What is AML (Angiomyolipoma) on a kidney ultrasound?

Angiomyolipoma (AML) on Kidney Ultrasound

Angiomyolipoma (AML) typically appears as a hyperechoic and homogeneous mass on kidney ultrasound, though this appearance is not pathognomonic as up to 8% of renal cell carcinomas can also appear hyperechoic. 1

Characteristics of AML on Ultrasound

• AMLs are benign renal tumors composed of varying amounts of fat, smooth muscle, and blood vessels, giving them their characteristic hyperechoic appearance on ultrasound 1, 2
• Some fat-poor AMLs may appear isoechoic and can be difficult to detect on ultrasound 1
• Ultrasound has high accuracy in patients with small body habitus but is operator-dependent and less accurate in patients with larger body habitus 1
• Contrast-enhanced ultrasound may help better characterize solid kidney lesions, particularly in pediatric patients 1

Limitations of Ultrasound for AML Detection

• Ultrasound alone cannot definitively differentiate between AML and renal cell carcinoma (RCC), especially for fat-poor AMLs 1, 3
• A common pitfall is overlooking fat-poor AMLs that may be difficult to distinguish from renal cell carcinoma on ultrasound 3
• For lesions inadequately visualized by ultrasound, CT or MRI is recommended for further characterization 3

Alternative Imaging Modalities for AML

• CT can detect macroscopic fat that appears as an area of negative density, which is diagnostic of AML 1
• MRI is the preferred imaging technique for diagnosing and monitoring kidney tumors as it:
  • Does not deliver ionizing radiation 1
  • Has good soft tissue contrast even without contrast agents 1
  • Allows multiparametric assessment that can better characterize fat-poor AMLs 1, 4
• The same imaging modality should be used consistently for follow-up to ensure accurate assessment of growth 1, 3

Clinical Significance and Management

• AMLs can be sporadic or associated with tuberous sclerosis complex (TSC) 1, 5
• TSC-associated AMLs develop at a younger age, grow faster, and are more prone to bleeding complications than sporadic AMLs 5
• Surveillance recommendations based on AML size:
  • For AMLs 2-3.9 cm: ultrasound every 3 years 3
  • For AMLs 4-4.9 cm: MRI or CT every 6-12 months due to increased bleeding risk 3
  • For AMLs ≥5 cm: MRI or CT every 6 months due to higher bleeding risk 3
• Treatment is indicated when there is substantial risk of rupture, with options including:
  • Embolization for acute hemorrhage 1
  • mTORC1 inhibition as first-line treatment for fat-poor lesions requiring non-urgent treatment 1
  • Surgical intervention for histology-proven renal cell carcinoma 1

Rare Complications

• AMLs can occasionally extend into the renal vein or inferior vena cava, though intracardiac extension is rare 6, 7
• The major complication of AML is intra-tumoral or retroperitoneal hemorrhage due to rupture, which can be life-threatening 5