Bosniak classification system of renal cystic masses

The Bosniak classification system of renal cystic masses divides renal cystic masses into five categories based on imaging characteristics on contrast-enhanced CT. It is helpful in predicting a risk of malignancy and suggesting either follow up or treatment.

There has been a recent proposal (2019) in Radiology  for updating the current classification which does not yet have validation. This update incorporates MRI in addition to CT, establishes definitions for previously vague imaging terms, and enables a greater proportion of masses to enter lower risk classes.


Bosniak I
  • simple cyst
    • imperceptible wall, rounded
    • work-up: none
    • percentage malignant: ~0%
Bosniak II
  • minimally complex
    • a few thin <1 mm septa or thin calcifications (thickness not measurable); 
    • non-enhancing high-attenuation (due to proteinaceous or hemorrhagic contents) renal lesions <3 cm
    • these lesions are generally well marginated
    • work-up: none
    • percentage malignant: ~0%
Bosniak IIF
  • minimally complex
    • increased number of septa, minimally thickened with nodular or thick calcifications
    • there may be perceived (but not measurable) enhancement of hairline-thin smooth septa
    • minimal thickening of the wall with perceivable enhancement
    • hyperdense cyst >3 cm diameter, mostly intrarenal (less than 25% of wall visible); no enhancement
    • requiring follow-up (the 'F' in 2F is for 'follow'): needs ultrasound/CT/MRI follow up - no strict rules on the time frame but reasonable at 6 months
    • percentage malignant: ~5% 
Bosniak III
  • indeterminate
    • thick, nodular multiple septa or wall with measurable enhancement
    • treatment/work-up: partial nephrectomy or radiofrequency ablation in elderly or poor surgical candidates
    • percentage malignant: ~55% 
Bosniak IV
  • clearly malignant
    • solid mass with a large cystic or a necrotic component
    • treatment: partial or total nephrectomy
    • percentage malignant: ~100%

History and etymology

It is named after Morton A Bosniak (1929-2016), who was professor emeritus in radiology at New York University (NYU) Langone School of Medicine .

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