A radioactive form of iodine, often used for imaging tests or as a treatment for thyroid cancer and certain other cancers. For imaging tests, the patient takes a small dose of radioactive iodine that collects in thyroid cells and certain kinds of tumors and can be detected by a scanner. For treatment of thyroid cancer, the patient takes a large dose of radioactive iodine, which kills thyroid cells. Radioactive iodine is also used in internal radiation therapy for prostate cancer, intraocular (eye) melanoma, and carcinoid tumors. The radioactive iodine is given by infusion or sealed in seeds, which are placed in or near the tumor to kill cancer cells.