What specialist should a patient with an incidentally found renal mass on a computed tomography (CT) scan be referred to for further evaluation and management?

Referral for Incidentally Found Renal Mass

Refer the patient to urology for evaluation and management of the incidentally found renal mass. While radiologists play a critical role in characterizing these lesions through appropriate imaging, urologists are the definitive specialists who determine treatment options including active surveillance, percutaneous ablation, or surgical intervention 1, 2.

Initial Imaging Characterization Before Referral

Before making the referral, ensure appropriate imaging characterization has been completed, as many lesions can be definitively diagnosed as benign and require no urologic consultation 3:

Lesions That Do NOT Require Referral

• Homogeneous masses <20 HU on unenhanced CT are benign simple cysts and require no further evaluation or referral 3, 4
• Homogeneous masses >70 HU on unenhanced CT are benign (typically hemorrhagic/proteinaceous cysts) and require no further evaluation or referral 3, 4
• Homogeneous cysts measuring 10-20 HU on contrast-enhanced CT are benign and require no referral 3, 4
• Homogeneous cysts measuring 21-30 HU on portal venous phase contrast-enhanced CT may also be considered benign and do not require referral 3, 4

Lesions That Require Further Imaging Characterization

If the initial CT shows an indeterminate mass (density 20-70 HU on unenhanced CT, or any heterogeneous mass), obtain multiphase CT or MRI with IV contrast before referral to better characterize the lesion 3:

• Multiphase CT with and without IV contrast is the primary imaging modality for characterizing indeterminate renal masses, with diagnostic accuracy of 79.4% for predicting renal cell carcinoma 3
• MRI with gadolinium-based contrast offers superior specificity (68.1% vs 27.7% for CT) and is particularly useful for complex cystic lesions and when CT findings are equivocal 3, 5
• Contrast-enhanced ultrasound (CEUS) is valuable when iodinated CT contrast or gadolinium-based MRI contrast is contraindicated, with accuracy of 90.2% for characterizing indeterminate lesions 3

When to Refer to Urology

Refer to urology when any of the following are present 3:

Solid Masses

• Any enhancing solid mass (>10-15 HU enhancement on CT or >15% enhancement on MRI) requires urologic evaluation 3, 5
• Masses containing macroscopic fat are typically benign angiomyolipomas, but lipid-poor angiomyolipomas cannot be differentiated from renal cell carcinoma on imaging alone and may require biopsy 3, 5

Complex Cystic Masses (Bosniak Classification)

• Bosniak IIF lesions: 10.9-25% progress to malignancy and require urologic consultation for surveillance planning 3
• Bosniak III lesions: 40-54% are malignant and require urologic evaluation 3
• Bosniak IV lesions: 90% are malignant and require urgent urologic referral 3

Indeterminate Masses Despite Optimal Imaging

• Lesions that remain indeterminate after multiphase CT or MRI require urologic consultation to discuss biopsy or treatment options 3

Important Caveats

Renal mass biopsy is not the initial workup but has expanded indications including small renal masses (T1a, <4 cm), suspected benign tumors like lipid-poor angiomyolipomas, and patients with limited life expectancy or significant comorbidities 3. Urologists determine when biopsy is appropriate, with significant complications occurring in only 0.9% of cases 3.

A common pitfall is inadequate imaging technique leading to mischaracterization—ensure dedicated multiphase renal protocols are used rather than single-phase contrast studies, as the latter cannot reliably distinguish enhancing masses from benign cysts 3, 1.

For small renal masses (<4 cm), active surveillance has become an accepted management option by urologists, as many demonstrate slow growth kinetics with low progression rates 3. This decision requires urologic expertise to balance oncologic outcomes with preservation of renal function.