Answer: Angiomolipoma

Histology: This tumor is composed of predominantly spindle cells which resemble smooth muscle cells. However, these spindle cells do not form fascicles as well as smooth musclecells, and have a cytoplasm which is less pink and more pale than smooth muscle cells. The vessels within this lesion are distinctive in that they have prominent eccentric hyalinization, and there is also fat within the lesion. There is no evidence of carcinoma, and the spindle cells lack significant mitotic activity.

Discussion: The key differential here is with a sarcoma, particularly on frozen section examination. The absence of significant cytologic atypia, mitoses and necrosis are the best features in making this distinction on the H&E, and the presence of HMB 45 immunoreactivity helps establish the diagnosis of angiomyolipoma. Mucinous tubular and spindle cell carcinomas are typically small lesions confined to the kidney, without retroperitoneal extension, and these would label for cytokeratin. Spindle cell components of such lesions are usually limited, with more overt myxoid stroma and tubule formations elsewhere. A sarcomatoid renal cell carcinoma, as in the previous case, would at least focally demonstrate areas of a recognized renal carcinoma, such as clear cell, papillary, or chromophobe, or collecting duct.