Angiomyolipoma

This benign tumor occurs most commonly in the kidney, but it may be seen in other locations as well, including the liver. The average age of the patients is about 50 years; some of them have tuberous sclerosis. Many of these tumors are not biopsied because they can be identified by imaging studies.

Cytologic diagnostic features

• Clusters of epithelioid and spindle cells (myoid cells)
• Fat cells
• Blood vessels
• Extramedullary hematopoiesis (40%)

The diagnosis rests on identifying the triad of fat, vessels and smooth muscle, but the myoid component is the only specific and diagnostic one.

Differential diagnosis

• Hepatocellular carcinoma
• Myelolipoma
• Nodular hematopoiesis
• Sarcoma
• Other carcinomas
• Granulomatous inflammation

The myoid cells of angiomyolipoma have a fibrillar cytoplasm and indistinct cell membranes, unlike the cells of hepatocellular carcinoma which have distinct cytoplasmic borders. The myoid cells are not seen in myelolipoma and in nodular hematopoiesis. Immunocytochemistry can be useful to distinguish the myoid cells from other spindle and epithelioid cell tumors: they are positive for HMB-45.