Cystic nephroma/mixed epithelial stromal tumor: a benign neoplasm with potential for recurrence.

Cystic nephroma (CN) is a rare, benign, renal neoplasm composed of epithelial and stromal elements. Only about 200 cases have been reported since 1892 and recurrence has rarely been observed. We report a 32-year-old Hispanic woman, with a history of a right, complex cystic, renal mass treated by robotic decortication 2 years ago, who presented with flank pain, hematuria, and recurrent urinary tract infection. A magnetic resonance imaging study showed a 3.4-cm multicystic lesion with thickened septa and enhancement at the right kidney. The partial nephrectomy specimen revealed a well-circumscribed, multicystic tumor abutting the renal pelvis, with thick septa and smooth walls, filled with clear fluid. Microscopic examination showed variably sized cysts lined by cuboidal epithelium with focal hobnailing, without significant cytologic atypia and mitosis. The epithelial lining was positive for CK19, high molecular weight cytokeratin, and α-methylacyl-CoA racemase suggesting a primitive tubular epithelial phenotype. Primitive glomeruli-like structures were also present. The ovarian-like stroma was condensed around the cysts and was variably cellular with areas of muscle differentiation and thick-walled vessels. The stroma was positive for desmin, estrogen receptor, progesterone receptor, and CD10. We suggest that CN represents a variable mixture of epithelial and stromal elements, immature glomerular, tubular, muscle, and vascular elements, which may be present in variable proportions creating a spectrum of lesions previously described as CN and mixed epithelial and stromal tumors (MEST). This case emphasizes that CN/MEST clinically/radiologically mimics other cystic renal neoplasms, especially cystic renal cell carcinoma and tubulocystic carcinoma, necessitating histopathological examination and immunohistochemial studies for definitive diagnosis. Additionally, CN has the tendency to recur when not completely excised initially.