Bosniak Classification

Version 2019 — cystic renal mass on CT or MRI, and its malignancy risk

Bosniak category

I — simple, benign cyst

II — minimally complex, benign

IIF — more complex, needs follow-up

III — indeterminate cystic mass

IV — clearly malignant cystic mass

Evidence

Original publication

Silverman SG, Pedrosa I, Ellis JH, et al. Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment. Revised the original CT-only classification to also apply to MRI, refined the category definitions to reduce inter-reader variability, and gave the approximate malignancy risk per category used here.

Radiology, 2019

Open source

Guideline

I and II are followed as any other benign finding; IIF is followed with periodic imaging (surveillance protocols vary by institution); III and IV are usually referred for surgery or ablation, though active surveillance can be appropriate for III in a patient whose comorbidities or life expectancy favour it, by shared decision with urology.

Last reviewed: 2026-09

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