Myoepithelioma of the retroperitoneum.

An abdominal mass detected in a 36-year-old man was thought from radiologic studies to be a renal neoplasm, but at surgery the kidney was found to be uninvolved. The 9.5-cm retroperitoneal tumor was resected totally and found to be encapsulated, and it appeared to be benign by light microscopy. The pattern of the spindle cells throughout the tumor suggested a cellular schwannoma, but immunocytochemical and ultrastructural studies did not support schwannian differentiation and instead revealed epithelial and smooth muscle features compatible with a myoepithelioma.