Rhabdomyosarcoma and undifferentiated round cell tumours of soft tissue in children, adolescents and young adults

Soft tissue sarcomas are a rare group of heterogenous entities which affect the tissues supporting, connecting and/or surrounding body structures and organs. Factors such as patient age and tumour location can influence how soft tissue sarcomas manifest with biological and clinical behaviour ranging from less aggressive to highly malignant. Recent advances in both diagnosis and treatment have led to new insights into the pathologic, histologic and genomic characterisation of sarcomas and these insights have diversified subtyping. One subtype, small round (blue) cell tumours (SRBCT), constitutes a divergent group of tumours which occur most commonly in the paediatric and young adult population. The clinical characteristics, prognostics and therapeutic approaches regarding these tumours differ greatly. Despite this variation the relative histomorphological uniformity of SRBCTs poses the histopathologist a diagnostic challenge. This challenge is being ameliorated by advances in molecular genetic profiling which is revolutionising diagnosis and providing insight into both biomarkers and therapeutic targets. The following review aims to address the pertinent morphological, immunohistochemical and molecular features of some of the more common SRBCTs, including variants, and hopes to provide the histopathologist with a diagnostic approach focussed upon the key diagnostic features.