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Bosniak v2019 Classifier

Assign a Bosniak class to a cystic renal mass by walking the v2019 features, most worrisome first (Silverman et al., Radiology 2019). The first feature present sets the class.

The class is set by the most complex feature; enhancement is the key discriminator.

Applies to a predominantly cystic mass — no more than ~25% solid. The 2019 update notes no single established threshold (the literature cites 75–90% cystic). First exclude infectious, inflammatory, or vascular causes and necrotic solid masses; if features span more than one class, the highest class is assigned.

Modality

Thickness: thin ≤2 mm, minimally thickened 3 mm, thick ≥4 mm. Protrusions: ≤3 mm with obtuse margins = irregular (III); ≥4 mm obtuse, or any size with acute margins = nodule (IV). Calcification of any amount does not change the class.

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Bosniak

Bosniak v2019 class definitions

Class I

Well-defined, thin (≤2 mm) smooth wall; homogeneous simple fluid (−9 to 20 HU on CT / CSF-like on MRI); no septa or calcification. Wall may enhance.

Class II (benign)

Thin (≤2 mm) smooth wall with: 1–3 thin enhancing septa (any calcification); or homogeneous hyperattenuating ≥70 HU (noncontrast CT); or homogeneous nonenhancing >20 HU (renal mass protocol CT); or homogeneous −9 to 20 HU (noncontrast CT); or homogeneous 21–30 HU (portal venous phase); or homogeneous low-attenuation too small to characterize. MRI: 1–3 enhancing septa; or markedly T2-hyperintense (≈CSF); or markedly T1-hyperintense (≈2.5× parenchyma).

Class IIF

Smooth minimally thickened (3 mm) enhancing wall, or 3 mm thickening of ≥1 enhancing septum, or many (≥4) smooth thin (≤2 mm) enhancing septa. MRI also: heterogeneously hyperintense on unenhanced fat-saturated T1.

Class III

One or more enhancing thick (≥4 mm) or enhancing irregular (≤3 mm obtusely-margined convex protrusion[s]) walls or septa. ~50% malignant.

Class IV

One or more enhancing nodule(s): ≥4 mm convex protrusion with obtuse margins, or a convex protrusion of any size with acute margins.

Management

I / II → benign, no follow-up. IIF → surveillance (CT or MRI without and with IV contrast at 6 and 12 months, then yearly for 5 years; morphologic change → refer). III / IV → refer for management. (Management per ACR Incidental Renal Mass white paper, Fig 3.)

Silverman SG, et al. Bosniak Classification of Cystic Renal Masses, Version 2019. Radiology 2019;292:475–488.