Review paper
Gastroenteropancreatic neuroendocrine tumors (GEP NET) – discussion around terminology and classification

Neuroendocrine tumours of the gastroenteropancreatic tract (GEP/NETs) represent a rare and heterogenous group of tumors. Firstly, Oberndorfer in 1907 introduced the term “carcinoid” to describe a group of ileal tumors with a distinct clinical and morphological features. In 1963, Williams and Sandler classified gut endocrine tumors based on their ontogenic origin into foregut (stomach, pancreas, duodenum and upper jejunum), midgut (lower jejunum, ileum, appendix, caecum) and hindgut (colon, rectum). In 1980, the WHO classification of endocrine tumors applied the term carcinoid to all tumors of the diffuse neuroendocrine system, excluding pancreatic endocrine tumor (islet cell tumor), medullary carcinoma of the thyroid, paraganglioma, small cell lung carcinoma and Merkel cell tumor of the skin. Finally, the term “carcinoid” was replace by the term “neuroendocrine tumors” for the designation of the totality of neoplasms with neuroendocrine features. In 2000, the new classification with follow comparable guidelines for GEP NETs was proposed. The first principle is that the tumor are distinguish according to the site of origin. The neuroendocrine differentiation of tumor cells is accept by the immunohistochemical neuroendocrinal markers as synaptophysin and chromogranin. The next principle is that the tumors are separate into groups according to histopathological parameters and behaviour. The classification distinguish well differentiated neuroendocrine tumors with benign behaviour, well differentiated neuroendocrine tumors with uncertain behaviour, well differentiated neuroendocrine carcinoma (low grade malignant), poorly differentiated neuroendocrine carcinoma (high grade malignant). The criteria for these separations are the tumors histological differentiated, size, Ki67 index, extension into surrounding tissues, angio-and perineural invasion.