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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 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, thin calcifications; non-enhancing high-attenuation (due to to proteinaceous or haemorrhagic fluid) 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 but requiring follow up
  • increased number of septa, minimally thickened or enhancing septa or wall
  • thick calcifications
  • hyperdense cyst, i.e.:
    • >3 cm diameter, mostly intrarenal (less than 25% of wall visible); no enhancement
  • work-up: needs ultrasound/CT follow up - no strict rules on time frame but could do at around 6 months.
  • percentage malignant: ~ 5% 6
Bosniak 3
  • indeterminate, thick or multiple septations, septal nodularity, hyperdense on CT (see 2F)
  • treatment/work-up: partial nephrectomy or radiofrequency ablation in elderly or poor surgical candidates
  • percentage malignant: ~54% 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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