Extraskeletal Ewing's sarcoma. A clinicopathologic and electron microscopic. Analysis of 8 cases.

This clinicopathologic study concerns 8 cases of extraskeletal Ewing's sarcoma, including electron-microscopic examination of one case. In three patients, autopsy was done. The age of the patients ranged from 12 to 31 years with a median of 16 years. The tumors mainly arose in the soft tissues of the trunk (4 cases) and the lower extremity (3 cases). Histologically, they were made up of closely packed uniform, small cells, arranged in sheets separated by strands of fibrovascular stroma. The tumor cells had round to oval nuclei with finely dispersed chromatin and scanty ill-defined cytoplasm almost invariably containing a fair amount of diastase-digested PAS-positive material. Ultrastructurally, the tumor cells were composed principally of undifferentiated mesenchymal cells, and contained prominent pools of glycogen in the cytoplasm. Aggregates of intermediate filaments were seen in a perinuclear location. These light- and electron-microscopic findings are indistinguishable from those of Ewing's sarcoma of the bone. Differential points from other soft-tissue small round cell sarcomas such as malignant neuroepithelioma (peripheral neuroblastoma), embryonal or alveolar rhabdomyosarcoma were briefly discussed.