1174 RADIATION BIOLOGY 



mammary carcinomas (Leitch, 1947). The incidence of benign mammary 

 tumors following X irradiation (Metcalf and Inda, 1951) was 10.8-34.6 

 per cent in female rats exposed to 0.1-10 r daily during a period of two 

 years and 8.3 per cent in the controls. There was a direct relation 

 between the daily dose of radiation and the number of benign tumors, 

 the smallest increase occurring among rats exposed to 0.1 r and the great- 

 est among those exposed to 10 r. Ovarian tumors were not seen in rats. 

 It is not likely that in the same species X radiation would induce benign 

 tumors, and neutrons, malignant tumors; the discrepancy is probably due 

 to differences in diagnoses. 



In guinea pigs of both sexes chronically exposed to 1.1 r for 8 hours 

 daily, mammary carcinomas occurred three to five times as often as 

 among the controls (Lorenz, 1950). These tumors were likewise not 

 associated with ovarian neoplasms. Thus the pathogenesis of the induc- 

 tion of mammary tumors in rats and guinea pigs requires an explanation. 



Those who practice "prophylactic" irradiation to prevent recurrence of 

 tumors after their surgical removal may find experimental support in the 

 work of Owen and Williams (1940), who exposed C3H mice, which have 

 a high spontaneous incidence of breast carcinoma, to 100-400 r at approxi- 

 mately four months of age. They observed a decrease of cancer incidence 

 from 65 per cent in the controls to 11-39 per cent in the irradiated mice, 

 the decrease in cancer incidence being a function of the dose. These 

 experiments are subject to different interpretations, and arguments can 

 be presented in favor of both indirect (hormonal) and local mechanisms 

 causing this change. Unfortunately, the animals were sacrificed too 

 soon. An increase in mammary tumor incidence occurs after a longer 

 period of time, when in the ovary the proliferation of estrogen-secreting 

 granulosa cells supersedes the degenerative changes (Furth and Furth, 

 1936). 



UTERUS 



Chronic irradiation significantly increased the incidence of uterine 

 carcinomas in rabbits chronically exposed to 7 radiation of radium 

 (Lorenz, 1950) ; the higher the dose rate, the earlier the tumor development 

 time. The tumors did not metastasize in the controls but did so in most 

 experimental animals, and it is possible that chronic irradiation may have 

 enhanced the dissemination process. All but one of twelve rabbits 

 exposed to 1.1-8.8 r daily developed a uterine carcinoma and seven of 

 these had metastases. Two of six controls had similar tumors but none 

 metastasized. None of three rabbits exposed to 0.11 r daily had tumors 

 because they died early of intercurrent disease. Kaplan and Murphy 

 (1949) described in mice a transplanted mammary tumor, which usually 

 does not metastasize but will do so when irradiated with doses insufficient 

 to destroy the tumor. Similarly, one of us (Furth, 1935) found that a 



