Ciliated muconodular papillary tumor (CMPT) is newly-defined low-grade malignant tumor, characterized as papillary tumor consisting of ciliated columnar, goblet, and basal cells. We present one case of pulmonary peripheral neoplasm misdiagnosed as mucinous adenocarcinoma. Pathologic findings showed, centrally, cystic papillary growth consisting of ciliated columnar, goblet, and basal cells, and peripherally, some tumor cells spread along the adjacent alveolar walls, in a lepidic fashion, and floating in pool of mucin. Tumor cells had bland nuclei, and no mitotic activity was observed. This had been initially misdiagnosed as mucinous adenocarcinoma in frozen section. We reviewed previous articles as well as permanent and frozen slides. In conclusion, in order to reduce the possibility of false positives, it is important to be aware that CMPT is a rare form of peripheral mucin-producing tumor with characteristic histologic findings.
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