• Shuli Liu, Nan Liu, Mingming Xiao, Liang Wang, and En-Hua Wang.
• Department of Pathology, the First Affiliated Hospital and College of Basic Medical Sciences, China Medical University.
• Medicine (Baltimore). 2020 Sep 25; 99 (39): e22322.

RationaleBronchiolar 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 ConcernsAn 82-year-old female patient was detected a 0.8 cm ground glass nodule in the left lower lobe of the lung.DiagnosesThe serum levels of tumor markers were normal.InterventionsThe patient underwent a segmentectomy of the left lower lobe.OutcomesThe 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.LessonsBA 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.

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