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Neuroendocrine Tumors of GI Tract : Classification

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Classification of Neuroendocrine Tumors (NETs) of Gastrointestinal Tract (continued from the previous slide). In 2018 the WHO proposed modifications to the classification of neuroendocrine neoplasms arising at various sites in an attempt reduce inconsistencies and contradictions in the terminology. The elements of the proposed classification are as follows:

The term neuroendocrine neoplasms (NENs) was adopted to include neuroendocrine tumors (NETs) and neuroendocrine carcinomas (NECs).

NETs are well-differentiated tumors that are graded into G1, G2, and G3 based on mitotic count and Ki-67 index as described previously. Majority of the cases meet the criteria for G1 or G2 tumors. Rare cases of well-differentiated NET may fall into Grade 3 category based on mitotic activity or Ki-67 labeling index.

Previously, G3 was used to described NECs. In the 2018 WHO proposal, NECs are not specifically graded as they are regarded as high-grade by definition.

Tumors containing both glandular and NEC components and are referred to as mixed adenoneuroendocrine carcinoma (MANEC). Each component should make up at least 30% of the tumor.

In addition to these well-defined entities, scattered neuroendocrine cells can be detected with immunohistochemical stains or in-situ hybridization for neuroendocrine markers in 15% to 50% of all colorectal adenocarcinomas, especially the mucinous variety.

The image shows a small well-differentiated neuroendocrine tumor (carcinoid) of rectum discovered incidentally on endoscopy.

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