[Mixed epithelial and stromal tumor of the kidney]

We describe a case of mixed epithelial and stromal tumor of the kidney. The tumor was identified in a 63 years old woman and retroperitoneoscopically resected. Immunohistochemistry, electron microscopy and fluorescence in situ hybridization were performed. Grossly, the tumor was well-circumscribed with a diameter of 8 cm and consisted of solid and cystic components. Microscopically, the spindle cell component ranged in appearance from scar-like fibrous tissue to leiomyoma-like fascicles. Epithelial elements were composed of tubules with variable lining. The tubules were focally transformed into large cysts. Estrogen and progesteron receptors were detected in the nuclei of the spindle cells. Electron microscopy confirmed a smooth-muscle differentiation of the spindle cells. Previously, such tumors were published as adult mesoblastic nephroma or cystic hamartoma of the renal pelvis. Its predominance in females with a history of long-term estrogen replacement or the history of long-term sex-steroid use in a male patient, combined with the expression of estrogen und progesteron receptors in the spindle cells suggest that hormonal influence is relevant for the development of these tumors.