URO60 • FRCS REVISION • RENAL IMAGING
Bosniak classification v2019
An imaging classification, not a numerical calculator. Learn the morphology, the evidence for IIF surveillance, and the pitfalls in quoting malignancy rates.
Visual progression: I → IV
Schematic only: thin septa are drawn pale; enhancing thick/irregular structures amber; enhancing mural nodule red. Actual v2019 classification also includes defined CT/MRI-specific features.
What Silverman et al. changed in 2019
The Radiology 2019 proposal formally incorporated MRI, specified measurable imaging features and clarified categories. Thin means ≤2 mm, minimally thickened 3 mm, thick ≥4 mm, and “many” septa means ≥4. IIF includes smooth minimally thickened enhancing walls/septa or many thin enhancing septa; III includes thick or irregular enhancing walls/septa; IV includes enhancing nodules. MRI has additional qualifying appearances.
| Class | Examinable principle |
|---|---|
| I / II | Benign imaging patterns; generally no surveillance. |
| IIF | Probably benign but insufficiently unequivocal to discharge; imaging follow-up. |
| III | Indeterminate enhancing thick/irregular wall or septa. |
| IV | Enhancing nodule; higher probability of malignancy. |
Silverman SG et al. Bosniak Classification of Cystic Renal Masses, Version 2019. Radiology. 2019.
Why follow Bosniak IIF?
The F stands for follow-up. Surveillance aims to detect a change in morphological class, not merely an increase in size. An enhancing nodule, irregularity or increased wall/septal thickness may warrant reassessment. Importantly, progression to III/IV does not prove a previously benign cyst became malignant: a cancer may have been present but not initially recognisable.
Radiological upgrading: a 2023 AJR study of 246 Bosniak v2019 IIF lesions found 4% upgraded during follow-up. Selected surgical malignancy rates: Silverman et al. described the profound selection bias in resected IIF lesions; a malignancy proportion in operated cases is not the risk of transformation in all surveilled IIF lesions. The true prevalence is uncertain.
Suggested follow-up: the 2023 Canadian Urological Association guideline suggests imaging at 6–12 months in the first year, then annually if stable, for a total of five years. This is conditional guidance, not a universal proof that all lesions need exactly five years.
Examiner trap: distinguish (1) prevalence of malignancy, (2) radiological class progression, (3) clinically aggressive disease and (4) true malignant transformation.
Short viva
Question: “A 62-year-old has a 3 cm Bosniak IIF lesion. Why not remove it, and what would make you change management?”
Model answer: “IIF is probably benign but has sufficient imaging complexity to merit surveillance. I would confirm a technically adequate v2019 CT/MRI classification, discuss uncertainty and competing risks, and follow morphology according to a local evidence-based protocol. Upgrade to a higher category, especially an enhancing nodule, would trigger MDT reassessment. I would not quote a malignancy percentage from surgically selected cysts as the chance of malignant transformation.”
References
Silverman et al., Radiology 2019 · Bosniak v2019 IIF surveillance cohort, AJR 2023 · CUA cystic renal lesions guideline update 2023 · EAU RCC guidelines 2026.