Fat-poor angiomyolipoma (AML) is a benign renal mesenchymal tumor containing no macroscopic fat or minimal fat below imaging detection threshold. It comprises approximately 5% of all AMLs. Because macroscopic fat cannot be detected on CT, it cannot be reliably distinguished from renal cell carcinoma (especially papillary RCC and chromophobe RCC) on imaging alone. Histopathologically, it contains smooth muscle, vessels, and small amounts of mature adipose tissue, but the fat component remains below imaging detection threshold. Typically presents as small (<3 cm), homogeneous, solid, well-circumscribed lesions. Homogeneous T2 hypointense signal on MRI is a characteristic finding. Definitive diagnosis usually requires biopsy; may be associated with tuberous sclerosis complex (TSC).
Age Range
20-70
Peak Age
45
Gender
Female predominant
Prevalence
Uncommon
Fat-poor AML is a benign mesenchymal neoplasm belonging to the perivascular epithelioid cell (PEComa) family. Like all AMLs, it contains smooth muscle, dysmorphic blood vessels, and mature adipose tissue; however, the fat component is very minimal or diffusely distributed, rendering it undetectable by imaging. In fat-poor AML, the dominant component is smooth muscle or epithelioid cell proliferation. The high nuclear-to-cytoplasmic ratio and dense cellularity of smooth muscle cells explain the characteristic homogeneous T2 hypointensity on MRI, while producing homogeneous hyperdensity on CT. HMB-45 and Melan-A positivity are diagnostically important. Cases associated with tuberous sclerosis complex (TSC) show bilateral and multiple AMLs; sporadic cases are typically solitary and small.
Homogeneous, markedly hypointense signal on T2-weighted MR images — the most characteristic finding of fat-poor AML reflecting smooth muscle dominance. Key differentiating finding from clear cell RCC (T2 hyperintense); however, papillary RCC can also be T2 hypointense.
Homogeneous hyperdense (>40 HU) solid renal mass on non-contrast CT. No macroscopic fat (pixel values >-10 HU). May be isodense or mildly hyperdense compared to surrounding renal parenchyma. Homogeneous internal structure without necrosis, hemorrhage, or cystic change.
Report Sentence
Homogeneous hyperdense solid lesion without macroscopic fat in the kidney on non-contrast CT; fat-poor angiomyolipoma and renal cell carcinoma should be considered in the differential diagnosis.
Shows homogeneous and avid enhancement in the corticomedullary phase. Enhancement degree is generally less than renal cortex and visually indistinguishable from solid RCC. Heterogeneous enhancement, necrosis, or cystic change is typically not seen — this homogeneity is a supportive finding for fat-poor AML.
Report Sentence
The lesion shows homogeneous enhancement in the corticomedullary phase; no necrosis or cystic change is observed.
Homogeneous hypointense signal on T2-weighted MR images — showing distinctly lower signal intensity than renal cortex. This is the most characteristic MRI finding for fat-poor AML, reflecting smooth muscle dominance. Papillary RCC can also be T2 hypointense but is generally more heterogeneous. Clear cell RCC is typically T2 hyperintense.
Report Sentence
Renal lesion shows homogeneous hypointense signal on T2-weighted sequences; this finding is consistent with fat-poor angiomyolipoma, though papillary RCC should be considered in the differential.
No signal drop on chemical shift (in-phase/opposed-phase) images — confirming the absence of macroscopic intralesional fat. In fat-poor AML, fat content remains below imaging detection threshold even at voxel level. However, some fat-poor AMLs may show mild signal drop due to minimal microscopic fat content — this finding makes distinction from clear cell RCC (signal drop due to intracellular lipid droplets) challenging.
Report Sentence
No significant signal drop on opposed-phase chemical shift images; no intralesional macroscopic fat detected.
Shows variable diffusion restriction on DWI — generally mild to moderate diffusion restriction is observed. ADC values may overlap with papillary RCC. High cellularity can cause diffusion restriction but this finding is not specific. ADC values are typically in the range of 1.0-1.5 x 10^-3 mm2/s, generally lower than clear cell RCC ADC values.
