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GENERAL FEATURES OF GASTROINTESTINAL NEUROENDOCRINE TUMORS

Gastrointestinal (GI) neuroendocrine tumors (NETs) are tumors derived from the diffuse neuroendocrine system of the GI tract, which is composed of amine- and acid-producing cells with different hormonal profiles, depending on the site of origin. NETs of the GI tract share many features with other NETs throughout the body and were historically divided into GI-NETs (in the GI tract) (also frequently called carcinoid tumors) and pancreatic neuroendocrine tumors (pNETs), although in newer pathologic classifications they are all classified as NETs (Table 80-1). These tumors originally were classified as APUDomas (for amine precursor uptake and decarboxylation), as were pheochromocytomas, NETs in other locations, melanomas, and medullary thyroid carcinomas, because they share certain cytochemical features as well as various pathologic, biologic, and molecular features. It was originally proposed that APUDomas had a similar embryonic origin from neural crest cells, but it is now known that the peptide-secreting cells are not of neuroectodermal origin.

TABLE 80-1Comparison of the Criteria for the Tumor Category in the ENETS AJCC TNM Classifications of Pancreatic and Appendiceal NETs (Top Panel) and the WHO/ENETS Grading and Classification (Bottom Panel)

CLASSIFICATION/PATHOLOGY/TUMOR BIOLOGY OF NETs

NETs generally are composed of monotonous sheets of small round cells with uniform nuclei, and mitoses are uncommon. They can be frequently recognized on routine histology; however, these tumors are ...

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