Fat-containing variant of solitary fibrous tumor (lipomatous hemangiopericytoma) arising on surface of kidney

Fat-containing variant of a solitary fibrous tumor is a recently recognized benign soft-tissue tumor that usually affects the thigh and retroperitoneum. We report a 51-year-old woman with a fat-containing variant of a solitary fibrous tumor that is the first reported case involving a visceral organ. The tumor was well delineated and seemed to arise from the renal capsule, radiographically and macroscopically. The tumor microscopically mimicked a solitary fibrous tumor but exhibited focal aggregates of fat cells. A fat-containing variant of a solitary fibrous tumor involving the kidney should be distinguished from spindle cell carcinoma, angiomyolipoma, gastrointestinal stromal tumor, and cellular schwannoma.