Answer: Angiomyolipoma

Histology: The tumor is extensively composed of mature adipose tissue. Scattered within the adipose tissue are vacuolated cells with filamentous cytoplasm. Also within the lesion are scattered blood vessels where the smooth muscle radiates out from the outer wall of these vessels into the surrounding adipose tissue.

Discussion: This lesion was seen at frozen section, where the clinical impression was that of a well-differentiated liposarcoma. On frozen sections, scattered atypical cells were seen within the adipose tissue, suspicious for well-differentiated liposarcoma. However, on the frozen section, one vessel was present similar to those illustrated here that had the typical features of angiomyolipoma. The finding of smooth muscle cells radiating out from the vessel into the adjacent adipose tissue is typical of angiomyolipoma. Furthermore, the finding of vacuolated cells within the adipose tissue, which represent abnormal smooth muscle cells, is also classic for this entity. This case is unusual for several reasons. First, this lesion is entirely exterior to the kidney, in contrast to most angiomyolipomas which primarily involve the kidney. Other primary sites, in addition to the retroperitoneum, include the liver, lymph nodes, fallopian tubes, and spleen. The second unusual feature of this case is its overwhelming predominance of adipose tissue with a very limited smooth muscle component and relatively sparse vessels. Although the typical angiomyolipoma contains a significant proportion of all three elements, some angiomyolipomas have a predominance of one cell type over the other. Just as the current case with a predominant fatty component can be confused with a lipomatous tumor, those angiomyolipomas predominantly consisting of smooth muscle may be confused with either a leiomyoma or leiomyosarcoma, depending on the presence or absence of cytologic atypia that may occasionally be seen within angiomyolipomas. There are even some angiomyolipomas which have a very prominent vascular component which may be confused with an angiomatous neoplasm.