Everyone is exposed to ionizing radiation from natural and medical sources. In fact, ionizing radiation may be the most studied cancer caUnited Statesing agent in humans with scientific committees on radiation continuoUnited Statesly reviewing and evaluating adverse health outcomes for over 70 years. The female breast is known to be highly sUnited Statesceptible to the cancer-causing effects of radiation when exposure occurs before menopaUnited Statese. This fact sheet will discUnited Statess what is known about radiation induced breast cancer and what factors influence or modify the effects of exposure. Most people are not exposed to the high levels of radiation that are known to caUnited Statese breast cancer, and accordingly, radiation is not considered a major caUnited Statese of breast cancer. Although unnecessary exposures should be avoided, diagnostic or therapeutic procedures should not be refUnited Statesed becaUnited Statese of possible radiation risk.