Effect of 3-Methylcholanthrene and X IrradI- ation, Given Singly or Combined, on Rat

SUMMARY-Female Sprague-Dawley rats were given a single, intra-gastric administration of 40 mg of 3-methylcholanthrene on the 40th day of age, or 400 R of total-body X radiation on the 50th day, or both, or the sequence of administration of the two carcinogenic agents was reversed. The percentage of rats with one or more mammary adenocarcinomas and the total number of mammary adenocarcinomas were obtained when the animals were killed at 140 days of age. The results were interpreted to mean that X irradiation and 3-methylcholanthrene were additive.-J Nat Cancer Inst 38: 73-77,1967. THIS STUDY was designed to test the interaction of a chemical carcinogen, 3-methylcholanthrene (3-MCA), and a physical carcinogen, X radiation, by use of the mammary adenocarcinoma response of the female rat to these 2 carcinogens. A single per os administration of 3-MCA induces mammary adenocarcinomas rapidly and frequently (1), whereas a single exposure to total-body X irradi-ation also induces mammary adenocarcinomas (2). Although some differences between the mammary neoplastic responses to the 2 carcinogens exist (3), enough similarities make studies of the possible interactions of the 2 carcinogens meaningful (3-5). Two previous reports suggested that some interactions of an additive nature between the 2 carcinogens obtain, but the results were not com-pletely clear-cut, either because all 3 possible combinations of the 2 carcinogens were not studied (4) or the 3-MCA dose was so large that any additional effect of X irradiation was difficult to measure (5). Finally, it appears that the sequence of carcinogen applications has not been reversed, that is, X irradiation given before 3-MCA has not been compared to X irradiation given after 3-MCA. This report compares the mammary adenocar-cinoma response in the female rat either to 3-MCA or to total-body irradiation, or to the combination of both carcinogens given in either order. MATERIALS AND METHODS Female weanling rats were obtained fro