Answer: Epitheloid angiomyolipoma

Histology: Areas of this lesion show the classic features of typical angiomyolipoma. These areas are composed of normal appearing adipose tissue. Admixed are abnormal vessels, where the smooth muscle from the vessel walls radiate out and merge in with the surrounding smooth muscle. The smooth muscle component has an epithelioid appearance. These epithelioid cells have vacuolated cytoplasm, which extends out from the nuclei as delicate eosinophilic strands resembling a “spider-web”. This smooth muscle component differs from normal smooth muscle, in that the cells are not arranged in long sweeping fascicles. Elsewhere in the lesion the morphology is quite different with epithelioid pleomorphic cells and occasional tumor giant cells. Most of the pleomorphic cells have densely eosinophilic cytoplasm, although some pleomorphic cells have similar vacuolated cytoplasm to that seen in the muscle component of the more typical angiomyolipoma foci.

Discussion: Patients with tubular sclerosis are at higher risk for renal cell carcinoma as well as angiomyolipomas. Consequently, especially in patients with tubular sclerosis, lesions such as the one illustrated here should be evaluated for the possibility of a collision tumor between renal cell carcinoma and angiomyolipoma. Our patient did not have tuberous sclerosis and there were morphologic transitional areas between the typical and epithelioid pleomorphic angiomyolipoma foci. We also performed stains for HMB-45 showing that the entire lesion was positive, which ruled out the possibility of a collision tumor. Ordinary angiomyolipomas may present in multiple locations including lymph nodes without causing morbidity or mortality; these cases represent multifocal disease rather than metastases. Rarely angiomyolipomas may exhibit truly malignant behavior, resulting the death of a patient. Some of these aggressive lesions have had a similar morphology to the case illustrated here. Other similar pleomorphic epithelioid angiomyolipomas reported in the literature have not behaved in a malignant fashion. In cases similar to the one presented here, the issue should be raised to the clinician that although many cases with similar such histology will not behave aggressively, this histology may rarely be seen associated with angiomyolipomas that have gone on to exhibit malignant transformation.