An Endometrial Stromal Tumor with Extensive Smooth Muscle Differentiation, Collagenous Rosettes and Focal Fibromyxoid Change: Case Report and Diagnostic Pitfalls

Endometrial stromal tumors (ESTs) are the second most common type of uterine mesenchymal tumors and endometrial stromal nodule (ESN) is a benign neoplasm within this category. ESNs can exhibit smooth muscle differentiation, epithelial patterns, or fibromyxoid stroma, which can complicate differential diagnosis. In this report, we present a rare case of ESN with featuring extensive smooth muscle differentiation and focal myxoid areas. Grossly, the tumor was intramural, well-circumscribed, and cystic. Histologically, the tumor consisted of cellular areas, myxoid areas, and extensive collagen roset formation. Immunohistochemical analysis showed that, the tumor cells were diffusely positive for B-catenin and WT-1, focally positive for CD56, smooth muscle marker, CAM5.2, PanCK and CD10. ESN with smooth muscle differentiation and collagenous rosettes should be considered in the differential diagnosis of intracavitary and intramural uterine mesenchymal tumors. Both fibromyxoid stroma and extensive smooth muscle differentiation have been documented in ESNs. Recognizing this morphologic variant of ESN is crucial for accurate tumor classification.