Hepatic angiomyolipoma (AML) is a rare benign mesenchymal tumor composed of fat tissue, smooth muscle cells, and abnormal blood vessels. It belongs to the PEComa (perivascular epithelioid cell tumor) family and is characterized by HMB-45 immunohistochemical positivity. It shares similar histology with renal AML, but tuberous sclerosis association is much rarer in the hepatic form. It is 4 times more common in women than men. Usually asymptomatic and discovered incidentally. Macroscopic fat content is the most important diagnostic clue.
Age Range
30-60
Peak Age
45
Gender
Female predominant
Prevalence
Rare
Hepatic AML is a tumor belonging to the perivascular epithelioid cell (PEC) family. PEC cells originate from embryonic neural crest and express both melanocytic (HMB-45+) and smooth muscle (SMA+) markers. The tumor consists of three components: (1) mature adipose tissue — produces negative HU values on CT and fat signal on MRI, (2) smooth muscle cells — form the solid enhancing component, (3) dysplastic thick-walled vessels — cause intense arterial phase enhancement. The proportion of these three components varies greatly between cases; fat-dominant types are easy to diagnose, but fat-poor (epithelioid) variants make differentiation from HCC challenging.
The triad of macroscopic fat density on CT (-20 to -100 HU), intense arterial enhancement in solid components, and absence of washout on delayed phases in a liver lesion is the signature finding of hepatic angiomyolipoma. This combination is nearly exclusively specific to AML among liver lesions.
Areas of macroscopic fat density within the lesion (-20 to -100 HU). These fat areas reflect the mature adipose tissue component of the tumor. ROI measurement is required for fat density confirmation.
Report Sentence
Areas of macroscopic fat density (__ HU) are observed within the lesion, consistent with hepatic angiomyolipoma.
Intense arterial enhancement in solid components. Prominent enhancement due to dysplastic thick-walled vessels and rich vascularity of smooth muscle component. Fat areas do not enhance.
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The solid components of the lesion demonstrate intense arterial phase enhancement.
Persistent enhancement in solid components on portal venous and delayed phases — no washout. This finding is critical in differentiating from HCC.
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The lesion does not demonstrate washout on portal venous and delayed phases, with persistent enhancement continuing.
Hyperintense areas on T1 — reflecting intratumoral fat component. Isointense/hypointense signal in areas with less fat content.
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The lesion contains hyperintense signal areas on T1, suggesting intratumoral fat component.
Significant signal dropout on opposed-phase imaging — demonstrating coexistence of intracellular fat and water protons in the same voxel. India ink artifact (at fat-water interface) may be seen at lesion margins.
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Significant signal dropout is observed on opposed-phase imaging, confirming the presence of intratumoral fat.
Heterogeneous signal on T2: fat components moderately hyperintense, smooth muscle areas hypointense/isointense, vascular structures show flow void. Signal loss in fat areas on fat suppression confirms the diagnosis.
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The lesion shows heterogeneous signal on T2 with different signal intensities reflecting fat, smooth muscle, and vascular components.
Hyperechoic mass — due to high acoustic impedance of fat component. May resemble hyperechoic appearance of hemangioma. Posterior acoustic shadowing may be observed.
Report Sentence
A hyperechoic mass measuring approximately ___ cm is seen in the liver, and a fat-containing lesion (angiomyolipoma) should be primarily considered; CT/MRI fat confirmation is recommended.
Criteria
Type where fat component constitutes >70%. Most commonly encountered AML variant.
Distinct Features
Prominent macroscopic fat on CT and MRI — diagnosis usually made by imaging, no biopsy needed. Presence of solid enhancing component is important in differential with lipoma.
Criteria
Type where smooth muscle component is dominant. Fat content is minimal or not macroscopically detectable.
Distinct Features
Solid mass with dominant hypervascularity — may be confused with HCC or hypervascular metastasis. Minimal signal loss on opposed-phase MRI, biopsy usually required.
Criteria
Rare variant where epithelioid cells dominate. Fat content very low or absent. Carries potential for PEComa malignancy.
Distinct Features
AML variant most confused with HCC — differentiation by imaging is difficult without fat finding. Biopsy mandatory. HMB-45+/SMA+ and Hepatocyte paraffin-1 (Hep Par-1) negative (Hep Par-1 positive in HCC). Risk of malignant transformation increases with >5 cm and/or necrosis.
Distinguishing Feature
Hemangioma shows peripheral nodular enhancement + centripetal fill-in and does not contain macroscopic fat. Macroscopic fat (-20 to -100 HU) in AML is pathognomonic. On MRI, hemangioma is light-bulb bright on T2 (fluid signal) while AML is heterogeneous on T2.
Distinguishing Feature
HCC shows arterial enhancement + portal venous washout (LI-RADS major feature). AML does not show washout. HCC develops in cirrhotic liver, AML is not associated with cirrhosis. HCC may contain microscopic fat but macroscopic fat is rare. HMB-45 is negative in HCC, Hep Par-1 is positive.
Distinguishing Feature
Hepatic adenoma (especially HNF1α-mutant type) may contain diffuse homogeneous fat — showing widespread signal loss on opposed-phase. Fat distribution in AML is more irregular and focal. Adenoma is associated with OCP use in young women. AML is positive for PEComa markers (HMB-45), adenoma is negative.
Distinguishing Feature
In focal steatosis, fat infiltration does not displace vessels (vessels pass through the lesion normally). In AML, the solid mass displaces vessels. Focal steatosis shows no mass effect, no enhancement, and has geographic margins.
Urgency
routineManagement
surveillanceBiopsy
Not NeededFollow-up
annualHepatic AML is a benign tumor and does not require treatment in most cases. In typical fat-dominant AML, CT/MRI findings are sufficient for diagnosis — no biopsy needed. Biopsy is recommended in fat-poor/epithelioid types for malignancy differential. Surgical resection or embolization may be considered in lesions >5 cm due to spontaneous hemorrhage risk. Small (<5 cm) and stable lesions are followed with annual US or MRI. Tuberous sclerosis association should be investigated (rarely may coexist with hepatic AML).
Hepatic AML is a benign tumor and does not require treatment in most cases. Surgical resection or embolization may be considered in large lesions due to bleeding risk. The epithelioid variant carries malignant potential and requires close follow-up. Screening for tuberous sclerosis is recommended.