The radionuclide molecular imaging and therapy of neuroendocrine tumors

Neuroendocrine tumors (NETs) represent a large group of neoplasms deriving from pluripotent stem cells or from differentiated neuroendocrine cells that are characterized by the expression of different peptides and biogenic amines. These rare tumors tend to grow slowly and are notoriously difficult to localize, at least in the early stages. Diagnostics involve blood, urine and biochemical examination as well as imaging modalities. Imaging is achieved by a variety of techniques such as radiological morphological imaging methods, for example, sonography, computerized tomography (CT)/magnetic resonance imaging (MRI), angiography and finally, nuclear functional imaging methods such as metaiodobenzylguanidine (MIBG), somatostatin receptor scintigraphy (SRS), vasoactive intestinal peptide receptor scintigraphy (VIPRS) and positron emission tomography (PET) using (18)F labeled deoxyglucose (FDG) and fluorinated dihydroxyphenylalanine ((18)F-DOPA) as a radioisotopic marker. (131)I-labeled MIBG is a well-established radiopharmaceutical for localization and therapy of phechromocytoma and paraganglioma. The majority of neuroendocrine tumors possess a high density of somatostatin receptors. This observation provided the basis for the development of various radiolabeled somatostatin peptide analogs as imaging agents and therapeutics in nuclear medicine. FDG-PET is now performed in a wide variety of tumors and indications, including diagnosis, staging, re-staging and evaluation of the response to treatment. (18)F-DOPA-PET may be useful if (18)F-FDG-PET scan result is negative. (99m)Tc-pentavalent dimercaptosuccinic acid ((99m)Tc-DMSA-V) or (99m)Tc sestamibi ((99m)Tc-MIBI) or (99m)Tc-tetrofosmin is used only for diagnosis of certain NETs such as medullary thyroid cancer. The expiences with other nuclear medicinie imaging and therapy modalities such as cholecystokinin (CCK)-B/gastrin-receptors, bombesin/gastrin-releasing peptide receptor scintigraphy are still limited, and further clinical studies are needed. The studies using vascular endothelial growth factor (VEGF) for tumor angiogenesis imaging, annexin-V for imaging apoptosis and agents for hypoxia imaging are still in an early stage and the clinical role for these agents needs to be defined. In conclusion, no single imaging technique identifies all the metastatic sites of NETs. The best results may be obtained with a combination of functional imaging such as PET or/and SRS and morphologic imaging with CT and/or MR imaging. Many molecular imaging and therapy modalities fur NETs are recently under investigation or being developed, the usefulness of these modalities, however, has to be evaluated by well-designed and multicentre studies.