Histologic grading scheme for gastrointestinal and pancreatic neuroendocrine neoplasms from the WHO and AJCC/UICC

Cellular pleomorphism per se is not a useful feature for grading NETs. The following grading scheme has been proposed for gastrointestinal NETs.
Grade	Definition
GX	Grade cannot be assessed
G1	Mitotic count (per 10 HPF)* <2 and Ki-67 index (%)^¶ <3
G2	Mitotic count (per 10 HPF) = 2 to 20 or Ki-67 index (%)^¶ = 3 to 20
G3	Mitotic count (per 10 HPF) >20 or Ki-67 index (%)^¶ >20

In cases of disparity between Ki-67 proliferative index and mitotic count, the result indicating a higher-grade tumor should be selected as the final grade. For example, a mitotic count of 1 per 10 HPF and a Ki-67 of 12% should be designated as a G2 NET.

WHO: World Health Organization; AJCC: American Joint Committee on Cancer; UICC: Union for International Cancer Control; NET: neuroendocrine tumor; HPF: high-power field.
* 10 HPF = 2 mm²; at least 50 HPFs (at 40 times magnification) must be evaluated in areas of highest mitotic density in order to adhere to WHO 2010 and 2017 criteria.
¶ MIB1 antibody; % of 500 to 2000 tumor cells in areas of highest nuclear labeling.

Used with permission of the American College of Surgeons, Chicago, Illinois. The original source for this information is the AJCC Cancer Staging Manual, Eighth Edition (2017) published by Springer International Publishing.

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Histologic grading scheme for gastrointestinal and pancreatic neuroendocrine neoplasms from the WHO and AJCC/UICC