Small round-cell type of malignant peripheral nerve sheath tumor.

Five cases of primitive, small, round-cell tumor that are a type of hitherto unclassified neurogenic sarcoma are described. The tumors were located deep within the soft tissue of the trunks and limbs without association with major nerves. The histologic features consisted mainly of uniform, small, round, tumor cells with scanty cytoplasm. Foci of uniform, short, spindle-shaped tumor cells arranged in whorl-like patterns were observed in some areas. Although the immunoreactivity for the neural markers, Leu-7 and MIC-2, was not marked, cell processes and fragmentous basal laminae, which are ultrastructural neural features, were found in both spindle-shaped and round tumor cells. In four cases, EWS chimeric transcripts were analyzed by reverse-transcription polymerase chain reaction. EWS chimeric mRNA (EWS-FLI-1, EWS-ERG, EWS-E1AF, EWS-ETV1, EWS-FEV) was not detected in any cases. The tumors were not consistent with peripheral primitive neuroectodermal tumor. We propose them as a small round-cell type of MPNST that might differentiate toward the immature neural cells.