A 65-year-old male smoker develops dyspnea and a peripheral coin lesion in the right upper lobe. Biopsy shows tumor cells growing along the pre-existing alveolar framework without stromal invasion (lepidic growth). CK7+, TTF-1+, napsin A+, CK20-, CDX2-. What is the MOST likely diagnosis?

• A Squamous cell carcinoma of the lung — central, p40+, CK5/6+
• B Large cell carcinoma — lacks squamous or glandular differentiation markers
• C Metastatic colorectal carcinoma — CK20+, CDX2+
• D Adenocarcinoma in situ (AIS) — pure lepidic growth, formerly called BAC, no stromal invasion ✓

Explanation

Adenocarcinoma in situ (AIS, formerly bronchioloalveolar carcinoma) is characterized by purely lepidic growth — tumor cells spread along intact alveolar septa without stromal, vascular, or pleural invasion. The immunoprofile CK7+, TTF-1+, napsin A+ is typical of pulmonary adenocarcinoma. AIS has an excellent prognosis (near 100% 5-year survival if resected) and is distinguished from minimally invasive adenocarcinoma (<5 mm invasion focus) and invasive adenocarcinoma by the complete absence of invasion.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.