What are the next steps if a computed tomography (CT) scan shows a size discrepancy between the kidneys, with the right kidney being larger than the left, and cancer is a possible diagnosis?

Management of Kidney Size Discrepancy on CT Scan

When a CT scan reveals a size discrepancy between kidneys with the right kidney being larger than the left, a dedicated multiphase contrast-enhanced CT scan should be performed as the next step to properly characterize the finding and rule out malignancy. 1, 2

Diagnostic Approach

Initial Evaluation

• A dedicated multiphase CT protocol with intravenous contrast is the optimal imaging approach for evaluating renal masses, including unenhanced, corticomedullary (arterial), and nephrographic phases 2
• The multiphase protocol allows for accurate detection of concerning features such as enhancement patterns, mural nodules, wall thickening, and calcifications 1, 3
• Thin-slice acquisition should be used to improve detection of small lesions and reduce partial volume averaging 2

CT Protocol Components

• Unenhanced phase: Essential as a baseline to assess pre-contrast density (masses with <20 HU or >70 HU are typically benign) 2
• Corticomedullary phase (arterial): Obtained 25-70 seconds after contrast injection to evaluate vascularity 2
• Nephrographic phase: Obtained 80-180 seconds after contrast injection, superior for detecting small renal masses (<3 cm) 2

Alternative Imaging Options

• MRI with intravenous contrast is an excellent alternative when CT is contraindicated, with higher specificity than CT (68.1% vs 27.7%) for characterizing renal lesions 2
• MRI offers excellent soft tissue contrast that may better characterize complex cystic features 3
• Contrast-enhanced ultrasound (CEUS) can be considered when both CT and MRI contrast agents are contraindicated 2

Interpretation of Findings

Benign Causes of Kidney Size Discrepancy

• Normal anatomic variation (one kidney may be up to 1.5 cm longer than the other) 1
• Simple renal cysts (homogenous renal masses measuring <20 HU or >70 HU on non-contrast CT) 1
• Compensatory hypertrophy (if the contralateral kidney has reduced function) 1

Concerning Features Requiring Further Evaluation

• Enhancement of solid components after contrast administration 1, 2
• Irregular borders or infiltrative appearance 1
• Presence of calcifications 1
• Cystic masses with enhancing nodules, walls, or thick septa (Bosniak classification) 1

Risk Assessment and Management

Low Risk Findings

• Simple cysts (Bosniak I) have essentially no risk of malignancy 1
• Minimally complex cysts (Bosniak II) have a very low risk of malignancy 1

Intermediate Risk Findings

• Bosniak IIF cysts have approximately 10.9-25% risk of malignancy and require follow-up imaging 1
• Bosniak III cysts have approximately 40-54% risk of malignancy 1

High Risk Findings

• Bosniak IV cysts have approximately 90% risk of malignancy 1
• Solid enhancing masses are concerning for renal cell carcinoma 1

Follow-up Recommendations

• For indeterminate findings, a repeat CT scan in 3-6 months is appropriate to assess for stability or growth 1
• If malignancy is suspected based on imaging characteristics, referral to urology for consideration of biopsy or surgical management is warranted 1
• For patients with high risk of chronic kidney disease progression (eGFR <45 ml/min/1.73 m²), nephrology consultation should be considered prior to additional contrast studies 1

Special Considerations

• Patients ≤46 years of age with suspected renal malignancy should be referred for genetic counseling 1
• Genetic counseling should also be considered for patients with multifocal or bilateral renal masses, or if personal or family history suggests a familial renal neoplastic syndrome 1
• Partial nephrectomy should be prioritized over radical nephrectomy for the management of cT1a renal masses when intervention is indicated 1

Remember that kidney size discrepancy alone is not diagnostic of cancer, but a thorough evaluation is necessary to rule out malignancy, especially when the finding is new or has concerning features on imaging.