Background: Low-grade endometrial stromal sarcoma (LG-ESS) is a rare uterine mesenchymal tumor, constituting only 0.2% of all female genital tract neoplasms. While LG-ESS typically presents as an indolent uterine mass, extrauterine manifestations are exceedingly uncommon and diagnostically confounding. We report a unique and diagnostically challenging case of LG-ESS with extensive extrauterine spread, initially misinterpreted as a primary ovarian malignancy.

Case Summary: A 36-year-old woman presented with abdominopelvic fullness and bilateral adnexal masses. Elevated CA-125 and imaging features mimicked advanced-stage ovarian carcinoma. Surgical exploration revealed bilateral ovarian involvement, extensive peritoneal and omental deposits, and an endometrial polypoid lesion. Histopathology revealed bland spindle cells in whorled architecture without mitoses or necrosis, raising differentials of granulosa cell tumor, leiomyosarcoma, and stromal sarcoma. Immunohistochemistry showed CD10, ER, PR, and Cyclin D1 positivity, suggestive of LG-ESS. The diagnosis was confirmed by FISH demonstrating JAZF1-SUZ12 gene rearrangement. The patient responded well to adjuvant hormonal therapy and remains disease-free.

Conclusion: This is the first reported case of JAZF1-SUZ12 fusion-positive LG-ESS presenting as a massive abdominopelvic mass with widespread metastases, clinically masquerading as an advanced ovarian carcinoma. The case highlights the critical role of integrated histopathology, immunoprofiling, and molecular diagnostics in identifying rare presentations of LG-ESS. This report not only expands the clinical spectrum of LG-ESS but also underscores the potential for misdiagnosis in atypical extrauterine settings, reinforcing the importance of molecular confirmation in equivocal cases.

Keywords: Low-grade endometrial stromal sarcoma, JAZF1-SUZ12, Ovarian mimic, Metastasis, immunohistochemistry, FISH, Diagnostic challenge