Bosniak classification to assess malignancy risk of renal cyst

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

Reference:

• 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