Synergistic Effect of Radiation on Colon Carcinogenesis Induced by Methylazoxymethanol Acetate in ACI/N Rats

The effect on colon and liver carcinogenicity in rats of a single X‐irradiation exposure given either before or after methylazoxymethanol (MAM) acetate was studied in ACI/N rats of both sexes. A single dose of X‐irradiation (3 Gy) was administered either 3 months before or after three weekly s.c. injections of MAM acetate (25 mg/kg body weight). At 365 days after the start, the incidence and multiplicity of MAM acetate‐induced intestinal tumors were enhanced by X‐irradiation either prior to or after the MAM acetate treatment. In addition, X‐irradiation before MAM acetate increased the incidence of hepatocellular foci in either sex. In females, X‐irradiation either before or after MAM acetate exposure decreased intestinal tumorigenesis. These findings suggest an apparent synergism of these agents in intestinal carcinogenesis of male rats.