Neuroectodermal differentiation in "extraskeletal Ewing's sarcoma".

A small round cell tumor of soft tissue arising in the retroperitoneum of an 18-year-old woman is reported. The light and electron microscopic features of the tumor were mostly indistinguishable from Ewing's sarcoma of the bone and in some parts from differentiating neuroblastoma with mature neurons and Schwann cells. An immunohistochemical study using anti-NSE antibody showed many widespread NSE-positive cells lying singly and, more often, in clusters including the undifferentiated areas. The tumor was not anatomically related to the paravertebral sympathetic ganglia or adrenal glands. The present findings strongly suggest that certain extraskeletal Ewing's sarcomas are very primitive neuroectodermal tumors.