Aggressive Clinical Course of Uterine Tumor Resembling Ovarian Sex Cord Tumor (UTROSCT) Harboring GREB1::SS18 Fusion Gene: A Case Report.

We report a fatal case of malignant uterine tumor resembling ovarian sex cord tumor (UTROSCT) harboring the GREB1::SS18 fusion gene. A 64-year-old woman presented with a uterine tumor extending into the inferior vena cava, along with multiple hepatic and bone metastases. Although imaging suggested low-grade endometrial stromal sarcoma, histopathology revealed sex cord-like structures. GREB1::SS18 fusion was identified by RNA sequencing and independently confirmed by RT-PCR and Sanger sequencing. Despite multimodal therapy, the patient died 26 months postoperatively. While most UTROSCTs follow a benign course, rare malignant cases exist. Literature review suggests GREB1-rearranged UTROSCTs are more aggressive, occurring in older patients with larger tumors compared to ESR1-rearranged tumors. This is the first report of a fatal outcome associated with the GREB1::SS18 fusion. This case underscores the importance of molecular diagnosis for predicting malignant potential in UTROSCT, especially when atypical aggressive features are present.