Endometrial stromal sarcoma (ESS) is the second most common malignant uterine mesenchymal tumor. It affects women primarily in the perimenopausal age group. ESSs are morphologically heterogeneous. The distinction between uterine smooth muscle tumors such as cellular leiomyoma and myxoid leiomyosarcoma and low-grade ESS can be problematic when stromal sarcomas show prominent smooth muscle differentiation and abundant myxoid stroma, respectively. We herein present a rare case of fibromyxoid variant of ESS, which was misdiagnosed as hydropic leiomyoma on intraoperative frozen section examination. Grossly, the uterine mass consisted of intracavitary and intramural portions. The intracavitary portion with extensive hydropic degeneration mimicked a hydropic leiomyoma. In contrast, the intramural portion displayed an obvious tongue-like myometrial invasion. Histologically, the tumor consisted of both cellular (20%) and myxoid (80%) areas. In the cellular areas, oval to spindle-shaped tumor cells with bland nuclear features were found to surround concentrically a rich vascular network of arterioles, a characteristic of ESS. In addition, two relatively well-circumscribed nodular lesions showing atypical bizarre nuclei were identified in the myxoid area. Immunohistochemically, the tumor cells were diffusely and strongly positive for CD10. The present case indicates a wide morphological spectrum of ESS. Fibromyxoid variant of ESS should be considered in the differential diagnosis of intracavitary and/or intramural uterine mesenchymal tumors with myxoid differentiation. It is important to avoid confusion between fibromyxoid ESS and myxoid leiomyosarcoma because of the differences in their clinical course, treatment, and prognosis.
Keywords: Endometrial stromal sarcoma; atypical bizarre nuclei; fibromyxoid variant.