Gastrointestinal neuroendocrine tumors (NETs) are tumors derived from the diffuse neuroendocrine system of the GI tract; that system is composed of amine- and acid-producing cells with different hormonal profiles, depending on the site of origin. The tumors historically are divided into carcinoid tumors and pancreatic endocrine tumors (PETs), although recent pathologic classifications have proposed that they all be classified as gastrointestinal NETs. In this chapter the term carcinoid tumor is retained because it is widely used. These tumors originally were classified as APUDomas (for amine precursor uptake and decarboxylation), as were pheochromocytomas, melanomas, and medullary thyroid carcinomas, because they share certain cytochemical features as well as various pathologic, biologic, and molecular features (Table 350-1). It was originally proposed that APUDomas had a similar embryonic origin from neural crest cells, but it is now known the peptide-secreting cells are not of neuroectodermal origin. Nevertheless, the concept of APUDomas is useful because the tumors from the cells have important similarities as well as some differences (Table 350-1). In this section, the areas of similarity between PETs and carcinoids will be discussed together and areas in which there are important differences will be discussed separately.

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Table 350-1 General Characteristics of Gastrointestinal Neuroendocrine Tumors [Carcinoids, Pancreatic Endocrine Tumors (PETs)] 

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