Cystic Lesions

Bosniak IIF

• Low but non-negligible malignancy risk.
• Typical surveillance: repeat CT or MRI at 6 months twice, then yearly for a total of about 5 years.
• Upgrade if new enhancing thick septa/wall, enhancing nodule, or meaningful morphologic progression develops.

Bosniak III / IV

• Urology consultation is usually appropriate.
• Consider surgery or ablation in fit patients, especially if > 3 cm or morphologically aggressive.
• Management can be individualized in older or comorbid patients.

Solid / TSTC

Too small to characterize

• If subjectively likely a benign cyst: no further workup.
• If indeterminate: MRI without and with contrast is preferred; renal mass CT is also acceptable.
• Typical follow-up window for indeterminate TSTC lesions is 6-12 months.

Solid renal mass

• < 1 cm: follow with MRI or CT beginning at 6-12 months, then yearly for about 5 years.
• 1-4 cm: refer for management; consider biopsy depending on imaging and treatment strategy.
• > 4 cm: refer for management.

Fat / Protocol / Sources

Fat-containing lesion

• Macroscopic fat strongly suggests angiomyolipoma.
• Fat plus calcification is suspicious for RCC rather than classic angiomyolipoma.
• Larger angiomyolipomas, symptomatic lesions, or lesions with prior hemorrhage may need treatment consideration.

Renal mass protocol

• Use renal mass protocol CT or MRI when enhancement or cyst complexity cannot be assessed on the initial exam.
• Enhancement and Bosniak class drive management for cystic lesions.
• Solid enhancing tissue makes Bosniak classification inapplicable and should be managed as a solid renal mass.

Sources

• ACR renal mass white paper
• CAR incidental renal mass recommendations
• Imaging and management review
• Small renal mass active surveillance review