Each year in the US, about 200 000 patients receive therapy with radionuclides, most commonly in the form of sealed sources for treating gynecological and head and neck cancers and radiopharmaceuticals for treating thyroid cancer. Known as brachytherapy, this kind of treatment has attracted a resurgence of interest in the medical world, primarily because it offers a simple procedure for delivering high radiation doses to a tumor but minimal doses to the surrounding healthy tissue. Brachytherapy can provide this optimal dose distribution because radiation sources are implanted either in the tumor or very close to it. (Brachys is Greek for “near.”) This advantage is not shared by external beam therapy, in which the source of radiation is about 1 m away from the patient.