Effect of radiation dosage on the synergism between radiation and estrogen in the production of mammary cancer in the rat.

An experiment was designed to determine the effect of radiation dosage on the synergism between radiation and estrogen in the production of mammary cancer. Female A x C rats had their left mammary chain exposed to graded doses of X-radiation while under anesthetic with urethan or sodium pentobarbital. The rats had previously been hysterectomized and exposed to estrogen by implantation with cholesterol pellets containing 25% diethylstilbestrol. The control animals were implanted with cholesterol pel lets containing no diethylstilbestrol or were not implanted with pellets. Some of the animals died in the 24 hr following irradiation. These deaths were independent of the anesthetic used, but there was a positive dose-re sponse curve for radiation. In terms of median survival time, the highest doses of radiation produced the shortest survival times, and those animals receiving estrogen also had the shortest survival times. Overall growth was also inhibited by both of the agents used. With radiation alone, there was an increase in benign tumors with a dose-response curve, but the increase in mammary cancers was not significantly pro portionate to dose. No significant difference was ob served in the incidence of tumor in any of the glands in each mammary chain. The cholesterol carrier in the pellets used did not affect the synergism between estrogen and radiation for mam mary carcinogenesis; however, synergism between radia tion and diethylstilbestrol administration is apparent. Ma lignant mammary tumors were relatively rare in groups that did not receive diethylstilbestrol. The synergism is dependent for its optimal expression on the amount of radiation given.