Mixed epithelial and stromal tumor of kidney with renal vein extension: an unusual case report and review of literature.

Mixed epithelial and stromal tumor of kidney (MESTK) is a rare but distinct renal complex neoplasm composed of a mixture of mesenchymal and epithelial elements with characteristic ovarian-type stroma. Due to its relative rarity, little is known about the histogenesis and prognostic factors of this tumor. Although most reported cases display bland histological features and benign clinical course, a few cases of malignant MESTK have been described. We report an unusual case of MESTK in a 50-year-old female patient with renal venous involvement. Macroscopically, the tumor was solid and unencapsulated in the central region of left kidney. There was a polypoid mass with slender pedicle found to extend into the renal vein forming an intravenous tumor thrombus. Histologically, both renal and intravenous mass were composed of bland spindle-shaped cells and round dilated tubules lined by epithelium without any cytological atypia. The spindle cells were diffusely positive for smooth muscle actin and desmin, while tubules were positive for pan-cytokeratin (AE1/AE3). A diagnosis of MESTK with renal vein extension was made. The patient received no adjuvant treatment after radical nephrectomy. There was no sign of recurrence or metastasis of tumor found in a period of 16-month regular follow-up. To our knowledge, this is the first case of MESTK with renal vein extension, but lacking malignant histological appearance. Additional studies of MESTK with vein involvement will be needed to determine whether this imparts any adverse behavior, similar to other benign renal tumors with vascular involvement.