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CLASSIFICATION
Classification
WHO 2019 GI NET grade (G1–G3)
WHO 2019 grading of well-differentiated gastroenteropancreatic neuroendocrine tumours by mitotic count and Ki-67 index.
Classification overview
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NET
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G1 · Grade 1 (low)
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G2 · Grade 2 (intermediate)
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G3 · Grade 3 NET (well-differentiated)
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Rule · Grading rule
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NEC
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NEC · Neuroendocrine carcinoma (reference)
NET
G1
Grade 1 (low)
Mitotic count
<2 per 2 mm²
Ki-67 index
<3%
Clinical
Indolent; endoscopic or local resection often considered for small lesions
G2
Grade 2 (intermediate)
Mitotic count
2–20 per 2 mm²
Ki-67 index
3–20%
Clinical
Intermediate behaviour; resection and surveillance per organ-specific guidelines
G3
Grade 3 NET (well-differentiated)
Mitotic count
>20 per 2 mm²
Ki-67 index
>20%
Differentiation
Well-differentiated NET G3 (not NEC)
Clinical
More aggressive than G1/G2; systemic therapy may be considered
Note: Poorly differentiated NEC is also high grade but a distinct entity from well-differentiated NET G3.
NEC
NEC
Neuroendocrine carcinoma (reference)
Definition
Poorly differentiated neuroendocrine carcinoma (small cell or large cell type)
Grade
High grade (G3) by definition
Note
Separate from well-differentiated NET G3 in WHO 2019
NET
Rule
Grading rule
Discordance
When mitotic count and Ki-67 fall in different grades, assign the higher grade
Counting
Mitotic count in 2 mm² (40×, 0.2 mm² fields); Ki-67 on ≥500–2000 cells
WHO 2019, Original
La Rosa 2021, Fig. 3, Not original, CC BY
Reference
Original article
WHO Classification of Tumours: Endocrine and Neuroendocrine Tumours, 5th ed, 2019
Original article
Klimstra DS et al.
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CLASSIFICATION
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Decision support only. Follow current guidelines and local protocols. Classification definitions and terminology follow the original publication language.