Cystic Renal Masses — Bosniak Classification (v2019)
| Bosniak | CT/MRI Features | Malignancy Risk | Recommendation |
|---|---|---|---|
| I | Simple cyst: imperceptible wall, water density, no calcification, no enhancement | <2% | No follow-up |
| II | Minimally complicated: 1–2 thin septa, fine calcification, <3 cm high-density cyst, no enhancement | <2% | No follow-up |
| IIF | Multiple thin septa, minimal smooth wall/septal thickening, no enhancing soft tissue; or ≥3 cm high-density cyst | ~6–15% | CT or MRI at 6 months, then annually × 5 years |
| III | Indeterminate: thick or irregular septa/wall with measurable enhancement | ~50% | Urology referral; excision vs. surveillance per clinical context |
| IV | Clearly malignant: enhancing soft-tissue component, nodular enhancement, irregular walls | >90% | Urology referral; surgical resection standard |
Bosniak v2019 updates the original 1986 system with more explicit CT and MRI criteria. MRI is preferred over CT for Bosniak IIF and III lesions — it is more sensitive for septal enhancement and better characterizes internal complexity.
Solid / Enhancing Renal Masses
| Mass Size and Type | Recommendation |
|---|---|
| Solid, enhancing, <1 cm | CT or MRI at 12 months; if stable × 2 → discontinue (most are benign) |
| Solid, enhancing, 1–3 cm | MRI with and without contrast to characterize; urology referral; consider biopsy |
| Solid, enhancing, >3 cm | Urology referral; surgical planning |
| Too small to characterize on CT (<1.5 cm) | MRI; if still indeterminate → CT or MRI in 12 months |
| Any mass, known malignancy or immunosuppression | MRI for characterization; urology/oncology referral at lower threshold |
Angiomyolipoma (AML)
AML is identified by macroscopic fat (−30 to −100 HU on CT, or T1 signal dropout on out-of-phase MRI). Management is size-driven due to spontaneous hemorrhage risk:
| AML Size | Recommendation |
|---|---|
| <4 cm, classic fat-containing appearance | No follow-up imaging needed |
| ≥4 cm or symptomatic | Urology referral (hemorrhage risk; embolization or resection may be indicated) |
Fat-poor AML can be difficult to distinguish from RCC on CT — MRI with chemical shift imaging is helpful; biopsy may be required.
Why This Matters
Simple renal cysts are found in over 50% of adults over 50 and are universally benign — recognizing a Bosniak I cyst correctly prevents unnecessary workup. The Bosniak IIF category exists specifically to capture the small subset of minimally complicated cysts that warrant surveillance (roughly 5% will upstage). For solid masses, small size (<1 cm) does not automatically mean benign — the decision for active surveillance vs. biopsy vs. resection involves urology and depends on patient comorbidity and mass characteristics.
References
Herts BR, Silverman SG, Hindman NM, et al. Management of the Incidental Renal Mass on CT: A White Paper of the ACR Incidental Findings Committee. J Am Coll Radiol. 2018;15(2):264–273.
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.