A Case of Ovarian Seromucinous Borderline Tumor With a Predominant Serous Component Suggesting Serous Borderline Tumor With Mucinous Differentiation

The current classification of ovarian seromucinous tumors is somewhat complex and may present challenges in routine diagnostic practice. In the fifth edition of the WHO classification, seromucinous carcinoma (SMC) is generally regarded as a subtype of endometrioid carcinoma (EC) because of its limited diagnostic reproducibility and evidence that most tumors meeting the morphologic criteria for SMC can be reassigned to EC or low-grade serous carcinoma (LGSC) with mucinous differentiation based on integrated morphologic, immunophenotypic, and genotypic analyses. However, SMC-like tumors reassigned to LGSC remain unaddressed in the current classification systems. Although seromucinous borderline tumor (SMBT) is generally regarded as a reproducible entity, diagnostic difficulties may arise in certain cases. We report the case of an 80-yr-old woman with a large multicystic ovarian tumor predominantly composed of serous cells with micropapillary architecture characteristic of a serous borderline tumor (SBT), admixed with Müllerian-type mucinous cells. The serous component showed diffuse and strong WT1 positivity, whereas the mucinous cells exhibited variable WT1 expression. Under the current WHO classification, this tumor is diagnosed as an SMBT because it fulfills the morphology-based criteria. However, the predominance of the serous component showing diffuse WT1 expression is unusual for this entity and raises the possibility that the tumor represents SBT with mucinous differentiation, a potential borderline malignant counterpart of LGSC with mucinous differentiation. This case suggests a potential diagnostic overlap between SMBT and SBT, underscoring the need for further accumulation of similar cases and integrated analyses to clarify whether such tumors may warrant distinction from typical SMBT.