An abdominal mass detected in a 36-year-old man was thought from radio
logic studies to be a renal neoplasm, but at surgery the kidney was fo
und to be uninvolved. The 9.5-cm retroperitoneal tumor was resected to
tally and found to be encapsulated, and it appeared to be benign by li
ght microscopy. The pattern of the spindle cells throughout the tumor
suggested a cellular schwannoma, but immunocytochemical and ultrastruc
tural studies did not support schwannian differentiation and instead r
evealed epithelial and smooth muscle features compatible with a myoepi
thelioma.