over a 10-day period than if given either as a single dose or divided over 

 40 days. Increasing doses are more effective until a saturation value is 

 reached (Brues et al, 1949). There is also evidence for a threshold dose 

 or dose rate below which leukemia is not induced. 



Lorenz et al. (1947), investigating the effect of chronic exposure to 7 

 radiation of radium, induced malignant lymphomas in mice at a rate that 

 was roughly dependent on the dose rate. The greater the dose rate, 

 the earlier the appearance of the tumors. Their results are illustrated by 

 Fig. 18-1. This shows that long-continued daily exposure of female 

 LAFi mice to 8.8 r daily (8 hours) to the 7 radiation of radium greatly 

 increased the incidence of leukemia and hastened its onset. The total 

 dose received by these animals over a period of twenty-three months was 

 5900 r. Daily exposure to 4.4 r had a similar but less marked effect, 

 while smaller doses (2.2 r daily or less) did not increase the incidence of 

 leukemia in comparison to untreated controls. The strain used had a 

 very high incidence of spontaneous leukemia appearing mainly late in 



life. 



Rats were exposed by Metcalf and Inda (1951) to doses of 0.1-10 r per 

 day during a period of two years. Their findings suggest that leukemia 

 may develop in this species as a result of such irradiations. 



P 32 has carcinogenic properties comparable to those of X radiation 

 (Brues et al, 1945). The rate of tumor production with both is roughly 

 proportional to the corresponding lethal doses. P 32 is effective whether 

 given in a single dose or in monthly divided doses. 



The carcinogenic effect of slow neutrons in the thermal column of the 



