Detection and treatment of pheochromocytomas and paragangliomas: current standing of MIBG scintigraphy and future role of PET imaging

Pheochromocytomas are rare tumors arising from chromaffin cells of adrenal medullary or extra-adrenal paraganglionic tissue. These tumors are characterized by synthesis, storage, metabolism and secretion of catecholamines. Similar to the sympathetic nervous system, pheochromocytomas express cellular norepinephrine transporters (NET) through which catecholamines can enter pheochromocytoma cells to be stored in vesicles. Metaiodobenzylguanidine (MIBG) resemblance to norepinephrine and its good affinity and uptake by NET resulted in its use in pheochromocytoma diagnosis from 1981. Both [(123)I]MIBG and [(131)I]MIBG (lower sensitivity) scintigraphy are used for localization of these tumors. Recent discoveries of different hereditary syndromes associated with pheochromocytomas led to the identification of several and new distinct genotype-phenotype associations. Importantly, with this distinction of clinical phenotypes, MIBG was found to have a different performance in subsets of pheochromocytoma patients. Reduced sensitivity of MIBG scintigraphy in some familial paraganglioma syndromes, malignant disease and extra-adrenal paragangliomas has been found. Therefore, newer compounds, especially for positron emission tomography (PET), such as [(11)C]hydroxyephedrine ([(11)C]HED), [(18)F]fluoro-2-deoxy-D-glucose ([(18)F]FDG), [(18)F]fluoro-dihydroxyphenylalanine ([(18)F] FDOPA) and [(18)F]fluorodopamine ([(18)F]FDA) have emerged and were found to be superior to MIBG in the localization of certain types of pheochromocytoma and paragangliomas. Finally, using [(131)I]MIBG represents an important treatment option in patients with malignant pheochromocytoma, but the development of newer treatment modalities is expected. In this review, we provide the reader with an overview of the current standing of [(123)I]- and [(131)I]MIBG in diagnosis and treatment of pheochromocytoma amongst the newer PET imaging agents.