Renal angiomyolipoma (AML) is the most common benign mesenchymal tumor of the kidney, composed of three histological components: mature adipose tissue, smooth muscle cells, and dysmorphic blood vessels — a hamartomatous lesion. It occurs with a prevalence of 0.3-3% in the general population, with a female-to-male ratio of 4:1. While 80% of cases are sporadic and typically solitary, 20% are associated with tuberous sclerosis complex (TSC) or pulmonary lymphangioleiomyomatosis (LAM) — in these patients, lesions tend to be bilateral and multiple. Macroscopic fat content on CT (< -20 HU) is a pathognomonic diagnostic criterion; a fat-containing renal mass is considered AML until proven otherwise. On MRI, the fat component shows hyperintense signal on T1-weighted sequences with marked signal loss on fat-suppressed sequences confirming the diagnosis. The India ink artifact (chemical shift artifact) creates a characteristic black line at the lesion-renal parenchyma interface. Clinically, the most significant complication is spontaneous hemorrhage; lesions >4 cm carry a significantly increased risk of retroperitoneal bleeding (Wunderlich syndrome). Treatment strategy is size-based: lesions <4 cm are followed with annual US, while >4 cm symptomatic or rapidly growing lesions are managed with selective arterial embolization or nephron-sparing surgery.
Age Range
20-70
Peak Age
45
Gender
Female predominant
Prevalence
Common
AML is a hamartomatous neoplasm originating from perivascular epithelioid cells (PEComa family). The three components — mature adipose tissue, smooth muscle cells, and thick-walled dysmorphic blood vessels — are present in varying proportions. The mTOR (mechanistic target of rapamycin) signaling pathway plays a central role in AML pathogenesis: in tuberous sclerosis complex, TSC1 (hamartin) or TSC2 (tuberin) gene mutations lead to excessive mTOR pathway activation, causing uncontrolled cell proliferation. Somatic TSC2 mutations can also be detected in sporadic AML. The dysmorphic blood vessels lack normal vessel wall architecture — these vessels without elastic lamina and with irregular muscular layers are prone to aneurysm formation. In lesions >4 cm, these aberrant vessels develop microaneurysms, creating risk of spontaneous rupture and retroperitoneal hemorrhage (Wunderlich syndrome). Macroscopic fat content is detected on CT as negative density values (< -20 HU). On MRI, chemical shift artifact occurs at the fat-water interface: the different precession frequencies of fat and water protons in in-phase and opposed-phase sequences cause signal loss at the boundary (India ink artifact). On T1-weighted sequences, the fat component appears hyperintense due to short T1 relaxation time; on fat-suppressed sequences (fat-sat, STIR), this signal is suppressed, confirming the presence of fat.
Detection of macroscopic fat tissue measuring < -20 HU within a renal mass on non-contrast CT is a pathognomonic diagnostic criterion for renal AML. When a fat-containing renal mass is found, AML is the first and strongest diagnosis — rare exceptions (Wilms tumor, liposarcoma, fat-containing RCC) should be excluded. ROI measurement should be performed on the lowest density area of the lesion using thin-section (≤3 mm) CT while avoiding partial volume effects. The MRI equivalent is the triad of T1 hyperintense signal + signal loss on fat-suppressed sequences + India ink artifact.
Macroscopic fat tissue measuring < -20 HU within the lesion on non-contrast CT. ROI (Region of Interest) must be carefully placed on the fat-containing portion of the lesion. Even small foci of fat are diagnostic. Thin-section (≤3 mm) CT should be used to avoid partial volume effects. Fat density typically ranges from -30 to -120 HU.
Report Sentence
A [size] mm mass in the [location] of the kidney containing macroscopic fat measuring [value] HU on non-contrast CT is identified, consistent with renal angiomyolipoma.
On contrast-enhanced CT arterial phase, the solid components (smooth muscle and vascular) of the lesion show enhancement while fat components do not enhance. The heterogeneous enhancement pattern reflects the varying proportions of the three components. In large lesions (>4 cm), dysmorphic vessels may be visible as vascular structures. In hemorrhagic AML, areas of acute bleeding appear hyperdense.
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On contrast-enhanced CT, the lesion demonstrates heterogeneous enhancement with non-enhancing fat-density areas interspersed among enhancing solid components.
