Endometrial Stromal Tumors: The New WHO ClassificationConklin, Christopher M. J. MD, FRCPC; Longacre, Teri A. MDAdvances In Anatomic Pathology: November 2014 - Volume 21 - Issue 6 - p 383–393 doi: 10.1097/PAP.0000000000000046 Review Articles Buy Abstract Author InformationAuthors Article MetricsMetrics Endometrial stromal tumors are rare uterine mesenchymal neoplasms that have intrigued pathologists for years, not only because they commonly pose diagnostic dilemmas, but also because the classification and pathogenesis of these tumors has been widely debated. The current World Health Organization recognizes 4 categories of endometrial stromal tumor: endometrial stromal nodule (ESN), low-grade endometrial stromal sarcoma (LG-ESS), high-grade endometrial stromal sarcoma (HG-ESS), and undifferentiated uterine sarcoma (UUS). uterine sarcoma. These categories are defined by the presence of distinct translocations as well as tumor morphology and prognosis. Specifically, the JAZF1-SUZ12 (formerly JAZF1-JJAZ1) fusion identifies a large proportion of ESN and LG-ESSs, whereas the YWHAE-FAM22 translocation identifies HG-ESSs. The latter tumors appear to have a prognosis intermediate between LG-ESS and UUS, which exhibits no specific translocation pattern. This review (1) presents the clinicopathologic features of endometrial stromal tumors; (2) discusses their immunophenotype; and (3) highlights the recent advances in molecular genetics which explain their pathogenesis and lend support for a new classification system. *Department of Pathology, Surrey Memorial Hospital, Surrey, BC, Canada †Department of Pathology, Stanford University, Stanford, CA The authors have no funding or conflicts of interest to disclose. Reprints: Christopher M. J. Conklin, MD, FRCPC, Histopathology, Surrey Memorial Hospital, 13750 – 96th Ave, Surrey, BC, Canada, V3V 1Z2 (e-mail: Chris.Conklin@fraserhealth.ca).All figures can be viewed online in color at http://www.anatomicpathology.com. Copyright © 2014 Wolters Kluwer Health, Inc. All rights reserved.