Answer: Angiomyolipoma

Histology: The tumor consists of a spindle cell smooth muscle component. The tumor is seen within perirenal soft tissue. In areas, adipose tissue is identified within the tumor. Admixed within the tumor are abnormal thick walled vessels where the smooth muscle from the vessel walls radiate out and merge in with the surrounding smooth muscle.

Discussion: A typical angiomyolipoma consists of three components, which include thick walled vessels, smooth muscle cells, and adipose tissue. However, variation in the proportion of the components may be seen in various tumors. The nuclear pleomorphism and areas where the tumor spindles raised the possibility of a sarcomatoid renal cell carcinoma and a metastatic melanoma. Immunohistochemical stains are helpful in differentiating these lesions. Angiomyolipomas are intensely positive upon immunostaining with HMB45 while stains for S100 and cytokeratins are negative.

20% to 50% of angiomyolipomas are seen in association with patients who have tuberous sclerosis. Angiomyolipomas may present in multiple locations, including lymph nodes, which is thought to represent multi-focal disease rather than metastasis. The majority of angiomyolipomas are benign lesions and do not metastasize. However, there are rare reported cases of angiomyolipomas with malignant behavior, usually with an epithelioid morphology. In cases of epithelioid angiomyolipomas, the issue should be raised to the clinician that, although many cases with similar histology have not behaved aggressively, this histology may rarely be associated with aggressive behavior.

References:
L’hostis H, Deminiere C, Ferriere JM, Coindre JM. Renal angiomyolipoma: a clinical pathologic immunohistochemical and follow-up study of 46 cases. Amer J Surg Path; 1999; 23: 1011-1020.