A solitary fibrous tumor of the kidney.

A solitary fibrous tumor (SFT) is an uncommon spindle cell neoplasm that usually occurs in the pleura, but may occur in extrapleural sites. Its occurrence in the kidney is rare. We report a SFT, clinically thought to be a renal cell carcinoma arising in the kidney of a 68-year-old female. The tumor was well-circumscribed and composed of a mixture of spindle cells and dense collagenous bands. Immunohistochemical studies revealed reactivity for CD34, CD99, and Bcl-2 protein, with no staining for keratin or muscle markers, confirming the diagnosis. The immunohistochemical study was the key to diagnosis. Several younger members of her family had colorectal and lung cancers suggesting the possibility of a familial or genetic susceptibility.