A 70-year-old man with a 40 pack-year smoking history has a peripheral lung mass. Biopsy shows cells with abundant pink granular cytoplasm and prominent nucleoli growing in a lepidic pattern along alveolar walls without stromal invasion, vascular invasion, or pleural invasion. The tumor measures 1.8 cm. According to WHO 2021 classification, what is this lesion?

• A Invasive adenocarcinoma, lepidic predominant subtype — requires adjuvant chemotherapy
• B Minimally invasive adenocarcinoma (MIA) — lepidic-predominant with ≤5 mm invasive focus; near-100% 5-year disease-specific survival after resection
• C Atypical adenomatous hyperplasia (AAH) — a precursor lesion measuring >5 mm but not yet meeting criteria for AIS
• D Adenocarcinoma in situ (AIS) — purely lepidic growth ≤3 cm without invasion; near-100% 5-year survival after complete resection; no adjuvant therapy needed ✓

Explanation

Adenocarcinoma in situ (AIS) per WHO 2021 is defined as a solitary adenocarcinoma ≤3 cm with purely lepidic growth pattern — tumor cells spread along pre-existing alveolar septa without stromal, vascular, or pleural invasion, and without acinar, papillary, micropapillary, or solid components. AIS achieves near-100% 5-year disease-specific survival after complete surgical resection and is considered a preinvasive lesion. Minimally invasive adenocarcinoma (MIA) is similar but has a small invasive focus ≤5 mm; this tumor at 1.8 cm without any invasive component qualifies as AIS. AAH is a proliferation ≤5 mm and is the earliest precursor. Lepidic-predominant invasive adenocarcinoma has invasive foci >5 mm.

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

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Written and medically reviewed by the StethoPrep medical team.