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

ABSTRACT 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.