First case of bronchiolar adenoma lined purely by mucinous luminal cells with molecular analysis: A case report

RATIONALE: Bronchiolar adenoma (BA) is a newly designated rare entity of the lung, including both the currently designated ciliated muconodular papillary tumor (CMPT) and so-called non-classic CMPT. The most prominent histological feature of BAs is the bilayered cell structures composed of the continuous basal cell layer and the luminal layer which consists of different proportion of mucinous cells, ciliated cells, Clara cells and/or type II pneumocytes. BA purely covered by mucinous cells without other components in the luminal layer has never been reported. PATIENT CONCERNS: An 82-year-old female patient was detected a 0.8 cm ground glass nodule in the left lower lobe of the lung. DIAGNOSES: The serum levels of tumor markers were normal. INTERVENTIONS: The patient underwent a segmentectomy of the left lower lobe. OUTCOMES: The postoperative pathological diagnosis was BA. Molecular analysis revealed that the tumor harbored ALK rearrangement and BRAF mutations simultaneously. There was no recurrence in 17 months of follow-up. LESSONS: BA can be lined only by mucinous cells, without any cuboidal and/or ciliated cells in the surface layer. This sets a dangerous pitfall in differentiation diagnosis with invasive mucinous adenocarcinoma especially during intraoperative frozen pathological diagnosis.