An iodine radioisotope (MIBG, iodine-131-meta-iodobenzylguanidine) is another type of a radionuclear scan using a substance called MIBG. MIBG is very similar to the hormone dopamine or the hormone adrenaline, which are secreted by many of the neuroendocrine tumors.MIBG is particularly useful in localization of the primary tumors or metastatic lesions in pheochromocytoma and related tumors of the autonomic nervous syetem such as paraganglioma, and can also be used to find a rarer type of tumor like medullary carcinoma of the thyroid. How good is the MIBG? It has a very high specificity. How does MIBG scan compare with OctreoScan in carcinoid patients? For most carcinoid tumors MIBG is not as good but it is used in some patients where OctreoScan is unavailable or is negative.