Primary Endometrioid Adenocarcinoma in the Right Fallopian Tube Mimicking Female Adnexal Tumor of Probable Wolffian Origin: A Diagnostic Pitfall

Female adnexal tumor of probable Wolffian origin (FATPWO) is a rare neoplasm that morphologically mimics endometrioid carcinoma and lacks specific immunomarkers or molecular alterations, posing a significant diagnostic challenge. We report a case of a 51-year-old woman with an adnexal mass initially diagnosed as endometrioid adenocarcinoma arising in the right fallopian tube. The tumor closely resembled FATPWO both morphologically and immunophenotypically, exhibiting loss of PAX8 and cytokeratin 7 expression, with no identifiable recurrent molecular alterations. However, the extensive tubular and focal papillary growth patterns observed are uncommon in FATPWO. Notably, PAX8 negativity can occur in endometrioid carcinoma involving adnexal tissue with low-grade morphology, as seen in this case. Endometrioid carcinoma of the fallopian tube can closely mimic FATPWO, particularly when PAX8 and CK7 are both negative. Molecular testing is limited in distinguishing between the two due to the absence of disease-defining alterations in FATPWO and the lack of specific mutations in some endometrioid carcinomas.