Peritoneal Fluid Cytology of Mesonephric‐Like Adenocarcinoma of the Fallopian Tube: A Case Reflecting Diverse Histologic Architecture

Mesonephric-like adenocarcinoma (MLA) is a rare gynecologic cancer, and primary involvement in the fallopian tube is extremely uncommon. We report a case of fallopian tube MLA with detailed peritoneal fluid cytology findings and a comparison of conventional smear and liquid-based cytology preparations. A 57-year-old woman presented with pelvic pain and ascites. Histopathologic examination confirmed stage IIIC MLA of the fallopian tube. Peritoneal fluid cytology, using both conventional smears and liquid-based cytology, showed moderate to high cellularity, with three-dimensional clusters exhibiting tubular, cribriform, papillary, and solid architectural patterns. The tumor cells were relatively uniform, and occasional cribriform structures were observed. Histologically, the tumor exhibited predominantly tubular and solid growth patterns, with focal areas resembling mesonephric tubules. The tumor cells showed focal elongated nuclei with overlapping nuclear grooves, resembling papillary thyroid carcinoma-like nuclear features characteristic of MLA. Immunohistochemically, the tumor cells demonstrated diffuse nuclear positivity for GATA3 and TTF-1, with widespread PAX8 expression. This case demonstrates that peritoneal fluid cytology can closely reflect the histologic heterogeneity of MLA and emphasizes the importance of correlating cytologic architectural features with immunohistochemical findings, particularly in this extremely rare entity.