Retroperitoneal mucinous cystadenoma: report of two cases and review of the literature.

Primary retroperitoneal cystic tumor is extremely rare, and its histogenesis and biological behavior remain speculative. Two surgical cases of retroperitoneal mucinous cystadenoma (Case 1, an 18-year-old woman; and Case 2, an 85-year-old woman) are reported. The cystadenomas in these cases were mainly lined by a monolayer of columnar or thin flat cells. Case 1 was positive for mucin and epithelial membrane antigen, whereas Case 2 was positive for a mesothelial marker (calretinin). Ciliated epithelium was also interspersed in Case 2. Some parts showed papillary projections, resembling well-differentiated papillary mesothelioma. Within the cyst walls of both cases, ovarian-like stroma that was positive for both estrogen and progesterone receptors was found. Interestingly, focal nodular hyperplasia of the liver was also detected in Case 1. We believe the retroperitoneal mucinous cystadenoma might have arisen from the peritoneum via mucinous epithelial metaplasia with a phenotype of extragenital Mullerian system.