Answer: Angiomyolipoma

Histology: none provided

Discussion: Sections show a cellular neoplasm composed of spindled and epithelioid tumor cells that have open, vesicular nuclei with small nucleoli. Rare intranuclear inclusions are seen but mitoses are not readily identified. The tumor contains admixed adipose tissue and also surrounds residual tubules of the renal parenchyma indicative of involvement of the renal cortex.
Angiomyolipomas (AML) are rare renal neoplasm that contain a mixture of smooth muscle, fat and blood vessels. Not all three components may be present and cases with pure epithelioid morphology can occur. When all three components are present the radiologic appearance in the proper clinical context is diagnostic and does not warrant removal of the lesion. Approximately one quarter of AML exhibit invasion of the renal parenchyma a findings that is not indicative of aggressive biologic behavior. Angiomyolipomas may also cause severe rarely fatal retroperitoneal hemorrhage. Malignant tumors also occur but are rare. Immunohistochemically, the tumor cells are HMB45 (sometimes only focally) and Melan-A positive.