Angiomyolipoma is the most common mesenchymal neoplasm of the kidney, classically composed by a variable mixture of adipocytes, smooth muscle cells, and abnormal thick-wall blood vessels. The perivascular epithelioid cell is considered to be the cell of origin for this and other related tumors.
Angiomyolipoma accounts for approximately 1% of surgically removed renal tumors.
Sporadic angiomyolipoma occurs in the fourth to sixth decades of life, whereas tuberous sclerosis in the third and fourth decades of life.
Sporadic angiomyolipoma has a female predominance; in patients with tuberous sclerosis, there is no gender predilection.
There is no site predilection in the sporadic angiomyolipoma, whereas those occurring in patients with tuberous sclerosis are usually bilateral, small, and multifocal.
Partial nephrectomy and less frequently, radical nephrectomy is the standard treatment.
Angiomyolipoma is a benign neoplasm. Multifocality and regional lymph node involvement can occur, especially in patients with tuberous sclerosis, and the latter finding is considered to represent a multifocal growth pattern rather than metastasis.
Angiomyolipomas usually are well demarcated, but not encapsulated. The color ranges from yellow to pink-tan, depending on the relative proportions of the various tissue components. Prominent cystic or pseudocystic changes may very rarely be present.
Angiomyolipoma is characterized by a coexpression of melanocytic markers (HMB45, Mart1/Melan-A, and microphthalmia transcription factor) (Pea et al. 1991), cathepsin K, and smooth muscle markers (smooth muscle actin, muscle-specific actin, and calponin) (Martignoni et al. 2012). Staining for CD68, S-100 protein, estrogen and progesterone receptors, and desmin may also be present, whereas epithelial markers are always negative.
The alterations of two genes are known to cause tuberous sclerosis. The TSC1 gene is located on chromosome 9q34 and encodes hamartin; the TSC2 gene is located on chromosome 16p13 and encodes tuberin. These two proteins interact with each other, forming a cytoplasmic complex. Angiomyolipoma frequently shows loss of heterozygosity (LOH) of variable portions of TSC2 gene locus and less frequently TSC1 gene locus in both sporadic and inherited tumors. Recently, TSC2 mutation but not TSC1 mutation has been reported in sporadic angiomyolipoma (Qin et al. 2011).
Tumors prevalently composed by smooth muscle cells and adipocytes may mimic a leiomyoma/leiomyosarcoma and lipoma/liposarcoma, respectively. The expression of melanocytic markers is extremely useful for the diagnosis.