Histological features of this case. A and B: The tumor was surrounded by a thick fibrous capsule and well-demarcated from the surrounding renal parenchyma. C: It was characterized by an alternation of cellular and paucicellular areas. D: The paucicellular areas were mainly composed of fibrous stroma containing reticular vascular channels, hemosiderin pigment, and rare stromal cells. E: The cellular areas were composed of stromal cells surrounded by a rich capillary network. F and G: The cytoplasm of stromal cells is weakly eosinophilic, and locally contains lipid droplets (arrow) but no hyaline globules. And the stromal cells also showed ovalnuclei, delicate chromatin, and inconspicuous nucleoli. H: Occasionally, pleomorphic, or bizarre tumor cell nuclei was seen.