Distal‐type bronchiolar adenoma of the lung expressing p16(INK4a) – morphologic, immunohistochemical, ultrastructural and genomic analysis – report of a case and review of the literature

Bronchiolar adenoma (BA) of the lung is a rare benign neoplasm. Because of a chest abnormal shadow indicated by health checkup, a 77‐year‐old female nonsmoker underwent computed tomography, revealing an 8 mm ground glass nodule in the peripheral field of the right lower lobe. Wedge resection of the nodule was performed, with a frozen diagnosis of primary lung adenocarcinoma. The localized, 8 × 4 × 3 mm‐sized, jelly‐like mass microscopically revealed a lepidic‐growing lesion composed of ciliated columnar cells, mucous cells and basal cells surrounded by mucin pool. Neither nuclear atypia nor mitotic activity was noted. Immunohistochemically, the ciliated, mucous and basal cells were positive for TTF‐1 and p16(INK4a). Mucous cells were positive for napsin A and focally expressed MUC5AC. MUC6 was negative. Basal cells were positive for CK5/6, p40, p63 and podoplanin. Human papillomavirus genome was undetectable by in situ hybridization. Ultrastructurally, the bronchiolar epithelial tubules consisted of two layers, the inner nonciliated microvillous cells and the outer basal‐like cells, and some of the inner cells were filled with mucin granules in cytoplasm. Molecular analysis of the tumor failed to show driver mutations. The final diagnosis was distal‐type BA. The postoperative course was uneventful for 6 months.