Induction of colonic adenocarcinoma in the rat by X-irradiation

In work to determine whether X-radiation could be used to induce tumors of the colon in outbred Holtzman rats, a technique was devised so that only the descending colon could be irradiated with a collimated X-ray beam and tumorigenic exposures in the kilo-Roentgen range were delivered. Ninety male Holtzman rats were divided into 6 exposure groups of 15 animals each. In the first group only the colon was exposed to 2500 R; in the second group the colon was exposed to 3500 R and the exposures were increased by 1000-R increments up through 6500 R; the sixth group was sham irradiated. Animals in each of the irradited groups developed mucoid adenocarcinomas with the highest incidence (47%) obtained inthe 4500-R group. Histopathologically, the induced tumors bear a close resemblance to human colon adenocarcinoma, and the development of pulmonary metastases in one of the animals with multiple primary tumors demonstrated their malignancy. The usefulness of the animal model is discussed.