Ciliated muconodular papillary tumors of the lung: Cytologic features and diagnostic pitfalls in intraoperative examinations.

Ciliated muconodular papillary tumors (CMPTs) of the lung are rare, likely benign neoplastic lesions. Here we describe a case of a CMPT, focusing on its cytologic features, which to our knowledge have not been reported previously. Owing to dull back pain, a 69-year-old male non-smoker underwent CT, which revealed a 1.3 × 1.3-cm solid nodule in the peripheral field of the left lower lung lobe. A wedge resection of the nodule was performed, with the provisional diagnosis being primary lung cancer. Macroscopic examination of a resected specimen showed a 1.2-cm grayish nodule. Touch imprint smear cytology revealed ciliated columnar cells and mucous cells, as well as abundant extracellular mucin on inflammatory background of lymphocytes and histiocytes. Histologic examination revealed a nodular papillary tumor composed of ciliated columnar cells, mucous cells, and basal cells surrounded by a mucin pool. No nuclear atypia or mitotic figures were identified. The final diagnosis was CMPT. The postoperative course was uneventful, with no recurrence at 8 months after surgery. Although a CMPT is a rare lung tumor, it should be considered when cytological or histological examination of a solitary peripheral lung nodule shows non-atypical ciliated cells and mucous cells surrounded by mucin.