498 L. H. GRAY 



significantly raised the incidence of lymphoma in mice (7). Whole 

 body exposure to 7 radiation also resulted in a raised incidence of 

 lung tumors, ovarian tumors, and tumors of the mammary gland in 

 mice (8). Radiation-induced tumors were also observed in guinea 

 pigs and rabbits, two species in which spontaneous neoplasms are 

 rare. The incidence of ovarian tumors in mice was particularly 

 striking as this was already evident when the dose received each 

 day was only 0.1 r. 



It has long been known that bone sarcomas in man, as well as in 

 experimental animals, may result from the fixation in the skeleton of 

 quantities of radium of the order of a few micrograms and it has also 

 been suspected that lung cancer among the workers in the uranium 

 mines at Joachimsthal is an industrial disease resulting from exposure 

 to the radiations from inhaled radioactive material. In neither of 

 these cases is it possible to say with certainty whether the a rays or 

 the j8 rays are the effective agent. 



Lisco, Finkel, and Brues (9), however, have demonstrated that a 

 high incidence of osteogenic sarcoma may result from chronic ex- 

 posure to the iS rays of Sr^^ and that Pu^^^, which emits only a rays, 

 frequently gives rise to malignant fibrosarcomas at the site of injec- 

 tion. There can thus be no doubt that both types of radiation are 

 carcinogenic but little is known regarding the relation between the 

 radiation dose and the tumor incidence. It is noteworthy, however, 

 in the experiments in which Sr^' was administered, that although 

 there was a latent period during which no tumors were seen, there was 

 no evidence of the existence of a threshold dose rate below which 

 tumors were not produced at all. Moreover, the proportion of ani- 

 mals in which tumors were produced was found to increase as the 

 square of the time from the end of the latent period. If the same is 

 true in man, significant tumor incidence may result from chronic 

 exposure to very low dose rates. For this practical reason, if for no 

 other, the subject is one that requires much more investigation and 

 it is highly desirable that both a and jS radiation should be employed 

 for such investigations, and that sources of radiation should be used 

 that deliver well defined doses of radiation to the tissues under inves- 

 tigation. 



6. Ionizing Radiations as ]Mutagenic Agents 



Exposure to X rays and other ionizing radiations may cause mu- 

 tations at a rate greatly exceeding the spontaneous rate. This was 



