Round cell sarcomas - biologically important refinements in subclassification.

Round cell sarcomas are a heterogeneous group of tumors that often affect children and young adults and, if untreated, often pursue a very aggressive clinical course. Specific subtypes of round cell sarcoma, like Ewing sarcoma or rhabdomyosarcoma, respond to well-defined therapeutic regimens so that proper classification is crucial for appropriate patient management. A subset of round cell sarcomas, however, lack specific clinical, morphologic, and immunophenotypic features and cannot be unequivocally classified based on such features. Systematic application of cytogenetics and molecular genetic techniques has allowed for the identification of an increasing number of genetically defined subgroups within this category of undifferentiated tumors. Although the clinical relevance of these molecular categories is yet to be proven, the systematic identification of lesions that share reproducible biologic, and often morphologic and immunophenotypic features, has great impact in terms of biologic understanding and coherent classification schemes, and will help to guide the potential development of rational new therapies. In this review we discuss the main categories of undifferentiated round cell sarcoma, in relation to Ewing sarcoma and its molecular variants, with particular emphasis on the genetic and biologic features of recently described entities including desmoplastic small round cell tumor and CIC-DUX4 as well as BCOR-CCNB3-associated round cell sarcomas. This article is part of a Directed Issue entitled: Rare Cancers.