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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 (<1 mm) 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, 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.
  • % 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 or poor surgical candidates
  • % malignant: ~54%6

Bosniak 4:

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

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