Differentiation | Grade | Mitotic index | Proliferative rate |
Well-differentiated NET | G1 (low-grade) | <2 mitoses/10 hpf | <3% Ki-67 index |
G2 (intermediate-grade) | 2–20 mitoses/10 hpf | 3%–20% Ki-67 index | |
G3 (high-grade) | >20 mitoses/10 hpf | >20% Ki-67 index | |
Poorly differentiated NEC | G3 (high-grade) | >20 mitoses/10 hpf | >20% Ki-67 index |
Small cell type | |||
Large cell type |
NET = neuroendocrine tumor; NEC = neuroendocrine carcinoma; hpf = high-power field.