Carcinoid tumors of the lung are well-differentiated neuroendocrine neoplasms. According to the WHO 2021 classification of thoracic tumors, the distinction between typical carcinoid (TC) and atypical carcinoid (AC) is based primarily on:
- A TC: <10 mitoses/2mm²; AC: >10 mitoses/2mm²
- B TC: synaptophysin negative; AC: synaptophysin positive
- C TC: no necrosis, <2 mitoses/2mm²; AC: necrosis or 2-10 mitoses/2mm² ✓
- D TC: Ki-67 <1%; AC: Ki-67 5-20%
Correct answer: C. TC: no necrosis, <2 mitoses/2mm²; AC: necrosis or 2-10 mitoses/2mm²
Explanation
Per WHO 2021 Classification of Thoracic Tumours, typical carcinoid (TC) is defined by: <2 mitoses per 2mm² (10 HPF) AND absence of necrosis. Atypical carcinoid (AC) requires: 2-10 mitoses per 2mm² OR foci of necrosis (often punctate). Both are low to intermediate grade neuroendocrine tumors, distinct from high-grade small cell and large cell neuroendocrine carcinoma (>10 mitoses and/or extensive necrosis, Ki-67 often >50%). TC has 5-year survival ~90%; AC ~60-70%.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.