External radiation and thyroid neoplasia.

Radiation remains the only factor that has been shown unequivocally to cause (nonmedullary) thyroid cancer. Recent advances include the analysis of the dose-response relationship using data pooled from multiple studies. This analysis confirms that radiation-induced thyroid cancers continue to occur, with a maximum risk at approximately 30 years after exposure. Physicians asked to evaluate patients with a history of radiation exposure should attempt to estimate the dose from the history and should be familiar with the other risk factors. For some individuals, screening should include thyroid imaging, but the results of such imaging, especially with thyroid ultrasound, should be interpreted with caution. The treatment of radiation-induced thyroid cancers is based on the observation that they appear to be no more aggressive than thyroid cancers not associated with radiation. In the future, more information should emerge about the role of cancer genes and susceptibility factors in radiation-induced thyroid cancer.