Damiano Arciuolo, Sara Patrizi, Rita Alaggio, Antonio Travaglino, Giulia Scaglione, Silvia Vallese, Angela Santoro, Lucia Pedace, Claudia Nardini, Giuseppe Maria Milano, Franco Locatelli, Isabella Giovannoni, Sabina Barresi, Luigi Pedone Anchora, Camilla Fedeli, Francesca Ciccarone, Frediano Inzani, Evelina Miele, Gian Franco Zannoni
High-grade endometrial stromal sarcoma (HGESS), which was not recognized as a distinct entity in the third edition of the World Health Organization (WHO) classification and was therefore included in the undifferentiated uterine sarcoma (UUS) category, was reestablished as a separate entity in the fourth edition. HGESS shares BCL-6 corepressor (BCOR) alterations with undifferentiated small round-cell sarcoma (USRCS) of the soft tissue and clear cell sarcoma of the kidney (CCSK). This study aims to perform a comparative morphological, immunohistochemical, and molecular analysis of HGESS, UUS, USRCS, and CCSK. Consecutive uterine sarcomas diagnosed as HGESS or UUS were reviewed and compared to BCOR-altered USRCS (n = 28) and CCSK (n = 10) (including both previously published and new cases). Molecular, DNA methylome (DNAm), and copy number variation (CNV) analyses were performed. DNAm data of 93 previously analyzed uterine and round-cell tumors of several different types were used for clustering analysis. Twenty-five uterine sarcomas (six HGESS and 19 UUS) were included; five of six HGESS cases showed fusions associated with BCOR alterations (YWHAE::NUTM2A/B, EPC1::KDM2B, ZC3H7B::BCOR); UUS showed no fusions (n = 15) or fusions unrelated to BCOR alterations (n = 4). All USRCS and CCSK cases showed either BCOR-ITD or BCOR alteration-related fusions. BCOR-altered HGESS showed broad morphological and immunophenotypic overlap with USRCS and CCSK. DNAm analysis showed that BCOR-altered HGESS, USRCS, and CCSK clustered together, separately from all other tumor types. The non-BCOR-altered HGESS showed a JAZF1::SUZ12 fusion and arose from a low-grade endometrial stromal sarcoma component. CNVs were found in 15/16 uterine sarcomas and in 16/36 USRCS/CCSK cases. BCOR-altered HGESS appeared to be closely related to BCOR-altered USRCS and CCSK rather than to other uterine sarcomas. We suggest that HGESS with BCOR alterations may warrant reclassification as 'BCOR-altered uterine sarcomas'. © 2026 The Pathological Society of Great Britain and Ireland.