Version July 2025
| Terminology | Differentiation | Grade | Mitotic rate* (mitoses/2 mm2) | Ki-67 index* (%) |
| NET, G1 | Well differentiated | Low | <2 | <3 |
| NET, G2 | Well differentiated | Intermediate | 2 to 20 | 3 to 20 |
| NET, G3 | Well differentiated | High | >20 | >20 |
| NEC, small cell type (SCNEC) | Poorly differentiated | HighΔ | >20 | >20 (often >70) |
| NEC, large cell type (LCNEC) | Poorly differentiated | HighΔ | >20 | >20 (often >70) |
| MiNEN | Well or poorly differentiated¶ | Variable¶ | Variable¶ | Variable¶ |
G1: grade 1; G2: grade 2; G3: grade 3; LCNEC: large cell neuroendocrine carcinoma; MiNEN: mixed neuroendocrine-non-neuroendocrine neoplasm; NEC: neuroendocrine carcinoma; NET: neuroendocrine tumor; SCNEC: small cell neuroendocrine carcinoma.
* Mitotic rates are the number of mitoses/2 mm2 (ie, 10 high-power fields at 40× magnification and an ocular field diameter of 0.5 mm). To assess the mitotic rate, count in 50 fields of 0.2 mm2 (over a total area of 10 mm2). To assess the Ki-67 proliferation index, count at least 500 cells in the regions of highest labeling (using scanning magnification). The final grade is determined by the value (mitotic rate or Ki-67 proliferation index) that places the tumor in the category with the higher grade.
¶ In most MiNENs, both the neuroendocrine and non-neuroendocrine components are poorly differentiated. Additionally, the mitotic rate and Ki-67 proliferation index of the neuroendocrine component usually fall into the same range as an NEC. However, in some MiNENs, either (or both) components could be well-differentiated so they should be graded separately, if possible.
Δ Poorly differentiated NECs are not formally graded but are considered high grade by definition.