A drug containing a form of radioactive iodine called I 131 that is used to treat malignant pheochromocytoma and paraganglioma. It is used in patients who need treatment with systemic anticancer therapy and whose tumor cells take up radioactive iobenguane and are detected by a scanner. 131I-MIBG is used to treat pheochromocytoma and paraganglioma that cannot be removed by surgery, are locally advanced, or have spread to other parts of the body. It is also being studied in the treatment of other types of cancer. 131I-MIBG builds up in tumor cells and gives off radiation that may kill the tumor cells. It is a type of radioconjugate. Also called Azedra, iobenguane I 131, and iodine I 131-metaiodobenzylguanidine.