Answer: Angiomyolipoma

Histology: This lesion is centered on the renal capsule, and consists of poorly formed fascicles of spindle cells with features resembling smooth muscle. There is extensive stromal hyalinization between the poorly formed fascicles, and the cells have eosinophilic to clear cytoplasm. Nuclear pleomorphism or mitotic activity is not appreciated. By immunohistochemistry, the tumor cells label for muscle markers actin and desmin, but also show labeling for HMB45. This supports classification of this lesion as a spindle cell predominant angiomyolipoma. Lesions like this have traditionally been considered renal capsular leiomyomas; however, their immunoreactivity for HMB45 suggests that they are in fact more closely related to angiomyolipoma.

Discussion: Neither leiomyoma nor leiomyosarcoma should label for HMB45. These true smooth muscle lesions typically have better formed fascicles of spindle cells than angiomyolipoma, and the cells lack the cytoplasmic clearing of angiomyolipoma. Leiomyosarcomas, as well as sarcomatoid renal cell carcinomas, show greater pleomorphism and mitotic activity than the spindle cell angiomyolipoma.