Effects of X‐Irradiation on N‐Methyl‐N‐nitrosourea‐induced Multi‐organ Carcinogenesis in Rats

The effects of X‐irradiation on N‐methyl‐N‐nitrosourea (MNU)‐induced multi‐organ carcinogenesis were examined in both sexes of ACI/N rats. At 6 weeks of age, rats in groups 1 (25 males, 25 females) and 3 (24 males, 23 females) received a single i.p. injection of MNU (25 mg/kg body weight), while those in groups 2 (25 males, 26 females) and 4 (25 males, 25 females) were administered the carcinogen at a dose of 50 mg/kg body weight. At 10 weeks of age, groups 3 and 4 were X‐irradiated at a dose of 3 Gy. Group 5 (24 males, 24 females) received X‐irradiation alone. Group 6 (21 males, 21 females) served as an untreated control. As a result, neoplasms developed mainly in the digestive tract, kidney, uterus, and hematopoietic organ in groups 1–5. The incidences of adenocarcinoma in small and large intestines of male rats of group 4 (50 mg/Ag MNU and X‐irradiation) (small intestine: 48%, large intestine: 32%) were significantly higher than those of group 2 (50 mg/kg MNU) (small intestine: 17%, P<0.05; large intestine: 8%, P<0.05), and also the frequency of adenocarcinoma in the large intestine of males of group 3 (25 mg/kg MNU and X‐irradiation) (22%) was significantly greater than that of group 1 (25 mg/kg MNU) (0%, P<0.O5). These results indicated that X‐irradiation enhanced the development of intestinal neoplasms induced by MNU in male ACI/N rats.