How should incidentally found renal cysts be monitored?

Management of Incidentally Found Renal Cysts

Simple benign cysts measuring <20 HU or homogeneous cysts measuring 10-30 HU on contrast-enhanced CT require no further imaging or monitoring. 1

Initial Characterization Strategy

Definitively Benign Cysts (No Monitoring Required)

• Homogeneous masses <20 HU on unenhanced CT are benign and require no further evaluation 1
• Homogeneous masses >70 HU on unenhanced CT are benign and require no further evaluation 1
• Homogeneous cysts measuring 10-20 HU on contrast-enhanced CT are benign and require no further evaluation 1
• Homogeneous cysts measuring 21-30 HU on portal venous phase contrast-enhanced CT may also be considered benign and do not require further evaluation 1

Indeterminate Cysts Requiring Further Evaluation

• Any mass with density 20-70 HU on unenhanced CT warrants further evaluation 1
• Any heterogeneous mass on unenhanced CT is considered indeterminate and requires additional imaging 1

Monitoring Protocol for Complex/Indeterminate Cysts

Initial Workup

For cysts that cannot be definitively characterized as benign, perform multiphase CT or MRI with IV contrast to assess for enhancement and complexity 1. This dedicated protocol is the mainstay for evaluating indeterminate renal masses and establishing baseline characteristics.

Surveillance Imaging Schedule

For small renal masses (<4 cm) that remain indeterminate or show concerning features:

• Initial follow-up within 6 months to establish growth rate 1
• Annual imaging thereafter with CT, MRI, or ultrasound 1, 2
• Measurement consistency is critical: Use the same modality when possible, as interobserver variability can be 2.3-3.1 mm and different modalities produce inconsistent measurements 1

Alternative Imaging Modalities

Contrast-enhanced ultrasound (CEUS) is valuable for patients with contraindications to iodinated CT contrast or gadolinium-based MRI contrast 1. The microbubble agents are not renally excreted and allow real-time evaluation of microvasculature for differentiating cystic from solid lesions 1. However, CEUS may assign higher Bosniak classifications compared to CT and does not evaluate both kidneys completely 1.

Unenhanced MRI has advantages over unenhanced CT for characterizing cysts when contrast is contraindicated 1. Simple cysts or those with thin septations can be characterized on T2-weighted imaging based on homogeneous and very high T2 signal intensity 1.

When to Discontinue Surveillance

Proceed to Biopsy or Treatment When:

• Growth rate accelerates or mass demonstrates interval change in complexity 1, 2
• Enhancement pattern suggests malignancy (>15% enhancement on MRI or >10-20 HU enhancement on CT) 1
• Development of solid nodules, thick septations, or irregular walls 1

Role of Biopsy

Percutaneous biopsy should be strongly considered when imaging features suggest a benign mass (such as fat-poor angiomyolipoma) but cannot definitively exclude malignancy 1. Biopsy has expanded indications for small renal masses given that one-third may be benign and many small renal cell carcinomas demonstrate slow growth with low metastatic potential 1.

Critical caveat: Nondiagnostic biopsy results (occurring in approximately 20% of cases for masses <4 cm) cannot be considered evidence of benignity and require continued surveillance or repeat biopsy 1.

Special Populations

Patients with Family History of ADPKD

Multiple cysts in the absence of known family history require clinical workup for cystic kidney diseases including genetic testing 1. For at-risk individuals with positive family history, ultrasound criteria apply: ≥3 total cysts (ages 15-39), ≥2 cysts per kidney (ages 40-59), or ≥4 cysts per kidney (age ≥60) confirm ADPKD 1.

Solitary cysts in children with positive family history are likely signs of ADPKD and require follow-up imaging 1. If ultrasonography is normal in at-risk children, rescreening intervals should not be shorter than 3 years 1.

Contraindications to Contrast

When both iodinated CT contrast and gadolinium-based MRI contrast are contraindicated, CEUS with microbubble agents is the preferred alternative for characterizing indeterminate masses 1. Unenhanced MRI can characterize simple cysts and some hemorrhagic/proteinaceous cysts based on T1 and T2 signal characteristics 1.