What is an angiomyolipoma (AML)?

What is Angiomyolipoma (AML)?

Angiomyolipoma is a benign kidney tumor composed of three tissue components: blood vessels, smooth muscle, and adipose (fat) tissue in varying proportions. 1, 2

Tumor Composition and Classification

• AMLs belong to the perivascular epithelioid cell tumor (PEComa) family, characterized by myogenic and melanocytic differentiation 3
• The tumor is classified into three main types based on fat content and cellular composition 4, 5:
  • Classic AML: Contains detectable macroscopic fat (most common presentation) 1, 3
  • Fat-poor AML: Smooth muscle-predominant with minimal or undetectable fat on imaging 1, 2
  • Epithelioid AML: Rare variant that can behave aggressively with potential for metastasis and mortality 4, 3

Clinical Presentation and Associations

• AMLs are the most common benign renal tumors, accounting for up to 25% of small renal masses 1, 4
• They occur in two distinct patterns 4, 3:
  • Sporadic AML: Isolated tumors in otherwise healthy individuals
  • TSC-associated AML: Multiple, bilateral tumors in patients with tuberous sclerosis complex that develop at younger ages, grow faster, and are more prone to bleeding complications 1, 4

Imaging Characteristics

• On ultrasound: Classic AMLs appear hyperechoic (bright) and homogeneous, though fat-poor variants may be isoechoic and difficult to detect 1, 2
• On CT: Macroscopic fat appears as areas of negative density (diagnostic feature) 1, 2
• On MRI: Preferred modality showing multiparametric assessment capabilities, particularly useful for characterizing fat-poor lesions 1

Clinical Significance and Complications

• The major complication is spontaneous hemorrhage due to fragile vessels lacking complete elastic layers and the presence of microaneurysms 1, 4
• Bleeding risk increases significantly with tumor size >4 cm or presence of aneurysms ≥5 mm 2, 3
• Rare complications include extension into the renal vein, inferior vena cava, or even intracardiac extension with potential pulmonary embolism 6
• TSC-associated AMLs with TSC2 pathogenic variants arise younger, grow faster, and bleed more frequently than sporadic lesions 1

Diagnostic Pitfalls

• Up to 8% of renal cell carcinomas can appear hyperechoic on ultrasound, mimicking classic AML 1
• Fat-poor AMLs are particularly challenging as they cannot be reliably distinguished from renal cell carcinoma on imaging alone 1, 2
• When small and containing macroscopic fat on noncontrast CT or MRI, conservative management is appropriate 1

Pathologic Features

• Microscopically shows spindle-shaped cells resembling mature smooth muscle surrounding vascular structures with very low mitotic activity 1
• Immunohistochemistry is diagnostic: positive for smooth muscle actin, vimentin, HMB-45, and Melan-A (melanocytic markers) 1, 6, 7
• Negative staining for epithelial membrane antigen, S-100, and glial fibrillary acidic protein helps differentiate from other tumors 1