Malignant Solitary Fibrous Tumor of the Kidney

Solitary fibrous tumor is a rare mesenchymal tumor first described in the pleura although it has been reported in other sites including the kidney. We present a case of a 46-year-old woman with a history of atypical meningioma diagnosed 9 years ago who presented with a chief complaint of hematuria. Physical examination was unremarkable, including a soft nontender abdomen without organomegaly. Laboratory findings demonstrated trace RBCs on urinalysis but were otherwise unremarkable. CT scan of the abdomen/pelvis revealed a well-circumscribed, 51 x 42 x 45 mm enhancing lesion with a hypodense central core located at the central to upper pole of the left kidney. The mass arose from the renal parenchyma proper and splayed into the collecting system with no signs of obstruction, vascular infiltration, or perinephric fat involvement. The patient underwent a radical left nephrectomy with an unremarkable postoperative recovery. Gross examination revealed a 5.5 cm, tan, well-circumscribed mass at the midpole of the kidney. The tumor was confined to the renal parenchyma without frank invasion of the renal vein. Microscopically, the tumor comprised a hypercellular mass delineated by a well-circumscribed fibrous capsule. The cells were primarily spindle shaped with patternless growth architecture. The cells had ovoid to elongated nuclei with eosinophilic cytoplasm. The tumor showed increased mitotic activity ranging from 6-8 mitosis/10 hpf with moderate nuclear atypia. Immunohistochemical stains were positive for CD99, Bcl-2, vimentin and CD34 with negative pertinent stains. Diagnosis of malignant SFT was rendered using the criteria reported by Enzinger and WeissSoft Tissue Tumors book - tumor size > 5cm, increased cellularity, atypia, > 3 mitosis/10 hpf and foci of necrosis. The only criterion missing in our case was necrosis. Literature on malignant solitary fibrous tumor in the kidney is scarce; however, it is a significant differential diagnosis for malignant spindle cell lesions in the kidney.