What does it mean if a computed tomography (CT) scan of the abdomen in an adult patient shows an area of low density in a segment of the liver with focal fatty infiltration high in the differential?

Understanding "Low Density Area with Focal Fatty Infiltration High in the Differential"

This CT finding means the radiologist has identified a localized area of the liver that appears darker than normal liver tissue, and the most likely explanation is a benign accumulation of fat in that specific region rather than a tumor or other concerning pathology. 1

What This Finding Represents

Focal fatty infiltration is a benign condition where fat accumulates in a localized area of the liver, creating a characteristic low-density appearance on CT imaging. 1 This is distinct from diffuse fatty liver disease because it affects only a segment or portion of the liver rather than the entire organ.

Key Imaging Characteristics

The radiologist is suggesting focal fatty infiltration based on specific CT features that help distinguish this benign finding from malignant lesions:

• Nonspherical, geographic shape rather than round or oval mass 1, 2, 3
• Absence of mass effect (doesn't push or distort surrounding liver structures) 1, 2, 3
• Low density on CT with attenuation values approaching water density 2, 3
• Common locations include areas adjacent to the falciform ligament, gallbladder fossa, or porta hepatis 1, 4

Clinical Significance and What Happens Next

Focal fatty infiltration is almost always a benign finding that requires no treatment, but confirmation of the diagnosis is important to exclude malignancy, particularly if you have risk factors such as cirrhosis or history of cancer. 1, 5

Recommended Diagnostic Approach

The American College of Radiology guidelines provide clear direction on how to proceed:

• If you have no history of cancer or liver disease: The finding is likely benign and may require only correlation with clinical context and possibly ultrasound for confirmation 1

• If you have a history of malignancy: Further imaging with multiphasic contrast-enhanced MRI is recommended to definitively distinguish focal fatty infiltration from metastatic disease, as MRI has superior soft tissue characterization with 76.7%-90.0% sensitivity and 87.1%-91% specificity for detecting fatty infiltration 1

• If imaging remains indeterminate despite advanced imaging: Image-guided biopsy may be considered, though this is rarely necessary for focal fatty infiltration given its characteristic imaging appearance 1

Important Distinguishing Features from Malignancy

The radiologist's confidence in focal fatty infiltration as the diagnosis is based on features that differ significantly from liver tumors:

Focal Fatty Infiltration vs. Metastases

• Focal fat: Geographic/wedge-shaped, no mass effect, very low density (close to water), vessels course through it normally 1, 2, 3
• Metastases: Round/oval shape, solid appearance with minimal vascular enhancement, typically higher density than focal fat 1

Focal Fatty Infiltration vs. Hepatocellular Carcinoma

• Focal fat: No enhancement pattern, nonspherical, occurs in any liver (though more common with underlying fatty liver disease) 1
• HCC: Enhances in arterial phase and becomes hypoattenuating in portal venous phase, almost always occurs in cirrhotic livers 1, 6

Common Clinical Contexts

Focal fatty infiltration is associated with several clinical scenarios that your physician should evaluate:

• Alcohol use (most common association) 7, 8
• Obesity and metabolic syndrome 1
• Diabetes mellitus 1
• Medications (corticosteroids, chemotherapy) 1
• Increasing age (prevalence increases significantly with age, reaching 25.6% in young adults aged 15-19 years) 4

Critical Pitfalls to Avoid

Several imaging pitfalls can lead to misdiagnosis, which is why the radiologist stated this is "high in the differential" rather than definitive:

• Single-phase CT without contrast is inadequate for definitive characterization if there's any concern for malignancy 6, 9
• Focal fatty infiltration can occasionally have atypical appearances including multinodular patterns or even mass effect in alcoholic liver disease, making differentiation from malignancy difficult 8, 2
• In patients with known cancer, benign liver lesions are still found in nearly 30% of cases, so the presence of focal fat doesn't exclude concurrent metastatic disease 9

Practical Next Steps

Your physician should:

1. Review your clinical context: History of alcohol use, metabolic syndrome, medications, and any known malignancies 1

2. Correlate with liver function tests: Focal fatty infiltration may be associated with mildly abnormal aminotransferases if moderate to severe 1

3. Consider additional imaging if: You have a history of cancer, the lesion has atypical features, or there's diagnostic uncertainty 1, 6, 5

4. Avoid unnecessary biopsy if imaging features are characteristic and clinical context is consistent with benign focal fat 1, 5

The phrase "high in the differential" indicates the radiologist is confident this is likely focal fatty infiltration based on its imaging characteristics, but is appropriately cautious given that CT has limitations in definitively diagnosing early or mild fatty infiltration. 1 This conservative language protects against missing alternative diagnoses while providing the most likely explanation for the finding.