Answer: Angiomyolipoma

Histology: The specimen is from a resected kidney which shows a tumor confined to the kidney. The tumor is composed of a mixture of blood vessels, fat, and smooth muscle. The blood vessels are somewhat thick-walled in appearance. The adipose tissue is mature looking. The smooth muscle component of this tumor does show some cytologic atypia with enlarged and sometimes multi-nucleated nuclei. Mitotic activity is not observed. The observation of adipose tissue, smooth muscle, and vessels in this case helps establish the diagnosis of angiomyolipoma.

Discussion: Angiomyolipoma is a relatively uncommon tumor composed of an admixture of fat, blood vessels, and smooth muscle. This tumor has a characteristic appearance on ultrasonography and CT scan and usually is clinically suspected based on these studies. Grossly this tumor, because of its yellow color, may be mistaken for a renal cell carcinoma. However, this tumor usually lacks the intraluminal hemorrhage and necrosis typical of renal cell carcinoma. In about one third of the cases, the tumors may be bilateral. The hypercellularity and pleomorphism that is seen in the smooth muscle component of this tumor may lead to a mistaken diagnosis of a malignant smooth muscle tumor. In addition, sometimes these tumors may have numerous epithelioid-like cells which may lead to further confusion with renal cell carcinoma. In addition to the characteristic histologic appearance, these tumors show consistent immunohistochemical staining for HMB45. This can be helpful in establishing the diagnosis. About one third of patients with angiomyolipomas of the kidney are found to have skin or neurologic findings diagnostic or suggestive of tuberous sclerosis. The incidence of tuberous sclerosis is actually higher in cases of multiple or bilateral tumors. The regional lymph nodes in renal angiomyolipoma may occasionally be involved with the process. This is considered to represent multicentricity of the lesion rather than actual metastasis. Morphologic features of angiomyolipoma may also be seen in other areas in addition to the kidney. These include primarily liver and retroperitoneal space.