Radionuclide imaging of neuroendocrine tumours: biological basis and diagnostic results

Neuroendocrine tumours have been defined as APUD-omas in the past by authors who identified common metabolic characteristics (amine precursor uptake and decarboxylation) in a group of tumours thought to originate from cells of the neural crest and to be able to produce biogenic amines. The identification of neuroendocrine tumours with APUD-omas was not confirmed by subsequent investigators. At present it is known that a group of neuroendocrine tumours derive from pluripotent stem cells or from differentiated neuroendocrine cells, and that they have a particular pattern of histology due to the presence of some secretory products and particular cytoplasmic proteins. Many radiopharmaceuticals have been successfully used in nuclear medicine to visualise neuroendocrine tumours; most of them are based on specific uptake mechanisms, but some are non-specific probes. This review is focussed on the clinical application of radiolabelled metaiodobenzylguanidine, indium-111 pentetreotide, radiolabelled vasointestinal peptide, radiolabelled monoclonal antibodies and positron-emitting tracers. While many different types of neuroendocrine tumours are identified today, only the most common histotypes and those tumours of major relevance for nuclear medicine are considered in this review (anterior pituitary tumours and neuroblastoma are excluded). New knowledge in molecular biology, relevant biological and histological patterns, and the physiological and clinical behaviour are described for neuroendocrine tumours of the lung, tumours of the gastroenteropancreatic tract, medullary thyroid carcinoma, tumours of sympatho-adrenal lineage, and multiple endocrine neoplasia. The nuclear medicine results in diagnostic imaging are presented, and the major comparative studies with different tracers are reported. The study of further possible diagnostic approaches addressing the biological characteristics of these tumours could open the way to various new therapeutic options.