Benign mixed epithelial and stromal tumor of the kidney with inferior vena cava tumor thrombus: A rare case report and review of literature.

Mixed epithelial and stromal tumor of the kidney (MESTK) is a rare renal tumor composed of epithelial and stromal cells. In this study, we report a rare case of MESTK, which was pathologically benign but complicated with renal vein and inferior vena cava tumor thrombus. The 50-year-old female patient was admitted to hospital for a mass on the left kidney. Computed tomography showed a 32 mm×18 mm mass with slight delayed enhancement in the left renal sinus, and neoplastic thrombus was detected in left renal vein and inferior vena cava. A preoperative diagnosis of renal leiomyoma was made by needle biopsy and a laparoscopic radical nephrectomy with thrombectomy was performed. Histologically, the tumor and thrombus were composed of proliferative spindle cells and a small amount of tubular structures. Both kinds of the cells were well differentiated with no atypia or mitosis of nuclei. Immunohistochemical staining showed positive for CK, Ki-67 in the tubular cells and desmin, actin, estrogen receptors, progesterone receptors in the spindle cells. Finally, the diagnosis of MESTK was established. No recurrence or metastasis was found in the patient with a followed-up period of 12 months after the surgery. Due to the difficulty in diagnosis of MESTK, documentation with more cases of MESTK is needed to further understand its pathogenesis, biological behavior, preoperative diagnosis and optimal management of patient treatment.