Cytological, Immunocytochemical, and Molecular Findings of Extraskeletal Round Cell Sarcomas in Paediatric Patients

BACKGROUND: Round cell soft-tissue sarcomas in the paediatric population are a common cytomorphological group of tumours. The most common entity in this group is rhabdomyosarcoma. Other representatives include Ewing sarcoma, other undifferentiated small round cell sarcomas (so-called Ewing-like sarcomas, such as CIC-rearranged sarcoma, sarcoma with BCOR genetic alterations, and round cell sarcoma with EWSR1-non-ETS fusions), and desmoplastic small round cell tumour. SUMMARY: The main cell population in round cell sarcomas is composed of small round cells. Other cell morphologies may be seen in specific entities, such as spindle cells in Ewing-like sarcomas or rhabdomyoblasts in rhabdomyosarcomas. Smear backgrounds may contain varying amounts and types of stroma, for example, fibromyxoid stroma in CIC-rearranged sarcoma or sarcoma with BCOR genetic alterations or desmoplastic stromal fragments in desmoplastic small round cell tumour. As cytological specimens are highly cellular, they are excellent material for molecular studies. KEY MESSAGES: Round cell sarcomas share many morphological similarities, so the differential diagnosis can be broad; however, subtle differences between tumours can aid the final diagnosis. Nevertheless, most round cell sarcomas require molecular studies for accurate classification, and cytological specimens are usually high-quality material for such ancillary techniques.