How version 2019 differs from the original
The 2019 update exists because the original classification was written for CT in 1986 and was being applied to MRI, to ultrasound, and to findings it was never defined against. The revision gives explicit, measurable definitions in place of descriptive ones.
The substantive changes: septa and wall thickness are given numbers rather than adjectives — thin means 2 mm or less, minimally thickened means 3 mm, thick means 4 mm or more. Calcification is largely removed as a discriminator. Explicit MRI criteria are defined. And the definitions were deliberately tightened to move more masses into the benign classes, because the original over-called class III.
Classification requires contrast-enhanced CT or MRI. Enhancement is the axis the whole system turns on, so a non-contrast study cannot produce a valid Bosniak class — not even a provisional one.
The classes
Classify by the single most advanced feature present. Work down the list and take the last one that applies — a mass with three thin septa and one enhancing nodule is class IV, not class II.
| Class | Defining feature | Malignancy risk | Management |
|---|---|---|---|
| I | Simple fluid, thin smooth wall, no septa or enhancement | ~0% | Benign. No follow-up |
| II | One to three thin (≤ 2 mm) enhancing septa, or fine calcification, or homogeneously hyperattenuating | ~0% | Benign. No follow-up |
| IIF | Four or more thin enhancing septa, or a smooth minimally thickened (3 mm) enhancing wall or septa | ~5% | CT or MRI at 6 and 12 months, then annually to 5 years |
| III | Enhancing thick (≥ 4 mm) or irregular wall or septa | ~50% | Urological referral — resection, ablation or surveillance |
| IV | Enhancing nodule (convex protrusion) | ~90% | Urological referral for definitive management |
What the classification does not do
Size does not change the class. A 2 cm and a 10 cm simple cyst are both class I. Size belongs in the report because it is the baseline against which growth is judged, but it is not an input to the classification and it does not upgrade anything.
The class is a statement about the imaging appearance and its associated risk, not a diagnosis. Class III in particular describes a group where roughly half the masses are malignant — it is explicitly a decision point requiring judgement, not a direction.
The system does not apply to a mass that is predominantly solid, to an infected or inflammatory cystic lesion, or to a patient with von Hippel-Lindau or another hereditary renal cancer syndrome, where surveillance follows the syndrome rather than the lesion.
Frequently asked questions
What is the Bosniak classification?
A system for categorising cystic renal masses on contrast-enhanced CT or MRI by their risk of malignancy, running from class I (simple, benign) to class IV (malignant in the large majority of cases). Version 2019 is the current revision.
What changed in Bosniak version 2019?
Wall and septal thickness were given explicit measurements — thin is 2 mm or less, minimally thickened is 3 mm, thick is 4 mm or more — in place of descriptive terms. Calcification was largely removed as a discriminator, explicit MRI criteria were added, and the definitions were tightened so that fewer masses fall into class III.
Can a Bosniak class be assigned without contrast?
No. The classification depends on whether wall, septa or nodules enhance, which cannot be assessed on a non-contrast study. A cystic renal mass seen without contrast needs a dedicated contrast-enhanced CT or MRI before it can be classified.
Does the size of the cyst affect its Bosniak class?
No. Size is not an input to the classification and does not upgrade a mass. It should still be recorded, because it is the baseline against which any future growth is measured.
What follow-up does a Bosniak IIF lesion need?
Surveillance imaging with CT or MRI at 6 months and 12 months, then annually to complete 5 years. The reported risk of malignancy for class IIF is around 5%.
What is the malignancy risk for each Bosniak class?
Approximately 0% for class I and class II, around 5% for class IIF, around 50% for class III and around 90% for class IV. These are pooled approximations and vary between published series.
How do I classify a mass with several different features?
By the single most advanced feature present. Work down the list of features in order and take the last one that applies — the presence of an enhancing nodule makes a mass class IV regardless of how many thin septa it also has.
Does this calculator send anything anywhere?
No. It runs entirely in your browser, makes no network requests, stores nothing and has no analytics. It works offline and from a USB stick on a reporting workstation.
Source
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.
This tool is an independent implementation and is not affiliated with or endorsed by the American College of Radiology or any other body named here. It is intended for qualified healthcare professionals, and it supports a reporting decision rather than making one. The published guideline remains normative.