Bosniak v2019
Classify a cystic renal mass with the Bosniak version 2019 CT criteria. Tick the features present and the calculator returns the class, the reported malignancy risk and the management that follows.
Features on CT
Class
IIF
Malignancy
~5%
Bosniak IIF · 34 mm
Bosniak IIF. The reported probability of malignancy is on the order of ~5%. Imaging follow-up, commonly at 6 and 12 months and then annually up to 5 years.
IIF is the category v2019 changed most: the criteria were tightened to pull back masses that used to be over-called as III. Surveillance is the management here, not surgery.
Remember the underlying condition: class IIF or higher requires enhancement. A thickened or irregular wall that does not enhance does not lift the mass.
Enhancement must be demonstrated rather than suspected: compare the same ROI on the unenhanced and enhanced series of a renal mass protocol. An increase within the noise of pseudo-enhancement is not septal enhancement.
This calculator follows the v2019 CT criteria. The MRI criteria differ in places, partly because high T1 signal and the assessment of septa behave differently there — so do not apply the CT criteria unchanged to an MRI.
How to read the result
The class answers a management question rather than a diagnostic one. Classes I and II end the conversation, IIF starts a surveillance clock, and III and IV send the patient to a urologist.
How the class is assigned
Class = the highest class for which any single feature is present
There is no scoring and no averaging. One enhancing nodule makes a mass class IV even if every other feature looks benign.
Enhancement must be demonstrated by comparing the same ROI on unenhanced and enhanced series of a renal mass protocol.
Wall and septal thickness are measured at the thickest point, and protrusions are judged on their margins as well as their size.
When to use it
- Reporting a cystic renal mass found on a renal mass protocol CT.
- Separating a mass that needs surveillance from one that needs a urologist, where the boundary sits between IIF and III.
- Putting a defensible class in the conclusion rather than a descriptive paragraph nobody can act on.
- Re-reading an older study classified under the 2005 criteria, where v2019 often gives a lower class.
⚠️ Pitfalls
- The classification applies to cystic masses, not to solid masses with a small cystic component. Forcing a predominantly solid mass into Bosniak understates it.
- Pseudo-enhancement in small intrarenal cysts mimics septal enhancement. Compare identical ROIs and be sceptical of small differences.
- These are the CT criteria. The v2019 MRI criteria differ, and applying the CT rules to an MRI changes classes in both directions.
- Version 2019 deliberately lowered many masses relative to the 2005 criteria. When comparing with an older report, state which version you used.
- A haemorrhagic or proteinaceous cyst can be hyperattenuating and still be class II. Attenuation alone never raises the class without an enhancing feature.
- Class IIF and above require enhancement. Version 2019 also asks that classes I and II be called cysts, and that the term cystic renal mass be reserved for IIF, III and IV.
Frequently asked questions
How is the Bosniak class determined?
By taking the highest class for which any single feature is present. It is not a score: one enhancing nodule places the mass in class IV regardless of its other features.
What is the difference between Bosniak IIF and III?
IIF covers smooth, minimally thickened enhancing walls or septa and many thin enhancing septa. III requires an enhancing thick wall or septum of 4 mm or more, or an enhancing irregular wall or septum. IIF is followed; III goes to a urologist.
What changed in version 2019?
The definitions were made explicit and measurable, with thresholds in millimetres and criteria for enhancement. The practical effect is that many masses previously called IIF or III now sit a class lower.
What is the malignancy risk per class?
Reported figures are under 1% for classes I and II, around 5% for IIF, around 50% for III and around 90% for IV. These vary between series and should be read as orders of magnitude.
Can I use these criteria on MRI?
Not unchanged. Version 2019 contains separate MRI criteria, and features such as high T1 signal behave differently there. Use the criteria that match the modality you reported.
References
📄Silverman SG, Pedrosa I, Ellis JH, et al. Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment. Radiology. 2019;292(2):475–488.
📄Bosniak MA. The current radiological approach to renal cysts. Radiology. 1986;158(1):1–10.
This calculator supports clinical judgement and does not replace it. Always check the result against your own measurements and the guideline in force.
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