VALUE OF IMMUNOHISTOCHEMISTRY IN A SUBSET OF SMALL ROUND BLUE CELL TUMORS

A patient with a retroperitoneal mass extending into the liver had a c ore needle biopsy that showed a small, round, blue cell tumor of uncer tain origin by light microscopy. The tumor cells stained with antibodi es against vimentin, neuron specific enolase, and the S-100 protein. T he cells did not stain with antibodies to muscle specific actin, chrom ogranin A, cytokeratin cocktail, or leukocyte common antigen. Based up on these histologic and immunohistochemical findings, consideration wa s given to either an extra-osseous Ewing's sarcoma or a peripheral neu roepithelioma, and a diagnosis of ''consistent with peripheral neuroep ithelioma'' was rendered. The role of immunohistochemistry in resolvin g this differential and the resulting prognostic implications are disc ussed.