Solitary fibrous tumor involving the renal capsule.

Solitary fibrous tumors are spindle-cell neoplasms that originally were described in the pleura but that can occur in a large variety of sites. We report a well-circumscribed tumor, apparently involving the renal capsule, clinically thought to be a renal-cell carcinoma or oncocytoma. It was composed of bland spindle-shaped cells with a patchy lymphoplasmacytic infiltrate, suggesting sarcomatoid renal-cell carcinoma, inflammatory myofibroblastic tumor, or solitary fibrous tumor; however, immunohistochemical stains were negative for keratin, alpha-smooth-muscle actin, and desmin but strongly positive for CD34. Ultrastructural examination revealed fibroblast-like cells without myofibroblastic or epithelial differentiation. The combined findings favor a diagnosis of a solitary fibrous tumor involving the renal capsule. To our knowledge, this lesion has not been reported in this location.