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Genitourinary
Demonstration data

Bosniak Classifier (2019)

Assigns a Bosniak class to a renal cyst from wall/septa, enhancement and septa features, with malignancy risk and management.

The Bosniak classification stratifies cystic renal masses by their likelihood of malignancy using wall and septal thickness, the number of septa, and the presence of measurable or nodular enhancement. The 2019 update sharpened the definitions of each class.

🧮 Interactive Calculator

📐 Formula

Feature mapping → Bosniak I / II / IIF / III / IV

When to Use

  • Characterising a cystic renal mass on contrast-enhanced CT or MRI.
  • Deciding between reassurance, surveillance and surgical referral.
  • Standardised communication of cyst complexity to urology.

How It Works

  • Assess the thickness and regularity of the wall and any septa.
  • Count the number of septa and grade any enhancement as none, minimal (thin septa) or measurable/nodular.
  • Map the worst feature to the corresponding Bosniak class.
  • Nodular or measurable enhancement upgrades the cyst regardless of other features.

Interpreting the Result

  • Bosniak I: simple cyst, ~0% malignancy — no follow-up.
  • Bosniak II: minimally complex (few hairline septa), <1% malignancy — no follow-up.
  • Bosniak IIF: minimally thickened smooth wall/septa or many hairline septa, ~5% malignancy — imaging surveillance.
  • Bosniak III: thick or irregular wall/septa with measurable enhancement, ~50% malignancy — surgical/urological referral.
  • Bosniak IV: enhancing soft-tissue nodule, ~90% malignancy — treat as malignant.

Limitations & Pitfalls

  • Requires a dedicated multiphase contrast study; non-contrast imaging cannot classify enhancement.
  • Inter-reader variability persists for IIF vs III boundaries.
  • This tool maps discrete inputs and cannot capture every nuance of the full 2019 criteria.

References

  • Silverman SG, Pedrosa I, Ellis JH, et al. Bosniak Classification of Cystic Renal Masses, Version 2019. Radiology. 2019;292(2):475-488.
  • Bosniak MA. The Bosniak renal cyst classification: 25 years later. Radiology. 2012;262(3):781-785.

⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.