Ovarian Mesonephric-like Adenocarcinoma With Multifocal Microscopic Involvement of the Fimbrial Surface: Potential for Misdiagnosis of Tubal Intraepithelial Metastasis as Serous Tubal Intraepithelial Carcinoma Associated With Ovarian High-grade Serous Carcinoma

Background/Aim: We describe a rare case of ovarian mesonephric-like adenocarcinoma (MLA) involving the fimbria and mimicking serous tubal intraepithelial carcinoma (STIC). Case Report: A 47-year-old woman presented with a 4.4-cm left ovarian mass. Histologically, the ovarian tumor showed papillary and solid architecture, severe nuclear pleomorphism, and increased mitotic activity. Some microscopic foci where the tumor cells spread horizontally along the fimbrial surface epithelium were noted, compatible with STIC. We initially considered the ovarian tumor to be high-grade serous carcinoma accompanied by a fimbrial STIC. However, immunostaining revealed nuclear immunoreactivity for paired box 2 and GATA-binding protein 3, but lacked expression of Wilms tumor 1. A thorough slide review and additional immunostaining revealed architectural diversity, densely eosinophilic intraluminal secretions, and lack of hormone receptor expression, supporting the diagnosis of MLA. Conclusion: Microscopic intraepithelial metastases of the MLA to the fimbria mimic STIC. We recommend ancillary tests, such as immunostaining, in patients with ovarian tumors whenever possible, particularly for those with differential diagnosis of MLA and high-grade serous carcinoma.