[Mixed epithelial and stromal tumor growing with polypoid pattern in the renal pelvis].

A 68-year-old woman was found incidentally to have right hydronephrosis and a renal pelvic mass by abdominal ultrasonography. Radiographic examinations revealed a heterogeneous renal pelvic tumor, and right nephroureterectomy was performed. The tumor was well circumscribed yellow-whitish solid mass with scattered cysts. Histologically, the tumor was composed of both mesenchymal and epithelial components. The mesenchymal elements consisted of fibroblasts and smooth muscle cells, and the epithelial elements of cystic and tubular structures lined by cuboidal epithelium. Atypia and mitoses were not identified. The patient was free of recurrence 42 months postoperatively. Mixed epithelial and stromal tumor of the kidney is a recently recognized neoplasm that occurs almost exclusively in perimenopausal woman. Similar tumors have been reported previously under various names, including adult mesoblastic nephroma and cystic hamartoma of the renal pelvis. Histogenesis of the tumor is still controversial.