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. 2017 Mar 13;189(10):E398–E404. doi: 10.1503/cmaj.160771

Table 1:

World Health Organization grading systems for neuroendocrine tumour18–20

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.