Radiation exposure is the only known etiologic factor clearly associated with an increased risk of thyroid cancer. External beam radiation treatment for medical therapy, acute gamma ray exposure from environmental sources (nuclear weapons, nuclear power plant accidents), and ingestion of short-lived radioactive iodine isotopes are the primary sources of radiation exposure, which increases the risk of benign and malignant thyroid neoplasia. Relative risk is directly proportional to the exposure dose and is inversely proportional to the age at exposure.