Uterine Sarcomas With KDM2B Fusion: Clinicopathological Analysis of Three Cases Supporting a Novel BCOR-Altered Entity.

High-grade endometrial stromal sarcomas (HGESS) with BCOR-alteration include sarcomas with YWHAE::NUTM2A/B fusion, ZC3H7B::BCOR fusion, BCOR-ITD, BCOR alternative fusion, and BCORL1 fusion. Recently, 4 cases of uterine sarcomas with KDM2B fusion have been described, which were suggested to belong to the category of BCOR-altered tumors. These tumors are characterized by recurrent fusions of the KDM2B gene with fusion partners including EPC1, EP400, and CITED1. Morphologically, some of them may have a nonspecific spindle-cell or round-cell appearance. In other tumors, however, peculiar morphologic features combining atypical round cells with at least focal sex cord-like features and myxoid stroma were found. In our study, we described 3 additional cases of uterine sarcomas with KDM2B fusion, with fusion partners including EPC1, EPC2, and CREBBP. Morphologically, these tumors showed either pure spindle-cell morphology, a combination of spindle and round cells, or a heterogeneous pattern with spindle cells, round cells, and prominent hyalinization, with only focal, vague sex cord-like features. Despite possible morphologic overlap with low-grade endometrial stromal sarcomas in some cases, these tumors seem to represent a variant of HGESS belonging to the category of BCOR-altered sarcomas and have a propensity for aggressive behavior, as 2 of our patients died of disease, and the third had pelvic recurrence before being lost to follow-up.