Bosniak classification of cystic renal masses (2019)
The 2019 revision of the Bosniak classification, class by class — the defining imaging features, the reported malignancy risk, and what each class means for management.
Summary
| Class | Malignancy risk | Management |
|---|---|---|
| Bosniak I | ~0% (benign) | No follow-up required. |
| Bosniak II | ~0% (benign) | No follow-up required. |
| Bosniak IIF | ~6% | Follow-up imaging. CT or MRI at 6 months, 12 months, then annually for a total of 5 years if stable. |
| Bosniak III | ~55% | Urology referral. Surgical or ablative management, or surveillance in selected patients. |
| Bosniak IV | ~90% | Treat as renal cell carcinoma. Urology referral. |
Class by class
Bosniak I
Bosniak I — malignancy risk ~0% (benign)
No follow-up required.
Defining features:
- Well-defined, thin (≤2 mm) smooth wall
- Homogeneous simple fluid (−9 to 20 HU on CT)
- No septa or calcification
- Wall may enhance
Bosniak II
Bosniak II — malignancy risk ~0% (benign)
No follow-up required.
Defining features:
- Few (1-3) thin (≤2 mm) septa, may enhance
- Calcification of any type
- Homogeneous hyperattenuating (≥70 HU) mass at noncontrast CT
- Homogeneous nonenhancing mass >20 HU at renal mass protocol CT
Bosniak IIF
Bosniak IIF — malignancy risk ~6%
Follow-up imaging. CT or MRI at 6 months, 12 months, then annually for a total of 5 years if stable.
Defining features:
- Many (≥4) thin (≤2 mm) enhancing septa
- Smooth minimally thickened (3 mm) enhancing wall or septa
- MRI only: cystic mass heterogeneously hyperintense at unenhanced fat-saturated T1-weighted imaging
Bosniak III
Bosniak III — malignancy risk ~55%
Urology referral. Surgical or ablative management, or surveillance in selected patients.
Defining features:
- One or more thick (≥4 mm) enhancing wall or septa
- One or more enhancing irregular walls or septa (≤3 mm obtuse-margined convex protrusion[s])
Bosniak IV
Bosniak IV — malignancy risk ~90%
Treat as renal cell carcinoma. Urology referral.
Defining features:
- One or more enhancing nodule — ≥4 mm with obtuse margins, or any size with acute margins
Using the 2019 revision
- The 2019 revision applies only to cystic renal masses imaged with CT or MRI at renal mass protocol. It is not validated for ultrasound or for masses that are predominantly solid.
- Enhancement may be measured (≥20 HU at CT; ≥15% signal increase at MRI) or perceived qualitatively; thin walls and septa are usually only perceived. Pseudoenhancement is a known pitfall in small intrarenal cysts on CT.
- Version matters: 2019 reclassified several categories relative to the 2005 version and narrowed III considerably. State the version in the report.
Related
Source. Silverman SG, et al. Radiology 2019;292(2):475-488. View on PubMed
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