Round Cell Sarcomas: Newcomers and Diagnostic Approaches.

Undifferentiated sarcomas of soft tissue and bone have been defined as tumors with no identifiable morphologic, immunohistochemical, or molecular features indicating tumor cell origin. In young patients, these tumors frequently have a round or spindle cell morphology. Recently described recurrent translocations within this category have led to the recognition of new molecular subtypes of round cell sarcomas, and several of them have a more aggressive clinical course and less chemosensitivity. Because these "newcomers" are diagnosed based on their molecular characteristics, molecular investigation is key in the diagnosis and optimal treatment of these challenging tumors.