Bosniak classification system of renal cystic masses

Dr Yuranga Weerakkody ◉ and A.Prof 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.

Classification

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 proteinaceous or haemorrhagic contents) 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
- there may be perceived (but not measurable) enhancement of a hairline-thin smooth septa
- 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 reasonable at 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