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Bosniak classification of kidney cyst (renal cysts)

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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The Bosniak classification of renal cystic disease

Category I:

  • malignant risk less than 1%; no follow-up required
    • uncomplicated, simple benign cyst
    • anechoic, posterior enhancement ( through transmission), round or oval shape, thin, smooth wall
    • homogeneous water content, sharp delineation with the renal parenchyma, no calcification, enhancement or wall-thickening

Category II:

  • malignant risk less than 3%; no follow-up required
  • cystic lesion with some abnormal radiological features
    • <1 mm septations (hairline thin)
    • fine calcifications within the septum or wall
    • <3 cm in diameter
    • hyperdense cysts (>20 Hounsfield units)

Category IIF:

  • malignant risk 5-10%; follow-up recommended
  • cystic lesion with increased abnormal findings
    • multiple thin septum
    • septa thicker than hairline or slightly thick wall
    • calcification, which may be thick
    • intrarenal, >3 cm
    • no contrast enhancement

Category III:

  • malignant risk 40-60%; surgical excision recommended
    • More complicated
      • uniform wall thickening/nodularity
      • thick/irregular calcification
      • thick septa
      • enhances with contrast

Category IV:

  • malignant risk greater than 80%; surgical excision recommended  
    • large cystic components
    • irregular margins/prominent nodules
    • solid enhancing elements, independent of septa


  • Israel GM, Bosniak MA. Follow up CT scan for moderately complex cystic renal masses (Bosniak category II F). Am J Roentgenol 2003;181:627-33
  • Whelan TF. Guidelines on the management of renal cyst disease.Can Urol Assoc J 2010;4(2):98-9

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