Radiation-induced thyroid and other head and neck tumors: occurrence of multiple tumors and analysis of risk factors.

In 2561 patients who received conventional external radiation treatment before age 15 yr for benign conditions of the head, neck, and upper thoracic area, we found an increase in the number of salivary gland tumors and benign neural tumors of the head and neck area. To determine whether patients who developed these relatively uncommon tumors were more susceptible to the long term effects of radiation, we analyzed how frequently these patients also developed benign and malignant thyroid neoplasms. Compared to the remainder of the irradiated population, the patients with salivary and neural tumors had an increased frequency of thyroid disease (19.4% vs. 11.2% for thyroid cancer and 52.4% vs. 39.9% for all thyroid nodules). Various risk factors were studied to determine whether they were related to the eventual occurrence of thyroid disease. Higher doses, being female, and lower ages at time of treatment were associated with increased risk. However, these factors did not fully explain the differences in the frequency of coexisting tumors, suggesting that additional factors, such as radiation sensitivity, may account for the increased risk.