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WHO Classification of Tumours
Neuroendocrine tumor G1 (NET G1) / Carcinoid
Anus and anal canal


A neuroendocrine tumour (NET) is defined as a well-differentiated, neuroendocrine neoplasm composed of cells with features similar to those of the normal gut endocrine cell, expressing general markers of neuroendocrine differentiation (usually diffuse and intense chromogranin A and synaptophysin) and hormones (usually intense but not necessarily diffuse) according to site, with mild- to-moderate nuclear atypia and a low number of mitoses.
The proposed grading based on proliferation with the following definitions of mitotic count and Ki67 index:
– G1 (NET G1): mitotic count, <2 per 10 high power fields (HPF) and/or ≤2% Ki67 index;
G2 (NET G2) : mitotic count 2–20 per 10 HPF and/or 3–20% Ki67 index

Commentary: Neuroendocrine tumours of the digestive system

Anal NETs
Neuroendocrine tumours may arise in the anus

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Fenger C, Lyon H (1982)
Endocrine cells and melanin-containing cells in the anal canal epithelium.
Histochem J 14: 631-9

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Hörsch D, Fink T, Göke B, Arnold R, Büchler M, Weihe E (1994)
Distribution and chemical phenotypes of neuroendocrine cells in the human anal canal.
Regul Pept 54: 527-42

. They are, however, conventionally classified as rectal. An immunohistochemical study of 17 rectal neuroendocrine tumours showed that most were of L cell type
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Chetritt J, Sagan C, Heymann MF, Le Bodic MF (1996)
[Immunohistochemical study of 17 cases of rectal neuroendocrine tumors].
Ann Pathol 16: 98-103

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Bosman FT, Carneiro F, Hruban RH, Theise ND (Eds.)
WHO Classification of Tumours of the Digestive System.
4th Edition
International Agency for Research on Cancer: Lyon 2010


Topographic definition of the anal canal, anal margin and perianal region