A fatty tumor on the kidney, called angiomyolipoma, is a benign mass composed of fat, blood vessels and smooth muscle. A benign tumor is noncancerous and cannot metastasize or spread to other parts of the body but it can grow and destroy other tissue, according to the Canadian Cancer Society.
An angiomyolipoma can be either sporadic, which accounts for 80 percent of cases, or it can be a sign of tuberous sclerosis, notes the Tuberous Sclerosis Alliance. Sporadic angiomyolipomas occur most often in women as a single tumor in one kidney. Tuberous sclerosis, which accounts for 20 percent of angiomyolipomas, can produce multiple tumors that are larger and affect younger people of both sexes in both kidneys.
An accurate diagnosis by ultrasound, CT or MRI of a fatty tumor depends on the fat content, explains the Tuberous Sclerosis Alliance. It can be difficult to differentiate between a malignant tumor and a benign angiomyolipoma. Patients with tuberous sclerosis and renal angiomyolipomas are more likely to develop malignant kidney tumors than people with renal angiomyolipomas and no sign of tuberous sclerosis. Benign fatty tumors often do not require treatment unless they grow larger than 4 centimeters, are malignant or cause an obstruction in the kidney.