Bosniak renal cyst classification

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

Bosniak 1

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

Bosniak 2

• minimally complex, single thin (< 1mm) septations, thin Ca++; non-enhancing high-attenuation 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 : ~ 5 %

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 : ~ 50%

Bosniak 4

• clearly malignant, solid mass with cystic spaces surgery
• treatment: partial / total nephrectomy
• % malignant : ~100%