Malignant solitary fibrous tumor in the kidney: a case report and review of the literature

Solitary fibrous tumor has been reported infrequently in the kidney. Malignant solitary fibrous tumor of the kidney is quite rare. Only 9 cases have been reported before. We report a case of malignant solitary fibrous tumor clinically diagnosed as renal cell carcinoma in a 63-year-old man. The patient underwent radical left nephrectomy. Grossly, the tumor which measured 4.5 × 4.0 × 3.0 cm was well circumscribed. Microscopically, the tumor was composed of spindle cells with frequent mitoses, monomorphic storiform patterns, and occasionally patternless architecture. Furthermore, the tumor showed increased cellularity and focally invaded out of the capsule to the surrounding fatty connective tissue and blood vessels. Immunohistochemical study revealed strong expressions of CD34, bcl-2 and vimentin, with no expressions of CD99, c-kit, S-100, smooth muscle actin and epithelial membrane antigen. The surgical line was free from the tumor and no metastasis was found in the left adrenal gland. Because of the increased cellularity, a high number of mitosis and invasion to the adipose tissue and blood vessels, the tumor was histopathologically recognized as malignant. As far as we know, this is the tenth case of renal malignant solitary fibrous tumor. Surgical resection is the first choice for primary solitary fibrous tumor. Histopathological characters and prognosis of malignant solitary fibrous tumor of the kidney are still unknown. Long-term follow-up is generally required for malignant solitary fibrous tumor in the kidney. © 2014, The Japan Society of Clinical Oncology.