Radiation-induced thyroid cancer Authors: Section Editors: Deputy Editor: INTRODUCTION Radiation exposure of the thyroid during childhood is the most clearly defined environmental factor associated with benign and malignant thyroid tumors. The risk of thyroid cancer following irradiation is related to radiation dose and age (greater for children exposed early in life), and the risk persists throughout life. Radiation exposure during childhood also increases the risk of benign thyroid nodules and hypothyroidism. The purpose of this topic is to review the evaluation of a patient with a history of childhood radiation exposure and how such a history affects treatment of patients with thyroid tumors. Radiation injury to other organs is reviewed separately. (See and .) RADIATION EXPOSURE Type of exposure -- Radiation exposure may be external or internal. *External - The predominant types of external radiation are diagnostic radiographs, therapeutic radiation for the treatment of cancer, and historical use of external radiation to treat a wide variety of nonmalignant conditions. External radiation also includes brachytherapy, whereby a sealed radiation source is placed adjacent to a treatment area. *Internal - Internal radiation exposure includes ingestion of foods or liquids contaminated with radioactivity or by inhalation of radioactive gases or paicles. Internal radiation occurs after exposure to nuclear fallout (from testing and accidents at operating nuclear power plants or above ground nuclear explosive testing) or after ingestion of radioiodine for diagnostic tests and for therapy of hypehyroidism.