[Ewing sarcomas and Ewing-like sarcomas : New aspects].

Sarcomas of the Ewing family of tumors are aggressive neoplasms occurring in bone and soft tissue of mostly children and young adults. Classical Ewing sarcomas are pathognomonically characterized by fusions between a gene of the RNA-binding TET family (EWSR1 or FUS) with a gene of the ETS-transcription family (FLI1, ERG, ETV1, ETV4 or FEV). Less frequent cases designated as Ewing-like sarcomas show different genetic rearrangements between EWSR1 and non-ETS genes (NFATC2, POU5F1, SMARCA5, PATZ, ZSG, SP3). Moreover, new molecular alterations biologically unrelated to Ewing sarcomas have recently been described in the category of undifferentiated round cell sarcomas including CIC-DUX4 fusions or BCOR alterations, each carrying unique gene expression signatures. In contrast to classical Ewing sarcomas, the morphologic spectrum of these tumor entities is much broader and includes round cell areas as well as spindled and myxoid components. The immunohistochemical profile with inconsistent CD99 positivity makes diagnosis more difficult and requires the use of a broad spectrum of antibodies and elaborate molecular work-up. Further studies for future therapeutic decision making in these newly described round cell sarcomas as well as for molecular subclassification of undifferentiated round cell sarcomas are ongoing.