Pathology of undifferentiated small round cell sarcomas of bone and soft tissues

In recent years, the emphasis on molecular studies has led to remarkable changes in the classification of undifferentiated small round cell sarcomas of bone and soft tissue. Whereas previously Ewing and Ewing-like sarcomas were distinguished, nowadays four molecularly well-defined entities belong to this group: Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusion, sarcoma with CIC rearrangement and sarcoma with BCOR genetic alteration. These tumours occur mainly in children or young adults, are associated with aggressive behaviour and poor prognosis. Although they are microscopically similar, each carries peculiar morphological features and a specific immunophenotype which can suggest the underlying genetic alteration.

Ewing sarcoma is the prototype of small round cell tumours, NFATC2 sarcomas are composed of more epithelioid cells embedded in fibro- or myxohyalin stroma, PATZ1 sarcomas are more characterised by a sclerotic stroma. While CIC sarcomas have focal pleomorphism and epithelioid morphology, BCOR sarcomas often show a more spindle cell appearance. In addition, the use of appropriate immunohistochemical reactions and molecular tests will help to make an accurate diagnosis, which is essential for choosing the right therapy.