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Bosniak renal cyst classification

The Bosniak classification system for CT evaluation of renal cysts is helpful in determining both malignant risk and required follow-up and/or treatment.

Bosniak 1

  • simple cyst, imperceptible wall, rounded
  • work up : nil
  • % malignant : ~ 0%

Bosniak 2

  • minimally complex, a few thin (< 1mm) septa, thin Ca++; 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
  • % malignant : ~ 0%

Bosniak 2F

  • minimally complex but requiring follow up. 
  • increased number of septa, minimally thickened or enhancing septa or wall  
  • thick Ca++, 
  • hyperdense cyst that is:
    • > 3 cm diameter, mostly intrarenal (less than 25% of wall visible); no enhancement
  • work up : needs ultrasound / CT follow up
  • % malignant : ~ 25 %6

Bosniak 3

  • indeterminate, thick or multiple septations, mural nodule, hyperdense on CT (see 2F)
  • treatment / work up : partial nephrectomy or RF ablation in elderly / poor surgical risk
  • % malignant : ~ 54%6

Bosniak 4

  • clearly malignant, solid mass with large cystic or necrotic component 
  • treatment: partial / total nephrectomy
  • % malignant : ~100%

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