Is a kidney lesion considered abnormal?

Is a Kidney Lesion Abnormal?

Not all kidney lesions are abnormal—many are benign simple cysts that require no further evaluation or treatment. The determination depends entirely on specific imaging characteristics that distinguish benign from indeterminate or potentially malignant lesions.

Defining Normal vs. Abnormal Kidney Lesions

Definitively Benign Lesions (Not Abnormal)

The following kidney lesions can be confidently classified as benign and require no further imaging or intervention:

• Homogeneous masses measuring <20 HU or >70 HU on unenhanced CT are considered benign and do not require further characterization 1
• Homogeneous renal masses measuring 10-20 HU on contrast-enhanced CT are benign cysts requiring no additional evaluation 1
• Recent evidence supports that homogeneous masses measuring 21-30 HU on portal venous phase contrast-enhanced CT may also be considered benign cysts 1
• Lesions containing macroscopic fat can be characterized as benign (typically angiomyolipomas) 1
• Simple cysts on ultrasound that are sonolucent, demonstrate posterior through-transmission, and have thin well-defined walls are benign 1

Indeterminate Lesions (Abnormal and Requiring Evaluation)

Any mass with density >20 HU and <70 HU, as well as any heterogeneous mass on unenhanced CT, is considered indeterminate and warrants further evaluation 1. These represent truly abnormal findings that cannot be dismissed.

Clinical Significance and Risk Stratification

The American College of Radiology emphasizes that indeterminate renal masses vary widely in biological aggressiveness, ranging from benign tumors to high-grade renal cell carcinomas 1. This heterogeneity is critical because:

• Small renal masses are increasingly detected as incidental findings in asymptomatic individuals 1
• Many small renal cell carcinomas demonstrate slow growth kinetics with low rates of progression 1
• Approximately one-third of biopsied indeterminate masses prove to be benign (such as lipid-poor angiomyolipomas or oncocytomas) 1

Recommended Diagnostic Algorithm

Step 1: Initial Characterization

If a kidney lesion is detected on any imaging:

• Assess density on unenhanced CT or enhancement pattern on contrast-enhanced imaging to determine if it meets criteria for a definitively benign lesion 1
• If the lesion is homogeneous and meets HU thresholds listed above, it can be dismissed as a simple cyst (not abnormal) 1

Step 2: Further Evaluation of Indeterminate Lesions

For lesions that cannot be definitively characterized as benign, MRI without and with IV contrast is the preferred next step 2:

• MRI demonstrates higher specificity than CT (68.1% vs 27.7%) for distinguishing benign from malignant masses 2, 3
• MRI is particularly useful for small cysts <1.5 cm and can better detect subtle enhancement patterns 2, 3
• Contrast-enhanced ultrasound (CEUS) is an alternative when iodinated CT contrast or gadolinium-based MRI contrast is contraindicated, with sensitivity of 100% and specificity of 95% for classifying benign versus malignant masses 1

Step 3: Management Based on Characterization

• Definitively benign lesions: No follow-up required 1
• Indeterminate solid masses: Consider biopsy, active surveillance, or surgical intervention based on size, patient comorbidities, and life expectancy 1
• Complex cystic lesions: Use Bosniak classification to guide management 3

Common Pitfalls to Avoid

• Do not assume all kidney lesions are abnormal—the majority of incidentally detected renal masses are simple benign cysts 1
• Avoid unnecessary imaging for lesions that meet definitive benign criteria on initial imaging, as this increases healthcare costs and radiation exposure without clinical benefit 4
• Do not rely on unenhanced CT or conventional ultrasound alone for indeterminate lesions, as these modalities cannot reliably assess enhancement patterns necessary for proper characterization 1, 3
• Remember that nondiagnostic biopsy results cannot be considered evidence of benignity—approximately 20% of biopsies are nondiagnostic, particularly for small masses 1