On T1-weighted sequences, hyperintense signal areas within the lesion are observed due to the fat component. In fat-rich AMLs, prominent homogeneous T1 hyperintensity is seen, while in smooth muscle-dominant lesions it is less conspicuous. T1 hyperintensity can be confused with subacute hemorrhage; therefore, confirmation with fat-suppressed sequences is mandatory. The fat component produces bright signal in vivid orange-yellow tones.
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On MRI T1-weighted sequence, hyperintense areas consistent with fat signal are identified within the lesion.
On opposed-phase (chemical shift) MRI, a conspicuous India ink artifact (black line) is observed at the lesion-renal parenchyma boundary. This artifact is characteristic of lesions containing macroscopic fat and strongly supports AML diagnosis. Importantly, the India ink artifact is seen at the lesion border (fat-water interface), not within the lesion (intravoxel). In clear cell RCC with intravoxel fat-water mixture, diffuse signal loss occurs on a voxel basis — this distinction is a critical differential diagnostic criterion.
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India ink artifact is observed at the lesion margin on chemical shift (opposed-phase) MRI, consistent with macroscopic fat content and supporting the diagnosis of angiomyolipoma.
On T1-weighted fat-suppressed sequence (fat-sat or STIR), marked signal loss is observed in the fat component of the lesion. Areas hyperintense on T1 without fat suppression lose signal and become hypointense when fat suppression is applied. This signal loss pattern definitively confirms the presence of intralesional macroscopic fat. The smooth muscle component is unaffected by fat suppression and maintains intermediate signal intensity.
Report Sentence
Marked signal loss in the hyperintense areas of the lesion on T1-weighted fat-suppressed sequence confirms the presence of intralesional macroscopic fat.
On B-mode ultrasonography, AML typically appears as a homogeneously hyperechoic mass. The hyperechogenicity is due to the high acoustic impedance mismatch created by the fat component. Small AMLs (<1.5 cm) can be confused with renal sinus fat. Posterior acoustic shadowing is usually absent. However, clear cell RCC is also in the differential of hyperechoic renal masses — CT or MRI confirmation is recommended especially for small hyperechoic lesions. In large AMLs, heterogeneous echopattern may be seen due to internal hemorrhage.
Report Sentence
A [size] mm homogeneously hyperechoic mass is identified in the [location] of the kidney, with angiomyolipoma considered as the primary diagnosis; CT or MRI is recommended for confirmation.
On T2-weighted sequences, AML shows variable signal intensity: fat component demonstrates intermediate-to-high signal, smooth muscle component shows low signal, and vascular component appears as flow voids. In fat-dominant AMLs, T2 appearance is isointense to subcutaneous fat. In muscle-dominant (epithelioid) variants, low T2 signal predominates and may be confused with malignancy. In the presence of hemorrhagic complication, T2 signal becomes more heterogeneous.
Report Sentence
On MRI T2-weighted sequence, the lesion demonstrates coexisting intermediate-to-high signal areas consistent with fat component and low signal areas reflecting smooth muscle component.
In large AMLs (>4 cm), spontaneous rupture can lead to acute retroperitoneal hemorrhage (Wunderlich syndrome). On non-contrast CT, perirenal and/or pararenal hyperdense collection (50-70 HU) is observed. Active bleeding focus may be detected on contrast-enhanced CT as contrast extravasation. The sentinel clot sign (most dense hematoma near the lesion) indicates the bleeding source. Hemorrhagic AML is an acute clinical presentation that may require emergent arterial embolization.
Report Sentence
Perirenal and pararenal hyperdense collection is observed around the left/right kidney, consistent with Wunderlich syndrome (retroperitoneal hemorrhage) due to spontaneous rupture of a large angiomyolipoma.
Criteria
Typical AML with macroscopic fat component < -20 HU on CT, containing all three components (fat, smooth muscle, dysmorphic vessels) in varying proportions. Constitutes 95% of cases.
Distinct Features
Macroscopic fat (< -20 HU) on CT is pathognomonic. T1 hyperintense + signal loss on fat suppression on MRI. Hyperechoic mass on US. Diagnosis is made by imaging, biopsy not needed. Annual US follow-up is sufficient for lesions <4 cm.
Criteria
AML without detectable macroscopic fat on CT (≥ -20 HU), with dominant smooth muscle component. Constitutes approximately 5% of cases. May appear as a homogeneously hyperdense mass.
