Carbohydrate Antigen 19-9 and Carcinoembryonic Antigen Immunostaining in Benign Multicystic Mesothelioma of the Peritoneum

A 58-year-old Italian man was incidentally discovered to have an elevated carbohydrate antigen 19-9 (CA-19-9) level of 132 U/mL on routine blood testing. Multisystem imaging studies revealed multiple benign-appearing cysts of the liver and single cysts in the pancreas and kidney parenchyma. Throughout 14 months, fluctuations were observed in the elevated serum CA-19-9 levels from 99 to 450 U/mL. Serum carcinoembryonic antigen (CEA) and other tumor markers were normal. Laparoscopy disclosed multiple cystic lesions on the surface of the liver, on the serosal surface of the ileum, and in the mesentery. Electron microscopy characterized the cells as mesothelial. The pathologic diagnosis was benign multicystic mesothelioma of the peritoneum (BMMP). Aspirated fluid from the liver cyst revealed CA-19-9 levels at 28 500 U/mL, strongly linking the elevated serum CA-19-9 levels with mesothelial cyst secretion. Immunostaining was positive for CA-19-9, CEA, and cancer antigen 125 (CA-125). We believe this is the first documented instance of CA-19-9 and CEA secretion in BMMP.