Answer: Angiomyolipoma

Histology: The tumor is composed of plump epithelioid and spindled cells which show moderate pleomorphism. Abnormal blood vessels with irregularly thickened walls and foci of fat are associated with the spindled cells. The tumor was immunoreactive for HMB-45 and Melan-A, and negative for HEPAR-1, supporting the diagnosis.

Discussion: Epithelioid angiomyolipomas of the liver are a great mimic of hepatocellular carcinoma, particularly when they have a trabecular pattern and show pleomorphism. Immunohistochemistry usually resolves this problem. Leiomyosarcoma can be mimicked by angiomyolipomas with prominent smooth muscle differentiation. The vacuolated nature of the smooth muscle cells in angiomyolipoma and the distinctive immunostaining pattern (positivity for both actin and melanocytic markers) are also helpful. Focal fat accumulation in hepatocytes can be mimicked by angiomyolipoma showing epithelioid cells and fatty differentiation. Attention to the large size of the lesion should prompt reconsideration of the diagnosis, and confirmatory immunohistochemistry.