A COMPARISON OF THE CARCINOGENIC EFFECT OF INTERNAL AND EXTERNAL IRRADIATION ON THE THYROID GLAND OF THE MALE LONG-EVANS RAT1

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Abstract
A single intraperitoneal dose of 400 [mu]c of I131 in male Long-Evans rats maintained on low-iodine diet induced symptoms of hypothyroidism and partial or almost complete ablation of the thyroid. Only one thyroid neoplasm was found in rats so treated. This was a benign follicular adenoma. X-ray irradiation of similar rats, 900r in 300r doses, delivered in 5 days, did not induce hypothyroidism, nor was atrophy of the gland a striking feature. A large number of benign adenomas and tumors thought to be carcinomas of the "solid" type, were found. The control series, not treated, had neoplasms of these same types, but in smaller numbers than in X-ray-treated animals. Papillary and follicular carcinomas were not found in controls but were found in X-ray-treated rats. The only malignant tumor with metastases was found in an X-ray-treated rat, a follicular carcinoma with lymph node metastases. The coexistence of multiple endocrine neoplasms was noted. Benign and malignant neoplasms in other sites are reported.