The gastroenteropancreatic neuroendocrine cell system and its tumors: the WHO classification.

Although well established in medical terminology, the term carcinoid is no longer adequate to cover the entire morphological and biological spectrum of neoplasms of the disseminated neuroendocrine cell system. Therefore, instead of carcinoid, the WHO classification published in 2000 uses the general terms neuroendocrine tumor and neuroendocrine carcinoma. In this review a classification of gastroenteropancreatic neuroendocrine tumors based on the WHO criteria is described. We also classify and comment on the most important tumor entities. On the basis of localization and of various morphological and biological criteria, we distinguish between benign neuroendocrine tumors, tumors with uncertain malignant potential, and tumors showing low-grade and high-grade malignancy.