Massive retroperitoneal angiomyolipoma. A lesion that may be confused with well-differentiated liposarcoma.

A benign massive retroperitoneal lipomatous tumor is described. The tumor, from a 72-year-old woman with increasing abdominal girth, consisted of a mixture of mature lipocytes, smooth muscle cells, and thick-walled medium-size blood vessels. Although the tumor focally involved the uterine serosa, suggesting the possible diagnosis of uterine leiomyoma with fatty change, the authors think that, because the bulk of the tumor was located in the retroperitoneum and because the tumor contained characteristic thick-walled blood vessels from which smooth muscle cells radiated, this tumor would be best classified as a retroperitoneal angiomyolipoma. Although both angiomyolipomas and leiomyomas with fatty change presenting as large retroperitoneal tumors are rare, and therefore are not well-recognized by surgical pathologists, they are benign and must be distinguished from liposarcomas.