A radioactive form of iodine, often used for imaging tests or as a treatment for cancer. For imaging tests, the patient takes a small amount of radioactive iodine that collects in thyroid cells and can be detected by a scanner. For treatment, 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 and intraocular (eye) melanoma. The radioactive iodine is sealed in seeds, which are placed in or near the tumor to kill cancer cells.