Mucinous Balls Tangled With Mesothelial Cells and MUC2‐Positive Cancer Cells in the Ascites of Pseudomyxoma Peritonei

Pseudomyxoma peritonei (PMP) is characterized by extensive mucinous ascites following rupture of mucinous neoplasms of an intra‐abdominal origin, and contain secreted gel‐forming mucins such as MUC2 and MUC5AC. We encountered a 66‐year‐old Japanese man complaining of abdominal distension. Ascites at the site was gelatinous upon gross examination, and needle aspirate smears showed histiocytes and many mucinous balls wrapped in spindle cells, which were positive for vimentin, pan‐cytokeratin, and podoplanin. The cell block showed several adenocarcinoma clusters, which were positive for MUC2, MUC5AC, CK20, and CDX‐2, and negative for CK7. From these findings, a diagnosis of PMP arising from colon cancer was indicated. Cytoreductive surgery was performed, and the cystic diverticulum was found to be infiltrated by tumor cells in the sigmoid colon that caused PMP. Mucinous balls surrounded by mesothelial cells and MUC2‐positive adenocarcinoma cells are useful clues in the diagnosis of PMP. Diagn. Cytopathol. 2016;44:628–631. © 2016 Wiley Periodicals, Inc.