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Bosniak classification system of renal cystic masses

Dr Henry Knipe and Dr Frank Gaillard et al.

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 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
    • increased number of septa, minimally thickened with nodular or thick calcifications
    • but no measurable contrast  enhancement enhancing 
    • 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 could do at around 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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