[Undifferentiated small round cell sarcomas of bone and soft tissue]

In the 2020 5th edition of the World Health Organization classification of soft tissue and bone tumours a major reorganization of Undifferentiated Small Round Cell Sarcomas (USRCS) took place based on the underlying molecular features. The classification now recognizes Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusions, CIC-rearranged sarcoma and sarcoma with BCOR alterations. The focus on these genetic alterations highlights the importance of molecular techniques in the diagnosis of these entities. Knowledge of these features can drastically reduce the time to diagnosis and avoid potential misdiagnosis. Molecular diagnostic capabilities should not be limited to an overall small number of centres worldwide as is reflected by the WHO's recognition of 'essential' and 'desirable' diagnostic criteria. A good knowledge of the usual histomorphology, uncommon variants and diagnostic pitfalls remains essential even in centres with access to a full molecular testing arsenal. This review aims to give an overview of the current classification of USRCS not by going over each entity, but instead going over the molecular, morphological, immunophenotypic and clinical features step by step to allow easy comparison of these features between the separate entities.