An unusual oncocytic renal tumor with sarcomatoid foci and osteogenic differentiation

A 67-year-old man with abdominal pain had a large neoplasm of the upper pole of the right kidney. The tumor was predominantly soft and brown but had one focus that was of bony consistency. On microscopic examination most of the tumor had the typical appearance of an oncocytoma, but rare papillary and sarcomatoid foci were identified, and there was focal metaplastic bone. Ultrastructural examination confirmed the oncocytic nature of most of the neoplasm. This case is further evidence that some predominantly oncocytic renal neoplasms have small foci that are incompatible with the diagnosis of oncocytoma.