Peritoneal inclusion cysts with mural mesothelial proliferation. A clinicopathological analysis of six cases.

The clinical and pathological features of six cases of a rare, hitherto unreported type of pelvic cyst are described. The cysts occurred in female patients (15-51 years of age) who presented with lower abdominal pain and evidence of a pelvic mass. All but one of the patients had a history of prior pelvic surgery. Laparotomy revealed dense pelvic adhesions and a cystic lesion that was interpreted by the surgeon as ovarian in origin. On gross examination, the cysts measured up to 15 cm in diameter, were uni- or multilocular and thin-walled, and contained bloody or serous fluid. They were adherent to the surface of the ovaries, but did not involve the ovarian parenchyma. On microscopic examination, the cyst walls were composed of markedly inflamed granulation and fibrous tissue in which were embedded mesothelial cells arranged in glands, nests, cords, and single cells. Because of this infiltrative pattern, as well as cytological atypia and mitotic activity within the mesothelial cells, a diagnosis of cancer was considered in several cases. All patients are alive with no evidence of disease at postoperative intervals of 6 months to 5 years. We propose that these cysts represent peritoneal inclusion cysts (benign cystic mesotheliomas) in which the histological appearance has been altered by an unusual degree of inflammation, fibrosis, and entrapment of mesothelial cells.