Cytomorphologic features of well-differentiated papillary mesothelioma in peritoneal effusion: a case report

Well-differentiated papillary mesothelioma (WDPM), a distinct subtype of diffuse malignant mesothelioma, usually occurs in the peritoneum and is seen most commonly in women of reproductive age. Histologic features of WDPM include papillary growth and stout fibrous cores surrounded by a single layer of tumor cells. We present the case of a 73-year-old woman without subjective symptoms who showed signs of peritoneal effusion during a routine examination and for whom cytologic examination of the ascitic fluid was performed. Many spherical clusters, with a smooth external surface composed of a single layer of uniform cuboidal cells, were observed. Within each cluster, a collagenous ball showed light green Papanicolaou staining. Immunohistochemistry of surgical specimens showed tumor cells positive for calretinin, D(2)-40, and HBME-1 staining. The histologic diagnosis was WDPM. The identification of a collagenous ball within these clusters is a useful cytologic finding for the diagnosis of WDPM. WDPM should be suspected when numerous collagenous balls are present by effusion cytology and isolated cells are not.