神经内分泌肿瘤是一组异质性罕见肿瘤，具有不同的形态特征、免疫表型、分子特征和临床表现。它们可以来源于全身任何神经内分泌细胞，但大多数NEC发生在胃肠道。根据激素分泌，可以将其分为两组，即功能性和非功能性NEC。第一组的特点是分泌特定物质，这定义了临床表现。功能性NEC可以分为类癌瘤，其过度产生血清素，以及功能性GEP NEC（主要发生于胰腺），可能分泌胰岛素、VIP、胃泌素、胰高血糖素或生长抑素。非功能性NEC约占NEC的85%。由于这些肿瘤缺乏特定症状，它们在临床上通常在体积较大或出现转移时被发现。除功能性NEC产生的特定生物标志物外，还有一些通用标记物对这些患者的诊断、预后和随访起到重要作用。这些标记物包括嗜铬粒蛋白（CgA）、神经元特异性烯醇酮酸（NSE）和5-羟基吲哚乙酸（5-HIAA）。

Neuroendocrine tumors are a heterogenous group of rare neoplasms with different morphological features, immunophenotype, molecular profile, and clinical presentation. They can derive from any neuroendocrine cell throughout the body, but the majority of NENs is developed in the gastrointestinal tract. They can be divided into two groups, based on hormone secretion, functioning and non-functioning NENs. The first group is characterised from the secretion of specific substances, defining the clinical manifestations. Functional NENs can be divided into carcinoid tumors, with serotonin overproduction, and functional GEP NEN's (mostly located in pancreas) that may secrete insulin, VIP, gastrin, glucagon or somatostatin. Non-functioning NENs, comprise approximately 85% of NEN's. As these tumors lack specific symptoms, they come to clinical attention later, when they have a large size or metastases. Apart from the specific biomarkers that functional NENs are producing, there are some general markers that are produced from all NENs and play a major role in the diagnosis, prognosis and follow up of these patients. These are chromogranin (CgA), neuron-specific enolase (NSE) and 5-hydroxyindolic acetic acid (5-HIAA).