Spindle and Round Cell Sarcoma With EWSR1-PATZ1 Gene Fusion A Sarcoma With Polyphenotypic DifferentiationChougule, Abhijit, MD*; Taylor, Martin S., MD, PhD*; Nardi, Valentina, MD*; Chebib, Ivan, MD*; Cote, Gregory M., MD, PhD†; Choy, Edwin, MD, PhD†; Nielsen, G. Petur, MD*; Deshpande, Vikram, MD*The American Journal of Surgical Pathology: February 2019 - Volume 43 - Issue 2 - p 220–228 doi: 10.1097/PAS.0000000000001183 Original Articles Buy SDC Abstract Author InformationAuthors Article MetricsMetrics The evolving classification of round cell sarcomas is driven by molecular alterations. EWSR1-PATZ1 fusion positive spindle and round cell sarcoma is one such new tumor entity. Herein, we report 2 EWSR1-PATZ1 fusion positive spindle and round cell sarcomas with overlapping histologic features and polyphenotypic differentiation. The intra-abdominal tumors affected female patients, 31-and 53-year old. Both tumors showed sheets and nests of round to spindle cells, fine chromatin, tiny conspicuous nucleoli, moderate cytoplasm, and thick bands of intratumoral fibrosis. On immunohistochemistry, both tumors showed positivity for CD99, desmin, myogenin, MyoD1, S100, Sox10, CD34, and GFAP and were negative for keratin. Fluorescence in situ hybridization revealed rearrangement at EWSR1 locus. Next-generation sequencing–based RNA fusion assay revealed EWSR1-PATZ1 fusion in both cases. EWSR1-PATZ1 fusion positive spindle and round cell sarcomas show abundant intratumoral fibrosis and polyphenotypic differentiation, thus mimicking a range of tumors including desmoplastic small round cell tumor. The precise classification of this spindle and round cell sarcoma and its relationship to the Ewing sarcoma family of tumors remains to be determined. *Department of Pathology †Center for Sarcoma and Connective Tissue Oncology, Massachusetts General Hospital, Boston, MA A.C. and M.T. contributed equally. Conflicts of Interest and Source of Funding: The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Correspondence: Vikram Deshpande, MD, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114 (e-mail: email@example.com). Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.