Mixed epithelial and stromal tumor of the kidney: report of a rare case and review of literature.

Mixed epithelial and stromal tumor of the kidney (MESTK) is a rare benign tumor composed of epithelial and stromal cells. We report a rare male case with detailed clinicopathological data and follow-up information. The patient presented with gross hematuria. Computed tomography (CT) and magnetic resonance imaging study showed a 60 mm×40 mm cystic lesion with thickened septa and minimal contrast enhancement at the lower pole of the right kidney. The patient underwent nephron sparing surgery (NSS). Intraoperative frozen section showed benign histological features without significant cytologic atypia and mitosis. By additional immunohistochemistry investigations, the epithelial component was positive for cytokeratin-7, high molecular weight cytokeratin, and PAX-8. The stromal component showed strong positivity for vimentin and smooth-muscle actin. This case emphasizes that it is difficult to establish a precise diagnosis of MESTK preoperatively due to lack of any typical radiological features. Thus, intraoperative frozen section is of great clinical significance for NSS with preservation of kidney function. Additionally, regular follow-up is necessary for the MESTK with malignant potential.