Angiomyolipoma

Angiomyolipomas are common benign tumours of the renal cortex, composed of a mixture of blood vessels, fat and smooth muscle , with macroscopic fat seen on CT in 95% of cases. They range in size from the tiny, often seen on US as small cortical echogenic foci, to massive retroperitoneal masses. When larger than 4 cm or so, they are prone to spontaneously bleed.
Usually AMLs are sporadic and solitary, although up to 20% are seen in association with tuberous sclerosis (80% of patients with TS have AMLs, 15% of patients with lymphangiomyomatosis have AMLs) , when they are very numerous.
They receive their blood supply from the kidney, a feature useful in distinguishing them from a retroperitoneal liposarcoma which has extended into the kidney.
For a brief discussion of the differential diagnosis, please visit Radiopaedia.org here.
Reference: StatDx.com, Amirsys
Credit: Dr Frank Gaillard
http://www.radpod.org