Distinct Features
Homogeneous enhancement on CT, difficult to differentiate from RCC due to inability to demonstrate fat content. T2 hypointense signal on MRI (smooth muscle dominant), minimal/no signal loss on opposed-phase. Non-contrast density may be equal to or slightly higher than renal cortex on CT. Definitive diagnosis usually requires biopsy or surgical pathology. Separate atlas file: kidney-aml-fat-poor.
Criteria
Rare AML variant with epithelioid cell predominance, potentially capable of malignant behavior. Classified as a separate entity in WHO classification. More frequently seen in tuberous sclerosis patients.
Distinct Features
Usually appears as a fat-poor or fat-free, solid hypervascular mass. May mimic RCC on CT. Can contain areas of necrosis and hemorrhage. Has potential for local invasion and metastasis — surgical resection required due to malignant potential. HMB-45 and Melan-A positivity on immunohistochemistry confirms the diagnosis.
Criteria
AML lesions seen in patients with tuberous sclerosis complex (TSC), typically bilateral and multiple. Renal AML develops in 80% of TSC patients. Associated with TSC1 or TSC2 gene mutations.
Distinct Features
Bilateral and multiple lesions (sporadic AML is usually solitary). Younger age of onset (20-30 years). Tendency for faster growth and higher hemorrhage risk. Risk of renal failure (due to multiple lesions). May respond to mTOR inhibitor therapy (everolimus). Other TSC findings: cortical tubers, subependymal giant cell astrocytoma, cardiac rhabdomyoma, facial angiofibroma, pulmonary LAM.
Distinguishing Feature
In fat-poor AML, macroscopic fat (< -20 HU) cannot be detected on CT, appearing as a homogeneously hyperdense mass. In classic AML, < -20 HU fat is pathognomonic. Fat-poor AML shows no significant signal loss on opposed-phase MRI and may demonstrate characteristic low T2 signal (smooth muscle dominant).
Distinguishing Feature
Clear cell RCC contains intracellular (microscopic) fat — diffuse intravoxel signal loss is observed on opposed-phase MRI (homogeneous decrease throughout the lesion). AML has macroscopic fat — India ink artifact is seen at the lesion BORDER, not within. RCC is prominently hypervascular, enhances intensely in arterial phase with washout. The presence of < -20 HU fat in AML is a definitive differentiator.
Distinguishing Feature
Papillary RCC is hypovascular — showing minimal enhancement in arterial phase (the vascular component of AML does enhance). Papillary RCC tends to be homogeneous and hypodense relative to renal cortex on CT. Contains no fat. May show low T2 signal but no signal loss on fat suppression. Calcification and hemosiderin deposition may be seen.
Distinguishing Feature
Oncocytoma is a benign tumor showing homogeneous enhancement and characteristic central scar (stellate pattern). Contains no fat — macroscopic fat (< -20 HU) is not detected on CT. Spoke-wheel vascular pattern is typical on arteriography. Mildly hypointense or isointense on T2, no signal loss on fat suppression. Unlike AML, has no fat component.
Urgency
routineManagement
surveillanceBiopsy
Not NeededFollow-up
annualClassic fat-rich AML is a benign tumor that can be definitively diagnosed by imaging when macroscopic fat is detected on CT or MRI — biopsy is not needed. Treatment strategy is size-based: (1) <4 cm asymptomatic: annual US follow-up; (2) >4 cm or symptomatic: selective arterial embolization or nephron-sparing surgery (partial nephrectomy); (3) Acute hemorrhage (Wunderlich syndrome): emergent angiographic embolization. In tuberous sclerosis patients, a nephron-sparing approach is particularly important due to bilateral/multiple lesions, and mTOR inhibitor (everolimus) may be used as a medical treatment option. In cases of rapid growth (>0.5 cm/year) or suspicion of epithelioid morphology, surgery or biopsy should be considered. In women planning pregnancy, hormonal stimulation can accelerate AML growth, so treatment should be considered for >4 cm lesions before pregnancy.
Fat-containing AML diagnosis can be confirmed by CT or MRI without biopsy. Lesions <4cm are monitored. For lesions >4cm, embolization or partial nephrectomy is considered due to hemorrhage risk. Bilateral multiple AMLs are common in tuberous sclerosis patients.