Well-differentiated papillary mesothelioma (WDPM) is a rare subtype of malignant mesothelioma, which is considered to have low malignant potential. Because of its rare occurrence in the pleura, cytopathologists are not familiar with the cytologic features of WDPM, and to date only one report regarding the cytomorphology of aspiration biopsies of WDPM in pleura has been released. The authors present the findings of fine needle aspiration cytology of WDPM in the pleura in a 53-year-old woman.

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... ration smears showed papillary clusters composed of one to three layers of surface tumor cells and a central hyalinized stromal core. Tumor cells were round, ovoid, and spindle like with minimally atypical nuclei and small conspicuous nucleoli. Mitotic activity was virtually absent. Excisional biopsy histologic and immunohistochemical findings were wholly compatible with WDPM findings. Knowledge of the specific cytologic findings of WDPM is crucial for accurate diagnosis and appropriate treatment.