Ovarian endometrioid carcinoma, more than any other type of ovarian epithelial malignancy, demonstrates a varied morphology which can cause problems in diagnosis. In tubo-ovarian tumor pathology, WT1 is a commonly used marker as it is consistently expressed in low-grade and high-grade serous carcinomas and is often considered a specific marker of a serous phenotype. However, ovarian endometrioid neoplasms may also express WT1 which may contribute to misdiagnosis. We report our experience with 23 ovarian endometrioid neoplasms (4 borderline tumors, 19 carcinomas), mainly received in consultation, which were WT1 positive (diffuse in 11 cases) which often contributed to misdiagnosis. Endometriosis was identified in the same ovary in 6 cases and squamous elements in 7. We describe strategies for distinguishing such neoplasms, which may exhibit morphologic overlap with serous tumors, from low-grade and high-grade serous carcinomas and stress that a diagnosis of HGSC is unlikely with two grossly and histologically normal fallopian tubes. We also stress that a panel of markers should always be used rather than relying on a single marker and that when the morphology is classical of an endometrioid carcinoma, diagnostic immunohistochemistry is not needed given the potential for confusion in cases showing "aberrant" staining. We also discuss the phenomenon of "aberrant" immunohistochemical staining in endometrioid carcinomas which appears more common than in other ovarian carcinomas.
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