Primary renal malignant fibrous histiocytoma is extremely rare. The prognosis is generally poor because of the high rate of local recurrence even if radical surgery is performed. We report a case of primary renal malignant fibrous histiocytoma in a 43-year-old woman.

A well-defined renal mass (10x9x7cm) with lobulated contour was revealed by abdominal ultrasonography and computerized tomography in our patient who presented with left-flank pain. The contralateral kidney and renal functions were normal. No other primary tumor focus was detected by screening performed to exclude a metastasis to the kidney. However, a nodule consistent with metastasis was identified in the liver. The patient underwent left radical nephrectomy. A huge, yellowish-white, partly necrotic neoplasm infiltrating the renal parenchyma, perirenal fat and adrenal tissue was revealed by gross examination. Microscopically, the tumor was composed of pleomorphic spindle to polygonal cells arranged in fascicles and storiform structures with extensive necrosis. Twelve mitotic figures were noted in 10 high power fields. Immunohistochemical staining for vimentin and CD68 were positive whereas pan-cytokeratin, smooth muscle actin, S-100, HMB-45 and desmin were negative. “A pleomorphic type of malignant fibrous histiocytoma” was diagnosed based on both the microscopic and immunohistochemical findings. The patient presented with multiple bilateral lung metastasis three years after nephrectomy.

Histopathological examination is essential since malignant fibrous histiocytoma cannot be differentiated clinically and radiologically from other lesions with kidney involvement. The overall prognosis is unfavorable with a recurrence rate of more than 50% and the 5-yearsurvival rate is only 14%.