Retrospective and prospective study of radiation-induced thyroid disease

Abstract

Two hundred and sixty-three patients with a history of head and neck irradiation only and 153 with a history of irradiation and possibility of a thyroid abnormality have been evaluated and followed up during a two- to 12-year period. Radiation exposure was primarily for enlargement of the tonsils or thymus or for acne. Average absorbed thyroid dose was 451 rads. After irradiation at a mean age of 7.1 years, patients were examined at a mean age of 33.5 years. The interval from irradiation to diagnosis of cancer was significantly less than from irradiation to examination for the entire group. Among patients seen because of a history of irradiation only, 29.7 percent had a physical abnormality. Physical examination detected an abnormality in all but one patient with thyroid carcinoma; this tumor was occult and detected at operation for hyperparathyroidism. Carcinoma occurred in 37 percent of single nodules found on physical examination and 18 percent of glands with multinodularity. Thyroid scintiscanning was significantly less effective in identifying abnormalities than was physical examination. Ten of 38 patients with carcinoma and 15 of 71 patients with single nodules had normal results on thyroid scanning. Findings on physical examination were normal in one of 41 with cancer and four of 63 with “cold nodules.” One hundred and thirteen patients underwent operation; in most cases, near-total or total thyroidectomy was performed. Complications of operation were minimal. Forty-one thyroid carcinomas were detected. The average size was 1.7 cm. The tumors were mainly papillary or mixed papillary and follicular, and no anaplastic or medullary tumors were detected. Nodes were involved in 25 percent, and parathyroidal invasion occurred In 17 percent. Fifty-nine percent of the lesions were multifocal. Thirty-seven percent of tumors were 1 cm or less in size, and three of these had nodal metastases. Fifteen percent were less than 0.5 cm, and five of these six tumors were recognized incidentally during operation for other reasons. The incidence of new malignancies appears to decline among patients more than 35 years after radiation exposure, but this relationship is not significant as of this time. During prospective evaluation over an average of five years In 88 patients with or without continuous thyroid hormone replacement therapy, the incidence of detection of new lesions was not different. Among patients followed up while receiving replacement therapy who Initially had a thyroid abnormality, subsequent examination revealed no evidence of thyroid disease in at least one third. The data suggest that the abnormality Induced by radiation occurs at or close to the time of original irradiation, and that carcinoma is detected earlier than benign lesions. Prophylactic therapy with thyroid hormone, started one or two decades after irradiation, has little value in preventing new disease. Physical examination alone is adequate for detection of almost all significant disease. The size, histologic type, and progression of thyroid cancer in the group are similar to those in patients of comparable age without a history of irradiation.