An Unusual Low-Grade Biphasic Pulmonary Neoplasm Harboring BRAF-V600E Mutation: A Potential Variant of Bronchiolar Adenoma.

We report a bronchiolar adenoma of the lung with unusual features. A 68-year-old woman presented with a right lower lobe lung mass found during routine screening. Positron emission tomography studies revealed a metabolically active 5.4 cm lesion. The patient underwent an endoscopic ultrasound-guided transbronchial fine needle aspiration. The cytologic material was interpreted as a basaloid neoplasm of uncertain malignant potential; focally, the cytologic features resembled adenoid cystic carcinoma. A lobectomy revealed a circumscribed biphasic tumor, composed of luminal epithelial cells and subjacent basal cells. The luminal cells consisted of ciliated cells and type II pneumocytes expressing CAM 5.2, TTF1, and NAPSA (napsin A aspartic peptidase). The basal cells were associated with a weakly basophilic matrix and basement membrane-like material; these cells expressed TP63 and expressed weakly with CAM 5.2 and calponin (CNN1). The tumor did not invade the pleura and did not show vascular or perineural invasion. The histopathologic and immunohistochemical findings, as well as the BRAF-V600E mutation supported its classification as an unusual variant of bronchiolar adenoma showing a prominent basement membrane-like material. This tumor was incidentally associated with a lung adenocarcinoma in situ. This presentation describes an unusual variant of bronchiolar adenoma and underscores the value of integrating the cytologic, histologic, and molecular findings.