Report Sentence
Mild diffusion restriction is observed in the lesion on diffusion-weighted sequences with low signal on ADC map.
Appears as a homogeneous isoechoic or mildly hyperechoic solid mass on B-mode ultrasound. No posterior acoustic shadowing or enhancement. Cannot be distinguished from clear cell RCC by ultrasound. Lesion margins are generally smooth and well-defined. Contains no calcification or cystic component.
Report Sentence
Homogeneous isoechoic/mildly hyperechoic solid lesion in the kidney; fat-poor AML and RCC cannot be differentiated by ultrasound, further imaging recommended.
Shows isointense or mildly hypointense signal relative to renal parenchyma on T1-weighted images. T1 hyperintensity may be seen in the presence of intratumoral hemorrhage but this is rare. Homogeneous signal pattern reflects the absence of necrotic or cystic changes.
Report Sentence
The lesion shows isointense signal to renal parenchyma on T1-weighted sequences with no evidence of intratumoral hemorrhage.
Criteria
Epithelioid cells predominant (>80%), may show atypia and mitotic activity; carries malignancy potential
Distinct Features
Tendency for larger size (>4 cm), heterogeneous enhancement, may have necrosis; strong association with TSC; classified by WHO as PEComa with malignant potential
Criteria
Contains smooth muscle, vessels, and minimal fat below imaging threshold; most common variant
Distinct Features
Small size (<3 cm), homogeneous T2 hypointensity, homogeneous enhancement; benign course; HMB-45 positive on biopsy
Criteria
Smooth muscle dominant component, minimal vessels and fat; phenotype that makes histopathological distinction from renal leiomyoma challenging
Distinct Features
Most pronounced T2 hypointensity, most homogeneous internal structure; distinguished from leiomyoma by HMB-45 positivity on immunohistochemistry; clinically entirely benign
Distinguishing Feature
Papillary RCC can also be T2 hypointense but generally has more heterogeneous internal structure, may show T2* signal loss due to hemosiderin deposition, and necrosis/cystic change develops with increasing size. Fat-poor AML remains homogeneous.
Distinguishing Feature
Chromophobe RCC may appear as homogeneous solid mass but is isointense or mildly hyperintense on T2 (unlike fat-poor AML). Segmental enhancement pattern (spoke-wheel) and central scar favor chromophobe RCC.
Distinguishing Feature
Oncocytoma can present as homogeneous enhancing solid mass and is difficult to differentiate from fat-poor AML. Central scar (in large lesions) and isointense/mildly hyperintense T2 signal favor oncocytoma. Segmental enhancement pattern can also be seen in oncocytoma.
Distinguishing Feature
Clear cell RCC is typically T2 hyperintense (fat-poor AML is hypointense), shows heterogeneous enhancement + washout, signal drop on chemical shift images (intracellular lipid), and contains necrotic areas. Hypervascularity is prominent in clear cell RCC.
Urgency
routineManagement
surveillanceBiopsy
NeededFollow-up
6-monthFat-poor AML cannot be reliably distinguished from RCC on imaging; therefore, percutaneous biopsy (core biopsy) is recommended for definitive diagnosis. HMB-45 and Melan-A positivity on biopsy is diagnostic. Biopsy-confirmed fat-poor AML is a benign lesion and does not require surgery; it is followed with periodic imaging surveillance. When biopsy cannot be performed or results are inconclusive, small (<3 cm) and stable lesions are followed with contrast-enhanced CT or MRI at 6-month intervals. For large (>4 cm), growing, or suspected epithelioid morphology lesions, surgical excision should be considered. Patients with tuberous sclerosis complex should be screened for bilateral multiple AMLs, and mTOR inhibitors (everolimus) should be evaluated as treatment options.
Fat-poor AML may not be reliably distinguished from RCC radiologically. Biopsy or surgical excision may be needed for suspicious lesions. Can be confidently monitored in tuberous sclerosis patients in the context of known AMLs.