A 68-year-old man with a 60 pack-year smoking history presents with a peripheral lung nodule. Biopsy shows a well-differentiated neuroendocrine tumor with organoid nesting pattern, uniform cells with salt-and-pepper chromatin, and less than 2 mitoses per 2 mm² with no necrosis. According to the WHO 2021 classification, this is best classified as:

• A Atypical carcinoid
• B Large cell neuroendocrine carcinoma
• C Small cell carcinoma
• D Typical carcinoid ✓

Explanation

Typical carcinoid is defined by less than 2 mitoses per 2 mm² and absence of necrosis. Atypical carcinoid (option A) has 2-10 mitoses per 2 mm² and/or foci of necrosis. Small cell carcinoma (option C) has high mitotic rate (>10 per 2 mm²), nuclear molding, and extensive necrosis. Large cell neuroendocrine carcinoma (option B) shows neuroendocrine architecture with high mitotic rate and necrosis, but with larger cells and prominent nucleoli.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.