Clinicopathological features and genomic analysis of bronchiolar adenoma.

Bronchiolar adenoma (BA) is a rare tumor of the bronchioles with a double-layer structure, including the basal cell layer and the superficial cell layer, and it has a good prognosis. However, the concept of a putative variant of BA has been proposed in the recent literature. Data on 17 cases of BA were collected from our center. The clinical data, morphology, immunophenotype, and molecular changes were retrospectively analyzed. We also collected the molecular changes in BA reported in the previous literature and summarized the putative driver mutations of BA. Out of 17 BAs, 13 were classic cases with a double-layer structure, including 9 proximal-type and 4 distal-type BAs. Of note, we also identified 3 cases that lacked a continuous basal cell layer, including 2 cases of mixed-type BA with monolayered lesions (basal cells were undetected in some areas) and 1 case of a monolayered BA-like lesion (basal cells were completely undetected). The immunohistochemical findings of monolayer cell lesions were closer to those of minimally invasive adenocarcinoma. We also found one case in which BA transformed into invasive adenocarcinoma accompanied by mutations in the TP53, JAK2, NF1 and RB1 genes. Combined with the previous literature, the most common putative driver gene mutations in 62 BA lesions were EGFR (25/62; 41%) and BRAF (21/62; 34.4%). Typical BA has a double-layer cell structure; however, there is also a putative variant of BA, which has a monolayer cell structure and lacks the basal cell layer. Transformation from BA into invasive adenocarcinoma is unusual but can occur.