World Health Organization grading systems for neuroendocrine tumour (18)– (20)
| Grade | NET of the lung and thymus | Gastroenteropancreatic NET | ||
|---|---|---|---|---|
| Nomenclature | Proliferative rate | Nomenclature | Proliferative rate | |
| Low (Grade 1) | Typical carcinoid | < 2 mitoses/10 hpf AND no necrosis | NET/pNET | < 2 mitoses/10 hpf AND < 3% Ki67 index |
| Intermediate (Grade 2) | Atypical carcinoid | 2–10 mitoses/10 hpf OR foci of necrosis | 2–20 mitoses/10 hpf OR 3%–20% Ki67 index | |
| High (Grade 3) | Small cell lung cancer, large cell neuroendocrine carcinoma | > 10 mitoses/10 hpf | Neuroendocrine carcinoma | > 20 mitoses/10 hpf OR > 20% Ki67 index |
Note: hpf = high powered field, NET = neuroendocrine tumour, pNET = pancreatic neuroendocrine tumour. The nomenclature for NETs varies by site. Per the World Health Organization grading system, low- and intermediate-grade tumours in the lung are still called typical carcinoid and atypical carcinoid, respectively; high-grade NETs in the lung are either small cell lung cancer or large cell neuroendocrine carcinoma. In the gastrointestinal tract, low- and intermediate-grade tumours are called NETs, whereas in the pancreas they are called pNETs. High-grade gastroenteropancreatic tumours are called neuroendocrine carcinomas rather than NETs.