Definition/General
Introduction:
Angiomyolipoma (AML) is a benign mesenchymal hamartoma composed of three tissue components
It contains blood vessels, smooth muscle, and mature fat
It represents 1-3% of all renal masses
FNAC shows characteristic mixed tissue elements.
Origin:
Arises from perivascular epithelioid cells (PECs)
The tumor demonstrates triple tissue composition
Smooth muscle component predominates in most cases
Vascular component consists of thick-walled blood vessels
Mature adipose tissue completes the triad.
Classification:
Classified as PEComa family tumor (WHO 2016)
Classic AML (contains fat, identifiable on imaging)
Epithelioid AML (aggressive variant, minimal fat)
Fat-poor AML (difficult to diagnose on imaging)
FNAC may sample different components variably.
Epidemiology:
Peak incidence in 4th-5th decades
Female predominance (4:1 ratio)
Associated with tuberous sclerosis complex (TSC) in 50% cases
Sporadic cases more common
Multiple and bilateral tumors in TSC patients
Indian population shows similar demographics.
Clinical Features
Presentation:
Asymptomatic (small tumors, incidental finding)
Flank pain (large tumors)
Acute hemorrhage (Wunderlich syndrome - rare but serious)
Palpable mass (large tumors)
Hematuria (uncommon)
Associated TSC stigmata.
Symptoms:
Acute flank pain and shock (retroperitoneal hemorrhage)
Chronic dull aching pain
Hematuria (gross or microscopic)
Hypertension (mass effect on renal vessels)
Constitutional symptoms rare
TSC-related symptoms (seizures, skin lesions).
Risk Factors:
Tuberous sclerosis complex (TSC1/TSC2 mutations)
Female gender
Estrogen exposure (pregnancy, hormone therapy)
No strong environmental risk factors
Family history of TSC
Age (peak in reproductive years).
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Gross Description
Appearance:
Well-circumscribed mass with yellow, red, and gray areas
Yellow areas represent mature fat
Red areas represent vascular component
Gray-tan areas represent smooth muscle
Size varies from few mm to >20 cm.
Characteristics:
Heterogeneous appearance reflecting tissue composition
Mature adipose tissue (yellow, soft)
Smooth muscle areas (gray-tan, firm)
Vascular areas (red, spongy)
No necrosis typically
Calcification uncommon.
Size Location:
Variable size (few mm to 30 cm)
Cortical location typical
Can be multiple (especially in TSC)
Bilateral involvement in TSC patients
Exophytic growth common
May extend into perinephric fat.
Microscopic Description
Immunohistochemistry
Positive Markers:
Melanoma markers (HMB-45, Melan-A) in epithelioid cells
Smooth muscle actin in smooth muscle component
Muscle-specific actin
Desmin (variable in smooth muscle)
S-100 protein in adipose component
CD68 (variable).
Negative Markers:
Cytokeratins (negative)
PAX8 (negative, excludes renal epithelial origin)
Chromogranin (negative)
Synaptophysin (negative)
CD10 (negative)
Inhibin (negative)
TTF-1 (negative).
Diagnostic Utility:
HMB-45 positivity in epithelioid cells is diagnostic
Smooth muscle actin confirms smooth muscle component
Combination of markers supports PEComa diagnosis
Melanoma markers distinguish from other mesenchymal tumors
Useful in fat-poor variants.
Molecular/Genetic
Genetic Mutations:
TSC1 or TSC2 mutations (TSC-associated cases)
Somatic TSC2 mutations (sporadic cases)
mTOR pathway activation
Loss of heterozygosity at TSC loci
No other recurrent genetic alterations in classic AML.
Prognostic Significance:
Classic AML has excellent prognosis
Epithelioid AML may have malignant potential
Large size increases hemorrhage risk
TSC association indicates multiple tumors possible
Complete excision usually curative.
Therapeutic Targets:
mTOR inhibitors (sirolimus, everolimus) for large/multiple tumors
Angiogenesis inhibitors under investigation
Surgical resection for symptomatic tumors
Embolization for acute hemorrhage
Active surveillance for small asymptomatic tumors.
Differential Diagnosis
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Specimen Information
FNAC from [kidney mass/renal lesion], [location], performed under [guidance method]
Specimen Adequacy
[Adequate/Inadequate] for cytological interpretation
Cytological Findings
Mixed cellular population showing [smooth muscle cells/epithelioid cells/mature adipocytes]. Cells show [benign cytological features/mixed morphology]
Tissue Components
[Smooth muscle component/Epithelioid cells/Adipose tissue/Vascular elements] identified
Background
Background shows [hemorrhage/mixed tissue elements/clean background]
Cytological Diagnosis
[Benign mesenchymal lesion] - Features consistent with angiomyolipoma
Morphological Variant
[Classic AML/Epithelioid features/Fat-poor variant] based on available material
Recommendations
[Histopathological examination/Immunohistochemistry/Clinical correlation] recommended. [TSC screening] if multiple lesions
Note
Final diagnosis requires histological examination. Clinical correlation for TSC association recommended