Extraskeletal Ewing sarcoma. An ultrastructural study.

Soft tissue tumors with the characteristics of Ewing sarcoma of bone have thus far only been studied by light microscopy. A pelvic tumor of this type in a 13-year-old girl was examined by electron microscopy. Comparison of its ultrastructural features with those of reported cases of bony Ewing sarcoma reveal much similarity. It is believed that they are probably identical and that the tumor cells are of immature mesenchymal type. However, their site of origin and the direction of their potential differentiation remain obscure.