A Novel EPC1::KDM2B Fusion in High-grade Endometrial Stromal Sarcoma.

The spectrum of endometrial stromal sarcoma (ESS) has expanded substantially since the publication of the most recent World Health Organisation (WHO) Classification of Female Genital Tumours and the advent of widely available genomic testing. We describe a uterine mesenchymal tumor harboring a novel EPC1::KDM2B fusion, best classified within the umbrella of high-grade endometrial stromal sarcoma (HGESS). This tumor was composed of a uniform population of spindled cells with some myxoid stroma, a mitotic rate of up to 21/10 high-power fields, and a largely pushing margin with focal vascular invasion. Immunohistochemistry showed strong and diffuse cyclin D1 positivity while CD10, WT1, DOG1, CD117, CD34, CD99, S100, MelanA, SMA, desmin, and h-caldesmon were negative. The tumor was confined to the uterus and no recurrence has been detected thus far, albeit with a short follow-up interval of 9 mo.