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Bosniak Classification

Created: 23/5/2005
Updated: 5/11/2006


Bosniak Classification


  • Approx. 50% of people >50 years have renal cysts
  • Simple cyst:
    • Fluid-filled, non-neoplastic, unilocular, smooth-walled, arise from cortex and contain straw-coloured fluid
    • Sharply marginated, round, smooth, homogenous, thin-walled, non-enhancing and have attenuation values of -10 to +20 Hounsfield Units (HU) on CT

Features of cysts suspicious of malignacy

  • Calcification- small amount in wall or septa may be benign
  • Abnormal density
    • Masses with a density >20 HU do not meet criterion for cysts
    • A change of >10 HU before and after contrast studies suggests vascularity
  • Septations- especially if irregular and thickness >1mm
  • Nodularity- solid tissue within cyst wall
  • Wall thickening
  • Small masses- due to imaging difficulties

Bosniak Classification

I Simple benign cyst with hairline thin wall which does not contain septa, calcification or solid components. Measures as water density and is non-enhancing.

II Benign cyst which may contain a few hairline septa. Fine calcification may be present in the wall or septa. Uniformly high attenuation lesions of <3cm that are sharply marginated and non-enhancing

IIF May contain more hairline septa. Minimal enhancement or minimal thickening of wall or septa can be seen. Cyst may contain calcification that might be nodular and thick but there is no enhancement. Lesions are generally well-marginated

Soft-tissue elements are non-enhancing

Includes intra-renal non-enhancing high attenuation lesions of =3cm

III Lesions are indeterminate cystic masses with thickened irregular walls or septa in which enhancement can be seen

IV Malignant cystic lesions containing soft-tissue enhancing components


  • Bosniak classification is good at classifying category I and IV lesions
  • It is not definitive and problems arise in diagnosing category II and III cysts
  • Category II and III cysts should be viewed with suspicion and followed appropriately
  • Methods to address this issue include:
    • Laparoscopy
    • CT-guided biopsy
    • PET-scanning

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