Answer: Angiomyolipoma, fat predominant

Histology: This is a fat predominant lesion consisting of variably sized fat cells. Intermixed with the fat cells are more epithelioid cells with pink cytoplasm, which appear to be clustered around blood vessels. On immunohistochemistry, these cell label for HMB45 and Melan A. This lesion focally connected with the kidney, and lipoblasts were not identified

Discussion: Liposarcoma is a major consideration for a fat predominant tumor in the retroperitoneum. The immunoreactivity for HMB45, the connection to the kidney, and the absence of lipoblasts argue against liposarcoma. Lipomas of the retroperitoneum are essentially nonexistent and any true lipomatous neoplasm of the retroperitoneum should be considered a liposarcoma until proven until otherwise. Metastatic prostatic adenocarcinoma is excluded by the absence of glandular formations, and the female gender of the patient.