Pulmonary Metastasizing Ameloblastomas Can Masquerade as Certain Bronchiolar Adenoma-Like Tumor With Squamous Epithelial Metaplasia.

Bronchiolar adenoma (BA) is a generally benign peripheral lung neoplasm with a bilayered structure of basal and luminal cells. Recently, an increasing number of bronchiolar adenoma-like tumor with squamous epithelial metaplasia (BATSM) have been reported, raising the question of whether they represent variant subtypes of BA or potentially novel tumor entities. Pulmonary metastasis of ameloblastoma (MA) shares morphological similarities with BATSM, exhibiting TTF-1/Napsin A-positive luminal cells and P40/P63-positive basal cells; this overlap can lead to misdiagnosis of MA as BATSM. We collected 5 MA and 10 BATSM cases, analyzing their morphology, immunohistochemistry, elastic fiber staining and molecular pathology. MA presents as well-circumscribed bronchiole-unrelated nodules with alveolar destruction, intraluminal serous secretions, and Calretinin-positive stellate reticulum-like cells. BATSM is bronchiole-associated, preserves alveolar architecture, and is Calretinin-negative with weak basal TTF-1 positivity. 80% of MA cases harbored BRAF V600E mutations; 30% of BATSM cases exhibited EGFR exon 20 insertions. This study highlights the need for integrating clinical history, morphology, immunohistochemistry and molecular testing to avoid misdiagnosing MA as BATSM.