Answer: Xanthogranulomatous pyelonephritis

Histology: The tumor is composed of a mixture of cell types. We can identify xanthoma cells focally within the tumor, in several sites associated with multinucleated giant cells and calcium oxalate crystals. Associated with these macrophages is a proliferation of spindled myofibroblasts associated with a chronic inflammatory infiltrate composed of lymphocytes and plasma cells. These spindle cells lack pleomorphism or significant mitotic activity. These features are characteristic of xanthogranulomas pyelonephritis, which on histologic and clinical grounds can simulate a high-grade renal malignancy.

Discussion: Sarcomatoid carcinoma would be characterized by more cellular atypia and mitotic activity than is seen in the current case, and would lack xanthoma cells. Collecting duct carcinomas can be spindled; however, they also would show atypia and mitotic activity, and usually they are associated with a desmoplastic stromal response. The xanthoma cells of xanthogranulomatous polynephritis can simulate the clear cells of renal cell carcinoma. The current lesion would be cytokeratin negative in contrast to clear cell renal carcinoma.