Thyroid Neoplasms in the Rat: A Comparison of Naturally Occurring and I¹³¹-induced Tumors

Summary

1. Radiation injury of the thyroid glands of 198 Long-Evans rats, 1.5–3 years following single injections of 10–400 µc. I¹³¹, is described.

2. The degree of follicular atrophy and of cellular and nuclear pleomorphism of the thyroid parenchyma varied with the dose of I¹³¹.

3. Single or multiple benign thyroid adenomas were found in twenty internally irradiated glands. The highest incidence of these adenomas was found in rats injected with 10 and 25 µc. I¹³¹.

4. Follicular and papillary carcinomas were observed in five of the irradiated animals injected with 10, 25, and 100 µc. I¹³¹.

5. The benign and malignant thyroid neoplasms found in I¹³¹-irradiated rats originated as foci of nodular regeneration and hyperplasia.

6. The follicular and papillary thyroid carcinomas in the irradiated rats appeared to have developed in most instances in previously benign adenomas. Parenchymal, capsular, and vascular invasion occurred in these malignant neoplasms, but regional or distant metastases were not demonstrable.

7. Naturally occurring alveolar carcinomas had an incidence of 30–35 per cent in normal, control Long-Evans rats. The incidence was the same in rats given injection of the lower doses of I¹³¹ (10–25 µc.), but was considerably lower in rats that received the higher doses (100–400 µc.) of radioiodine. These spontaneous carcinomas are unlike those resulting from irradiation with I¹³¹ and are characterized by an alveolar pattern and a low degree of malignancy.