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.

There has been a recent proposal (2019) in Radiology ¹¹ for updating the current classification which does not yet have validation. This update incorporates MRI in addition to CT, establishes definitions for previously vague imaging terms, and enables a greater proportion of masses to enter lower risk classes. 

Classification

Bosniak I

• simple cyst
  • imperceptible wall, rounded
  • work-up: none
  • percentage malignant: ~0%

Bosniak II

• minimally complex
  • a few thin <1 mm septa or thin calcifications (thickness not measurable); 
  • non-enhancing high-attenuation (due to proteinaceous or hemorrhagic contents) renal lesions <3 cm
  • these lesions are generally well marginated
  • work-up: none
  • percentage malignant: ~0%

Bosniak IIF

• minimally complex
  • increased number of septa, minimally thickened with nodular or thick calcifications
  • there may be perceived (but not measurable) enhancement of hairline-thin smooth septa
  • minimal thickening of the wall with perceivable enhancement
  • hyperdense cyst >3 cm diameter, mostly intrarenal (less than 25% of wall visible); no enhancement
  • requiring follow-up (the 'F' in 2F is for 'follow'): needs ultrasound/CT/MRI follow up - no strict rules on the time frame but reasonable at 6 months
  • percentage malignant: ~5% ⁶

Bosniak III

• indeterminate
  • thick, nodular multiple septa or wall with measurable enhancement
  • treatment/work-up: partial nephrectomy or radiofrequency ablation in elderly or poor surgical candidates
  • percentage malignant: ~55% ⁶

Bosniak IV

• 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.