A solitary fibrous tumor of the kidney in a 26-year-old man.

Background: Solitary fibrous tumors (SFTs) are rare spindle cell neoplasms that typically arise in the pleura. There are several reports of cases that arose from a variety of sites. SFT of the kidney is rare and, to our knowledge, only 25 cases have been published in the literature to date. SFTs in the kidney have similar morphologic and immunologic features and biologic behaviors as SFTs found elsewhere. In general, patients with SFTs of the kidney have a favorable prognosis.

Case: We report a case of SFT of the right kidney in a 26-year-old man. The tumor was localized in the upper and mid pole of the kidney. A nephrectomy was performed. The tumor was a well-circumscribed, solid mass attached to the renal capsule without necrosis or hemorrhage. Microscopically, a spindle cell neoplasm with alternating hypo- and hypercellular areas, storiform, fascilular and hemangiopericytoma-like growth pattern and less cellular dense collagen deposition was observed. Some glomeruli and renal tubules were entrapped by the tumor cells. There were no mitotic figures. Immunohistochemically, the tumor cells were consistently positive for CD34, CD99, and bcl-2. There was no evidence of recurrence after 6 months of follow-up.

Discussion: Although morphology is most important in formulating the initial differential diagnosis, the addition of immunohistochemistry is vital in arriving at the correct classification of renal spindle cell tumors.