Meta-iodobenzyl guanidine for detection and staging of neuroendocrine tumors

Gastroenteropancreatic neuroendocrine tumors (GEP-NET) are slow-growing neoplasms that arise from the neuroendocrine system of the gastrointestinal tract and pancreas. These may be classified based on location into the following: pheochromocytomas and parangangliomas; carcinoids; and pancreatic endocrine tumors. The majority of these tumors are nonfunctional, and thus, molecular imaging methods are critical in detection and staging of disease. Meta-iodobenzyl guanidine (MIBG) is a norepinephrine analog taken up by norepinephrine transporters that are overexpressed in the majority of GEP-NET. Radioactive MIBG can be used to image GEP-NET. The isotopes suitable for imaging include iodine-123 and iodine-131, using single-photon cameras, and iodine-124, using positron emission tomography (PET). Imaging is usually carried out a day or more after administration of the radiotracer, and serial and tomographic imaging may be necessary for optimal delineation. MIBG imaging is more useful for detecting pheochromocytoma, with reported accuracies greater than 80%, than for detecting carcinoid tumors, where the accuracy has been ∼70% and is reportedly higher in mid-gut tumors. MIBG imaging has been invaluable in the accurate staging of children with neuroblastoma, a lethal childhood tumor of the sympathetic nervous system. An important application of MIBG imaging is to demonstrate targeting of therapeutic I-131 MIBG. Imaging is thus useful in the detection of disease as well as in the demonstration of adequate targeting for therapy — either qualitatively or quantitatively with dosimetry. The latter will probably be feasible with PET using isotopes like iodine-124, and perhaps with single photon emission computed tomography/computed tomography. Imaging with MIBG will continue to be the mainstay for detection and staging of GEP-NET. More importantly, perhaps, imaging with MIBG will form part of an imaging continuum, including assessment of glycolytic rate and somatostatin receptor status, that will enable assessment of tumor phenotype and guide management.