How to explain a diagnosis of a complex cystic lesion in one kidney and a simple cyst in the other kidney to a patient?

Explaining Kidney Cyst Findings to Your Patient

You should explain that one kidney has a simple cyst (a benign fluid-filled sac requiring no treatment) while the other has a complex cyst (a cyst with unusual features that needs further evaluation with MRI to determine if it's benign or requires closer monitoring).1

Understanding the Two Different Findings

Simple Cyst (One Kidney)

• A simple cyst is a benign, fluid-filled sac that is extremely common and increases with age—it's essentially a "water balloon" in the kidney that causes no harm.2
• Simple cysts are usually solitary, asymptomatic, and require no treatment or follow-up unless they become complicated (which is rare).2
• You can reassure the patient that this finding is of no concern and needs no further action.3

Complex Cyst (Other Kidney)

• A complex cyst has unusual features that distinguish it from a simple cyst, such as septations (internal walls), calcifications, thickened walls, or debris inside the fluid.1, 4
• The key distinction is that complex cysts require further characterization because they can represent either a complicated benign cyst (one that has bled or become infected) or, less commonly, a cystic kidney cancer.4, 2
• The American College of Radiology emphasizes that complex features include septations, mural thickening, nodularity, calcifications, or hemorrhagic content.1

What Happens Next

MRI Is the Critical Next Step

• MRI is the most appropriate next imaging test because it has much higher specificity (68.1%) than CT (27.7%) for distinguishing between benign and malignant kidney masses.5
• MRI can detect subtle enhancement (blood flow) that might indicate cancer, characterize small cysts better, and provide detailed information about the internal structure without radiation exposure.5
• The American College of Radiology specifically recommends MRI for complex cysts with septations to detect any concerning characteristics and guide management.1

Risk Stratification

• Cysts with only fine septations generally have low risk, but the presence of septations does increase malignancy risk compared to simple cysts.1
• MRI may reveal additional features like increased septal thickness, wall enhancement, or nodularity that would change the classification and management plan.1
• The most aggressive feature determines management—if multiple concerning findings are present, the cyst is managed according to the worst characteristic.4

Management Algorithm Based on MRI Results

If MRI Shows Low-Risk Features

• Surveillance with periodic MRI is typically recommended to monitor for any evolution toward more suspicious characteristics.1
• Many complex cysts remain stable and benign over time.6

If MRI Shows High-Risk Features

• Features requiring surgical consideration include enhancement, wall thickening, nodularity, or multiple thick septations.4
• In ambiguous cases (Bosniak IIf-III), percutaneous biopsy is well-tolerated, accurate, and can provide definitive diagnosis without surgery.6

Key Reassurance Points

• The vast majority of kidney cysts—even complex ones—are benign.2
• The purpose of MRI is to avoid unnecessary surgery for benign lesions while identifying the small percentage that need intervention.6
• Having one simple cyst and one complex cyst does not indicate a systemic kidney disease, as these are typically isolated findings.2

Common Pitfall to Avoid

• Do not rely solely on initial CT or ultrasound findings for complex cysts, as MRI detects additional concerning features in 19% of cases that would change management.1 This is why MRI is essential before making any treatment decisions.