Bosniak classification system of renal cystic masses

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

Bosniak 4

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

History and etymology

It is named after Morton A. Bosniak (1929-2016), who was professor emeritus in radiology at New York University (NYU) Langone School of Medicine ^9,10.