Radiation-induced thyroid carcinoma. Clinical course and results of therapy in 296 patients.

Of 5379 patients who had received radiation treatment at our institution for benign conditions of the head and neck, 318 developed thyroid cancer 3 to 42 years later. We report follow-up observations (median interval from diagnosis to recurrence or last contact, 10 years; longest interval, 31) for 296 of these patients. Three patients died of thyroid cancer and 40 had recurrences. The rate of recurrence during the 10 years after the detection of thyroid cancer, determined by life-table analysis, was 1.1%/yr. Factors that correlated with the risk of recurrence were tumor size, histologic type, number of lobes involved, vessel invasion, and lymph node metastases. The time from radiation exposure to the discovery of thyroid cancer (latency) and the age at diagnosis both inversely correlated with recurrence. The administration of thyroid hormones reduced the number of recurrences in patients with combined papillary and follicular cancer, but no reductions could be associated with the extent of surgery done or the prophylactic use of radioactive iodine ablation. We conclude that the course of radiation-induced thyroid cancer is the same as that of thyroid cancer found in other settings. We advocate a conservative approach to the treatment of small tumors that have no associated risk for thyroid cancer other than that they developed many years after the patient had been exposed to radiation.