Bronchiolar adenoma with squamous metaplasia: a distinct phenotype

Aims Pulmonary bronchiolar adenoma is a benign lung tumour characterised by nodular proliferation of bilayered bronchiolar‐type epithelium with a continuous layer of basal cells. The aim of this study was to describe a distinct and rare histological type of pulmonary bronchiolar adenoma: bronchiolar adenoma with squamous metaplasia. Methods and results We examined the clinicopathological, immunohistochemical, and molecular characteristics of five cases (two cases from the same patient). The samples were histopathologically characterised by bilayered bronchiolar‐type cells with sheets like spindle‐oval and polygonal cells. Immunohistochemistry analysis revealed that columnar surface cells of the tumour were diffusely positive for TTF‐1 and Napsin A, while basal cells were positive for P40 and P63. Moreover, the squamous metaplastic cells in the stroma were positive for P40, and P63, while being negative for TTF‐1, Napsin A, S100, and SMA. Genomic analyses uncovered that all five samples had BRAF V600E mutations. Notably, both squamous metaplastic and basal cells were positive for BRAF V600E staining. Conclusion We discovered a distinct subtype of pulmonary bronchiolar adenoma termed bronchiolar adenoma with squamous metaplasia. It is composed of columnar surface cells, basal cells, and sheet‐like spindle‐oval cells with squamous metaplasia in the stroma. All five samples harboured the BRAF V600E mutation. Importantly, BASM may be misdiagnosed as pulmonary sclerosing pneumocytoma upon frozen sections analysis. It may need further immunohistochemistry staining.