Bosniak 2019

Bosniak v2019CTMRIKidney

Applies to cystic masses (<25% enhancing tissue) with dedicated renal mass protocol. Wall/septum: thin ≤2 mm, minimally thickened 3 mm, thick ≥4 mm. Nodule: ≥4 mm convex protrusion with obtuse margins, or any convex protrusion with acute margins.

Class	CT criteria	Management
I	Thin (≤2 mm) smooth wall; homogeneous simple fluid (−9 to 20 HU); no septa or calcification; wall may enhance	Benign
II	Six types: (1) thin smooth wall with 1–3 thin septa, may enhance, any calcification; (2) homogeneous hyperattenuating ≥70 HU unenhanced; (3) homogeneous nonenhancing >20 HU on renal protocol, may have calcification; (4) homogeneous −9 to 20 HU unenhanced; (5) homogeneous 21–30 HU portal venous; (6) homogeneous low attenuation too small to characterize	Benign
IIF	Smooth minimally thickened (3 mm) enhancing wall, or smooth minimal thickening of one or more enhancing septa, or many (≥4) smooth thin enhancing septa	Follow-up
III	One or more enhancing thick (≥4 mm) or enhancing irregular walls or septa (≤3 mm obtuse convex protrusion)	Surgery or surveillance
IV	One or more enhancing nodules (≥4 mm obtuse protrusion, or any size with acute margins)	Surgery or ablation

Pearls

Enhancement = ≥20 HU increase on CT, or unequivocal subtraction enhancement on MRI.
Calcification no longer drives classification in v2019.
IIF follow-up: imaging at 6 months, 12 months, then annually for 5 years. Malignancy rates fall to ~10% for IIF and ~50% for III in v2019 series.
Heterogeneous hyperattenuating masses or any mass not meeting cystic criteria need solid-mass workup.

Sources

Silverman SG, Pedrosa I, Ellis JH, et al. Bosniak classification of cystic renal masses, version 2019: an update proposal and needs assessment. Radiology. 2019;292:475–488.PubMed ↗

Radessential summarizes published classification systems for practicing radiologists. Summaries are condensed; confirm details against the cited source before relying on them for patient care.