Low-grade spindle cell carcinoma of the kidney.

The authors report a renal cell carcinoma composed largely of spindle cells of Fuhrman's nuclear grade II in which the bland appearance of the cells and low mitotic index were reminiscent of a benign or low-grade smooth muscle tumor. Keratin immunostaining was positive, but evidence of epithelial differentiation was obtained by electron microscopy. The tumor was an incidental finding and it did not invade the perirenal fat or the renal vein. Follow-up is only 24 months but the histological features suggest that the prognosis may be better than that of a classic sarcomatoid renal cell carcinoma.