Epithelioid angiomyolipoma of the kidney: a report of five cases with a prominent and diagnostically confusing epithelioid smooth muscle component.

We report five angiomyolipomas with a prominent component of epithelioid smooth muscle cells that occurred in patients from 20 to 48 (mean, 36) years of age. The tumors often posed problems in diagnosis, particularly with regard to distinction from renal cell carcinoma. Two patients had tuberous sclerosis. Two patients with more than 5 years' follow-up are alive and well. The epithelioid smooth muscle cells typically formed sheets that in two tumors were traversed by hyaline cords. The epithelioid cells ranged from medium sized and polygonal with slightly pleomorphic nuclei and eosinophilic cytoplasm to giant cells with prominent nucleoli. Hemorrhage, necrosis, and clusters of foamy macrophages were present in three tumors. Mitotic figures were easy to find in two of the tumors, but they were absent in the others. Obvious elements of typical angiomyolipoma were present in two tumors. The others contained only scattered, thick-walled blood vessels or a few fat cells suggestive of typical angiomyolipoma. None of the tumors was positive for low- or high-molecular-weight cytokeratins or epithelial membrane antigen. Actin was detected in the epithelioid areas in four tumors. Melanoma-associated antigens related to the gp100-cl gene product, HMB-45 and HMB-50, were present in all the tumors. Another melanoma-associated antigen, CD63 (NKI/C3), also was present in all the tumors, a finding suggesting that angiomyolipoma has features in common with melanoma beyond premelanosomal structures.