Hopkins, Mark

Abstract Introduction/Objective Ovarian endometrioid carcinomas can display varied morphologic and immunophenotypic features, complicating diagnosis and grading. We analyzed adnexal tumors with absent or limited PAX8 expression to distinguish endometrioid carcinoma from histologic mimics such as sex cord-stromal and carcinoid tumors. Methods/Case Report We retrospectively reviewed ovarian tumors with epithelioid morphology and absent or limited PAX8 expression. Detailed histologic evaluation and immunophenotyping were performed. Features including sex cord-like morphology, background endometriosis, associated borderline components, and squamous morular or mucinous differentiation were assessed. PAX8, CK7 and β-catenin, sex cord and neuroendocrine markers were evaluated. Results Sixteen cases were identified (age 33–76, median 69). Sex cord-like morphology was present in all. Endometriosis or adenofibroma (n = 5), squamous/mucinous differentiation (n = 5), and hyalinized/corded patterns (n = 5) were observed. Most cases (13/16) lacked PAX8 expression; 14 showed aberrant nuclear β-catenin. Sex cord markers were negative. CK7 was negative or focal in 7 cases; 3 showed neuroendocrine marker expression. Conclusion Some ovarian endometrioid carcinomas lack PAX8 and mimic sex cord or neuroendocrine tumors, imposing diagnostic challenges. Aberrant β-catenin and characteristic morphology aid diagnosis. Ancillary testing, including immunohistochemistry and CTNNB1 mutation analysis, is essential to avoid misclassification and guide appropriate management.