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.


Bosniak 1
  • simple cyst
    • imperceptible wall, rounded
    • work-up: nil
    • percentage malignant: ~0%
Bosniak 2
  • minimally complex
    • a few thin <1 mm septa or thin calcifications (thickness not measurable); non-enhancing high-attenuation (due to proteinaceous or haemorrhagic contents) renal lesions of less than 3 cm are also included in this category; these lesions are generally well marginated
    • work-up: nil
    • percentage malignant: ~0%
Bosniak 2F
  • minimally complex
    • increased number of septa, minimally thickened with nodular or thick calcifications
    • there may be perceived (but not measurable) enhancement of a hairline-thin smooth septa
    • hyperdense cyst >3 cm diameter, mostly intrarenal (less than 25% of wall visible); no enhancement
    • requiring follow-up: needs ultrasound/CT follow up - no strict rules on the time frame but reasonable at 6 months
    • percentage malignant: ~ 5% 6
Bosniak 3
  • indeterminate
    • thick, nodular multiple septa or wall with measurable enhancement, hyperdense on CT (see 2F)
    • treatment/work-up: partial nephrectomy or radiofrequency ablation in elderly or poor surgical candidates
    • percentage malignant: ~55% 6
Bosniak 4
  • clearly malignant
    • solid mass with a large cystic or a necrotic component
    • treatment: partial or total nephrectomy
    • percentage malignant: ~100%

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