Round cell tumours of bone.

Ewing's sarcoma is a very rare tumor which has, however, attracted much oncological interest since the dramatic improvement of its prognosis under chemotherapy. Its histogenesis has been discussed controversially for a long time, including a possible origin in immature reticulum, myogenous, endothelial and undifferentiated mesenchymal cells. Repeated reports have also suggested a possible neuroectodermal genesis. Convincing arguments, however, have only been brought forward during recent years, since it was found out that Ewing's sarcoma and malignant peripheral neuroectodermal tumor share a common chromosome translocation 11;22. In the meantime this hypothesis has been strengthened by numerous cell biological analyses. Histological differential diagnosis of Ewing's sarcoma has been improved by immunohistological methods. In most cases, they can be distinguished from lymphoma (leucocyte common antigen, B and T markers) and embryonal rhabdomyosarcoma (muscle specific actin, desmin). Apart from this, we now have an antibody specific for Ewing's sarcoma and malignant peripheral neuroectodermal tumors but not with neuroblastoma. Recent investigations regarding the prognosis under chemotherapy have shown that tumors with neural differentiation have a comparatively poor prognostic outcome. This is why in addition to the demonstration of Homer Wright pseudo-rosettes different neural markers such as neuron specific enolase, synaptophysin and chromogranin are to be analysed. Small cell osteosarcoma is a very rare sub-entity of osteosarcomas. Recent studies have shown that the tumor resembles Ewing's sarcoma with partial mesenchymal differentiation including osteoid formation.