A Comparison of the Carcinogenic Effect of Internal and External Irradiation on the Thyroid Gland of the Male Long-Evans Rat1

THERE has been increasing interest since the first case of thyrotoxicosis was treated with I131, in the possible carcinogenic effect of this form, or indeed any form, of irradiation of the thyroid gland. Evidence bearing on this possibility has been sought in various ways. Surveys have been made of patients, mostly adults, whose thyroidal disorders have been treated by radiation (1). Surveys have also been reported of the incidence of thyroid malignancy in patients who, as children, had received irradiation (2). Experimental series, comparable to the one to be presented, with somewhat conflicting results have also been published (3, 4, 5, 6, 7 and 8). In view of the possible significance of the first of these direct experiments with I131 (3) it was decided to repeat the work but also to extend the observations to compare the effect of internal and external irradiation.