A rare case of primary malignant mesothelioma originating from the rectovaginal tissue.

A 35 year-old female patient with pelvic malignant mesothelioma is described. The patient underwent total pelvic exenteration due to a pelvic tumor. Macroscopically, the resected tumor was located in the rectovaginal lesion with invasion into the rectal and vaginal wall, and around the internal urethral ostium. Light microscopically, the tumor predominantly consisted of sheets of plump round cells with acidophilic cytoplasm, and focally of tumor cells showing papillary growth pattern. The tumor cells showed remarkable cellular pleomorphism, and were both alcian blue and periodic acid-Schiff stain negative. Electron microscopically, these tumor cells had numerous long bush-like microvilli on their surface with increased length/width ratios. Positive staining with epithelial membrane antigen, cytokeratin, and vimentin, and negative staining with the carcinoembryonic antigen and S-100 protein were observed immunohistochemically. Based on these histological and immunohistochemical estimations, the tumor was diagnosed as a primary malignant mesothelioma originating from the rectovaginal tissue. Review of the literature confirmed the rarity of pelvic malignant mesothelioma. The possibilities of the pathogenesis of the tumor include the tumor's arising from the peritoneal remnant in the rectovaginal tissues, or from the epithelium of the secondary Mullerian system, which shares the same ancestry with the peritoneum.