Bosniak Renal Cyst Classification
Bosniak / ACR · v2019 · Updated v2019
Malignancy risk stratification of cystic renal masses using wall thickness, septations, calcifications, and enhancement.
IIF: annual imaging up to 5 y. III/IV: urology referral for possible surgery.
Cystic renal masses on contrast-enhanced CT or MRI · Adults with an incidentally detected renal cyst
Predominantly solid renal masses (< 25% fluid) — use renal mass pathway · Non-contrast studies (indeterminate — recommend dedicated imaging)
CT / MRI / US
Proposed 2019 update — expert consensus, growing validation
Class I–II cysts are benign, no follow-up. IIF are followed. III and IV need urology referral — malignancy risk climbs sharply from ~50% (III) to ~90% (IV).
Overview
The 2019 Bosniak update tightened definitions of wall thickness, septation number/thickness and 'enhancement' (increase of ≥15 HU on CT or clear enhancement on subtracted MRI). It expanded IIF to cover many previously III lesions and formalized MRI criteria.
Quick reference
| Class | Management |
|---|---|
| I — simple cyst | No follow-up |
| II — few thin septa, no enhancement | No follow-up |
| IIF — minimally complex | Imaging at 6 mo, 12 mo, then annually up to 5 y |
| III — thick / enhancing walls | Urology referral — surgery vs. active surveillance |
| IV — enhancing soft-tissue nodule | Urology referral — surgery |
Decision algorithm
Classify cystic renal masses by wall, septa, calcifications, and enhancement.
Structured report
Interactive template that produces a validated dictation ready for the report.
References
- Silverman SG et al. Radiology 2019
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.