Large Solitary Fibrous Tumor of the Kidney

Solitary fibrous tumors are spindle cell neoplasms frequently arising in the serosal surface as well as a variety of other sites. We report two cases of large solitary fibrous tumor arising in the kidney, clinically thought to be renal cell carcinoma, in 41- and 72-year-old men. Although large in size (13.0 and 14.0 cm in greatest dimension, respectively), both lesions were well circumscribed and composed of a mixture of spindle cells and dense collagenous bands with no areas of necrosis or cystic changes noted macroscopically or microscopically. Immunohistochemical studies revealed reactivity for vimentin, CD34, collagen IV, and bcl-2 protein in both cases, with no staining for keratin, S-100 protein, or muscle markers, confirming the diagnosis of solitary fibrous tumor of the kidney. Solitary fibrous tumor of the kidney is rare but may present as a large mass that may be clinically confused with carcinoma or sarcoma.