ES represents perhaps a more primitive, less differentiated neoplasm in the spectrum of peripheral primitive neuroectodermal tumors.
Patients typically present in the first two decades of life.
Morphologically, the neoplasm is characterized by monotonous sheets of hyperchromatic cells with round to oval nuclei, high nuclear to cytoplasmic ratios and scant cytoplasm. Cytoplasmic glycogen can often be detected by periodic acid schiff stain or electron microscopy. A filigree pattern of infiltration in soft tissue is associated with an adverse prognosis. The immunohistochemical profile usually demonstrates immunopositivity to vimentin and the MIC2 gene product. A fusion occurs between the EWS gene on 22q12 and the FLI-1 gene on 11q24.