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Neuroendocrine Tumors of GI Tract : Classification

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Classification of Neuroendocrine Tumors (NETs) of Gastrointestinal Tract: The WHO proposed the term carcinoid in 1980 for the most common type of neuroendocrine tumor of the GI tract.

The WHO classification of GI and pancreatobilitary neuroendocrine tumors was updated in 2000 and included well-differentiated neuroendocrine tumors (benign and low-grade), well-differentiated neuroendocrine carcinomas, and poorly-differentiated neuroendocrine carcinomas based on the degree of differentiation, depth of invasion, angiolymphatic invasion, and presence of metastasis.

The 2010 WHO Classification of NETs of the GI tract is a more simplified approach consisting of three grades. It is based solely on mitotic activity and Ki-67 labeling index and does not take into account tumor size, extent, or location. The defining criteria of various grades are as follows.

NET, Grade 1: Mitotic activity <2/10 HPFs and/or Ki-67 labeling index <3%.

NET, Grade 2: Mitotic activity 2-20/10 HPFs and/or Ki-67 labeling index of 3% to 20%.

Neuroendocrine Carcinoma (NEC, Grade 3). Mitotic activity >20/10 HPFs and/or Ki-67 labeling index >20%. NEC includes large cell neuroendocrine carcinoma with a more organoid appearance as well as small cell carcinoma with typical histology resembling those seen in the lung. Foci of glandular or squamous differentiation may be present. Continued in next slide.

The image shows NET, Grade 1 (carcinoid) discovered incidentally in the rectum. It is composed of a population of uniform cells arranged in ribbons, trabeculae and cords. Image courtesy of: @PatholWalker

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