Bosniak Classification
Classification of cystic renal masses by their likelihood of malignancy using CT or MRI. The Version 2019 update adds explicit, measurable definitions for walls, septa, and enhancing tissue, and is defined for both modalities. Assign the class based on the single most concerning feature.
2019 (v2019)·reviewed 2026-07-01
- The classic (2005 CT) Bosniak classification remains widely used; the 2019 update refines and standardizes measurable feature definitions and extends the system to MRI. Feature thresholds (≤2 mm thin, 3 mm minimally thickened, ≥4 mm thick) are specific to the 2019 version.
- Assign the class from the single most concerning feature.
- An enhancing convex protrusion ≤3 mm with obtuse margins is 'irregular' (a class III feature), not a nodule; a nodule (class IV) is a protrusion ≥4 mm with obtuse margins, or of any size with acute margins.
- Malignancy-risk percentages are commonly-cited estimates; the v2019 proposal did not publish validated per-class rates, and subsequent pathology series report higher malignancy rates for classes IIF–III.
- Bosniak applies only to cystic masses; a predominantly solid enhancing mass is not classified with Bosniak.
- Enhancement must be unequivocal; pseudoenhancement can occur with small cysts on CT.
Bosniak class
Class I~0% (benign simple cyst)
No follow-up; benign simple cyst.
Sources
- Silverman SG, et al. Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment. Radiology. 2019;292(2):475-488.source
Educational reference for clinicians — not medical advice. Verify against the primary source before acting on it; guidelines change. Final decisions rest with the interpreting physician and your institution's protocols.