A drug containing a form of radioactive iodine called I 131 that is used to find or treat certain types of tumors, including pheochromocytomas and neuroblastomas. It is also used to relieve pain caused by cancer that has spread to the bones. Radiation from I 131 may help kill cancer cells or show where they are in the body. 131I-MIBG is a type of radioconjugate. Also called iobenguane I 131 and iodine I 131-metaiodobenzylguanidine.