Angiomyolipoma (AML) on Kidney Ultrasound

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, 2
Ultrasound has high accuracy in patients with small body habitus but is operator-dependent and less accurate in patients with larger body habitus 1, 2
Contrast-enhanced ultrasound may help better characterize solid kidney lesions, particularly in pediatric patients 1

Ultrasound alone cannot definitively differentiate between AML and renal cell carcinoma (RCC), especially for fat-poor AMLs 3
For lesions inadequately visualized by ultrasound, CT or MRI is recommended for further characterization 3

CT can detect macroscopic fat that appears as an area of negative density, which is diagnostic of AML 1, 2
MRI is the preferred imaging technique for diagnosing and monitoring kidney tumors as it:
- Does not deliver ionizing radiation 1, 2
- Has good soft tissue contrast even without contrast agents 1, 2
- 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 5

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

When renal lesions exhibit atypical imaging features that raise concern for malignancy, percutaneous core biopsy should be considered to obtain definitive histopathology. 7

Imaging alone cannot definitively identify an epithelioid histologic component; therefore, reliance on radiologic appearance is insufficient for diagnosis. 7
Computed tomography (CT) and magnetic resonance imaging (MRI) cannot reliably differentiate fat‑free angiomyolipoma (which may be epithelioid) from renal cell carcinoma, limiting their utility in distinguishing benign from malignant renal masses. 7