Cytology of Undifferentiated Round-Cell Sarcomas of Bone and Soft Tissue: Ewing Sarcoma or Not Ewing Sarcoma , That Is the Question

Abstract

<b><i>Background:</i></b> Undifferentiated round-cell sarcomas (URCSs) of soft tissue and bone are a group of clinically heterogeneous tumors. Diagnosis of these malignancies is based mainly on recurrent genetic alterations. The most common and the best known representative of this group is Ewing sarcoma (ES) which is characterized by gene fusions including EWSR1 or FUS and ETS transcription factors family. Other newly described entities are CIC-rearranged sarcoma, sarcoma with BCOR genetic alterations, and round-cell sarcoma with EWSR1-non-ETS fusions. All these novel tumors are known as Ewing-like sarcomas. <b><i>Summary:</i></b> It is believed that morphologic features of ES and Ewing-like sarcomas vary only slightly or even that cytomorphology is not relevant. But differences are usually obvious, and some cytologic findings, such as spindle cells, connective tissue fragments, or myxoid stroma, are typical for Ewing-like sarcomas but not for ES. Each of these entities is also characterized by different immunoprofiles. The aim of this review was to summarize cytomorphologic and immunohistochemical features of URCS and compare them with other small round-cell tumors. <b><i>Key Messages:</i></b> Cytology can be successfully used in URCS diagnosis as a complementary tool for core-needle biopsy or even alone in selected cases, especially in recurrent and metastatic tumors. Knowing the morphologic and immunohistochemical differences between URCS is essential to provide appropriate ancillary studies and make a definitive diagnosis.