Answer: C
Histologic Description: This is a multicystic lesion that appears fairly well delineated but at its periphery intermingles with the native renal tubules. These cysts are surrounded by a cellular, “ovarian-like” stroma associated with ropy collagen. The lining cells are bland and sometimes hobnailed, consistent with collecting duct epithelium. These stromal cells label for estrogen receptor and inhibin. These are the typical features of mixed epithelial stromal tumor.
Differential Diagnosis: Angiomyolipoma with epithelial cysts would demonstrate three layers; a peripheral stroma rich angiomyolipoma component, a subepithelial blue cell component that resembles endometrial stroma, and entrapped native renal tubules with a frequently hobnailed appearance. The stromal predominant angiomyolipoma component resembles smooth muscle but has shorter fascicles, clearer cytoplasm, and a prominent lymphatic investment. AMLEC would label for melanocytic markers like HMB45. Tubulocystic carcinoma features an elastotic stroma and numerous small cysts, giving rise to a “bubble wrap” gross appearance. The lining cells have a hobnail appearance but demonstrate greater cytologic atypia and nuclear irregularities. Multilocular clear cell renal neoplasm of low malignant potential is also exclusively cystic, but there are characteristically small clusters of clear cells within the septa that do not form expansile nodules.