Five ovarian tumors were associated with florid mesothelial hyper- plasia that in four cases created initial problems in histologic classification and staging of the tumors. In two cases, small nests, cords, and gland-like arrangements of atypical mesothelial cells within the cyst wall of a serous or mullerian mucinous tumor of borderline malignancy were initially considered to possibly represent, or were misdiagnosed as. foci of stromal invasion, lymphatic invasion, or both. In the three other casesSummary:
an endometrioid adenocarcinoma, a struma ovarii, and a yolk sac tumorSummary:
papillary and sheet-like proliferations of mesothelial cells involved the extraovarian pelvic peritoneum: in one of the cases, necrosis of the mesothelial cells was prominent. These foci of extra- ovarian mesothelial hyperplasia were interpreted as metastatic tumor intraoperatively (three cases) and on initial microscopic examination (two cases). Factors that may have predisposed to the mesothelial hyperplasia included large size of the primary tumor (two cases), the presence of metastatic tumor elsewhere in the pelvis (three cases), ascites (two cases), and coexistent endometriosis (two cases). Distinction of mesothelial proliferations from invasive or metastatic ovarian tumor is obviously important in correct classification, staging, and management of the tumor. Awareness of the occasional occurrence of florid mesothelial hyperplasia in patients with ovarian neoplasms and attention to the histologic, histochemical. and immunohistochemical features of the mesothelial proliferation should facilitate the correct diagnosis.