Fat-forming Solitary Fibrous Tumor of the Kidney - A Case Report and Literature Review.

Fat-forming solitary fibrous tumors arising from the kidney are extremely rare, with only few cases reported so far. We report the case of a 49-year-old patient investigated for lumbar pain and diagnosed preoperatively with a large retroperitoneal tumor destroying the right kidney. The patient was submitted to surgery, the tumor being resected en bloc with right nephrectomy. The histopathological studies demonstrated the presence of a hemangiopericytoma-like tumor with adipocytary component; the immunohistochemical studies demonstrated the presence of cluster of differentiation 34 (CD34), while human melanoma black (HMB45) staining was negative. Therefore, the diagnostic of fat-forming solitary fibrous tumor of the kidney was established. The postoperative outcome was uneventful, the patient being discharged on the sixth postoperative day. In conclusion, although fat-forming solitary fibrous tumor of the kidney is a very rare entity, this diagnosis should be taken into consideration whenever a large retroperitoneal mass with high adipose content and renal invasion is described at the preoperative studies.