Distal‐type bronchiolar adenoma of the lung harboring an EGFR exon 21 p.L858R mutation: A case report

Here, we present a case of a distal‐type bronchiolar adenoma (BA) of the lung. BAs are benign lung tumors characterized by nodular proliferation of bilayered bronchiolar‐type epithelium with a continuous layer of basal cells. This patient underwent S3 segmentectomy following detection by computed tomography (CT) scan of a gradually enlarging ground‐glass nodule (GGO) over a five month period. Nodule morphology and immunophenotype were consistent with those of distal‐type BA of the lung. An epidermal growth factor receptor (EGFR) exon 21 p.L858R missense mutation was identified which, to the best of our knowledge, is the first case to be reported of a common gene mutation associated with non‐small cell lung cancer (NSCLC) being found in a BA lesion. Following surgery, the patient remains relapse‐free. KEY POINTS: SIGNIFICANT FINDINGS OF THE STUDY: Pathological assessment of a lung nodule confirmed a papillary tumor with a double‐layered cell structure, less than typical cytoplasm, and a mixture of ciliated columnar and globular cells, consistent with a distal‐type bronchiolar adenoma. WHAT THIS STUDY ADDS: This is the first report of an EGFR exon 21 p.L858R mutation in a bronchiolar adenoma.