What is a renal AML, and what are its CT and MRI features?
A renal AML is the most common benign mesenchymal neoplasm of the kidney and is composed of variable amounts of fat, blood vessels, and smooth muscle. It may arise sporadically (in up to 70% of cases), 4 times more commonly in women, or may occur in the setting of TS with equal frequency in men and women. Renal AMLs have a >50% risk of spontaneous hemorrhage when >4 cm in size. Therefore, it is recommended that renal AMLs be treated prophylactically with catheter arterial embolization when they exceed 4 cm in size.
On CT and MRI, visualization of the presence of macroscopic fat content within a renal mass is diagnostic of a renal AML. However, 5% of renal AMLs do not contain visible macroscopic fat and are known as lipid-poor AMLs. These may mimic other renal neoplasms such as RCC on cross-sectional imaging but tend to have homogeneous attenuation, homogeneous hypointense T2-weighted signal intensity relative to renal parenchyma, and homogeneous enhancement and often protrude from the renal parenchyma from a small parenchymal defect. Calcification is not typically seen ( Figure 35-9 ). A small RCC may be very difficult to distinguish from a renal AML by imaging alone.