Mixed epithelial and stromal tumor of kidney with malignant transformation: report of two cases and review of literature.

We present 2 cases of mixed epithelial and stromal tumor of the kidney with sarcomatous transformation. One patient was a 53-year-old woman who presented with macroscopic hematuria. The resected tumor involved the right renal parenchyma, measuring 13.0 x 8.0 x 4.0 cm, and extended to perirenal adipose tissue. The second patient was a 56-year-old woman who presented with right flank colic pain. The tumor measured 6.0 x 5.5 x 4.0 cm, with an intact capsule at the upper pole. Both tumors showed a well-circumscribed, multilocular, cystic, and focally solid mass. Sections of both tumors revealed benign and malignant components. The benign component consisted of multilocular cysts and fibrous stroma with a focally ovarian stromalike component. The malignant component in both cases was predominantly composed of undifferentiated cellular spindle cell sarcoma with frequent mitoses. One case showed additional heterologous malignant elements, including rhabdomyosarcomatous, chondrosarcomatous, and focal carcinomatous components. We report 2 additional cases of sarcomatous transformation in mixed epithelial and stromal tumor of the kidney.