Thyroid tumors associated with radiation exposure.

Epidemiologic studies of medically and environmentally exposed populations have been central to establishing ionizing radiation as a cause of malignant and benign thyroid tumors. Issues currently under investigation concern low dose effects, age sensitivity, the relative effectiveness of X-rays and iodine-131 in inducing thyroid cancer, and other risk factors. Excess thyroid tumors continue to appear in irradiated populations under study more than three decades after exposure.