A 55-year-old non-smoking woman presents with a peripheral lung nodule on CT. Biopsy shows a lepidic growth pattern with columnar cells lining alveolar walls without stromal invasion. TTF-1 is positive. This lesion is best classified as:

• A Adenocarcinoma in situ (formerly BAC) ✓
• B Squamous cell carcinoma in situ
• C Atypical adenomatous hyperplasia
• D Carcinoid tumor

Explanation

Adenocarcinoma in situ (AIS), formerly called bronchioloalveolar carcinoma, is defined as a solitary small (≤3 cm) adenocarcinoma with pure lepidic growth along pre-existing alveolar structures without stromal, vascular, or pleural invasion. TTF-1 positivity confirms the pneumocytic lineage. When fully resected, AIS has a near 100% disease-specific survival, making its correct classification prognostically critical.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.