Bosniak 2019: What Changed, and Why Your Report Should Say Which Version

The Bosniak classification has been in use since 1986, which means a substantial fraction of the cystic renal masses you report have a prior study classified under a different version of the same-sounding system. The 2019 revision changed enough that "Bosniak III" in a 2015 report and "Bosniak III" in a 2026 report are not reliably the same claim.

If your report does not name the version, nobody downstream can tell which one you meant.

What actually changed

Adjectives became millimetres

The original scheme described septa as "hairline-thin" and walls as "minimally thickened". Readers calibrated those words differently, and inter-reader agreement suffered accordingly.

The 2019 version assigns numbers:

• Thin means ≤ 2 mm.
• Minimally thickened means 3 mm.
• Thick means ≥ 4 mm.

An enhancing septum you would once have called "slightly thickened" now lands in a specific class depending on whether it measures 3 mm or 4 mm. That is a real threshold with a real consequence, and it rewards actually measuring rather than eyeballing.

"Many septa" got a number

Class II versus IIF used to turn partly on whether there were "a few" or "many" septa. The 2019 version sets the boundary at four. One to three thin septa is class II; four or more thin septa is IIF.

MRI is a first-class citizen

The original classification was a CT scheme that people applied to MRI by analogy. The 2019 revision defines criteria for both CT and MRI explicitly, including MRI-specific features. This matters because MRI detects septa and enhancement that CT misses, which historically pushed masses up a class purely because of modality — an artefact the revision is designed to remove.

Class III narrowed

This is the change with the most practical impact. The 2019 revision reclassified a number of masses that would previously have been class III, moving many into IIF. The intent was explicit: class III carried a roughly 50% malignancy rate under the old scheme largely because it was a catch-all, and a large share of those masses were being resected unnecessarily.

The practical consequence is that a mass called III on a prior study may be IIF today. That is not a change in the mass. Compare like with like, and say which scheme you used.

Enhancement was defined

Enhancement means a measurable increase in attenuation or signal, not a perceived one. On CT, pseudoenhancement in small intrarenal cysts remains a genuine pitfall — a simple cyst surrounded by enhancing parenchyma can appear to gain 10-20 HU without anything being there. The revision is explicit that this is not enhancement.

What the classes mean now

The IIF follow-up schedule is the part most often left out of reports: CT or MRI at 6 months, at 12 months, then annually to a total of five years if stable. "Recommend follow-up" without an interval puts the burden on someone who has less information than you do.

Where Bosniak does not apply

The 2019 revision is a CT and MRI classification for cystic renal masses, imaged at renal mass protocol.

• It is not validated for ultrasound. Ultrasound cannot assess enhancement, which is central to the system. Describing a cyst on ultrasound as "Bosniak II" is applying a system outside its evidence base — describe the features instead and recommend cross-sectional imaging if the appearance warrants it.
• It is not for predominantly solid masses. A solid enhancing renal mass is a renal cell carcinoma until proven otherwise and does not need a cyst classification.
• It assumes an adequate protocol. A single-phase portal venous CT often cannot establish enhancement.

Reporting it

Three things make a Bosniak report actionable:

1. The version. "Bosniak II (2019 version)."
2. The features that drove it, with measurements. "Two enhancing septa, each 2 mm."
3. The management that follows, with the interval spelled out.

That combination lets the next reader verify your reasoning instead of taking the class on trust — which matters more here than in most classifications, precisely because the same label has meant different things over time.

The full class-by-class breakdown

Our Bosniak 2019 reference sets out the defining features, malignancy risk and management for every class, with the version caveats that change how you report. Free, no sign-up, and no JavaScript required.

MyRadAgent applies Bosniak, LI-RADS, TI-RADS and the other ACR systems automatically as you dictate, with the version named in the recommendation — see how it works.