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.

Usage

The Bosniak classification is widely used by radiologists and urologists for addressing the clinical problem assessing renal cysts ³. It was last updated in 2005 ¹².  A Bosniak classification, version 2019 ¹¹ has been proposed to increase the accuracy and include MRI features but does not yet (c. 2021) have widespread validation.

Classification

Bosniak I

• benign simple cyst
  • hairline-thin wall of ≤2 mm
  • water density
  • no septa, calcifications, or solid components
  • no enhancement
  • work-up: none
  • percentage malignant: ~0% ^ref

Bosniak II

• benign cyst - "minimally complex"
  • few hairline thin <1 mm septa or thin calcifications (thickness not measurable)
  • perceived enhancement
  • non-enhancing high-attenuation (due to proteinaceous or hemorrhagic contents) renal lesions <3 cm
  • generally well marginated
  • work-up: none
  • percentage malignant: ~0% ^ref

Bosniak IIF

• minimally complex
  • multiple hairline thin septa or minimally smooth thickened walls or septa
  • perceived but no measurable enhancement of wall or septa
  • calcification can be present and may be thick and nodular
  • generally well marginated
  • high-attenuation lesion >3 cm diameter, totally intrarenal (<25% of wall visible); no enhancement
  • requiring follow-up (F for follow-up): needs ultrasound/CT/MRI follow up - no strict rules on the time frame but reasonable at 6 months, 12 months then annually for 5 years ³
  • percentage malignant: ~5% ⁶

Bosniak III

•  indeterminate cystic mass
  • thickened irregular or smooth walls or septa with measurable enhancement
  • treatment/work-up: partial nephrectomy or radiofrequency ablation in poor surgical candidates ^ref
  • percentage malignant: ~55% ⁶

Bosniak IV

• clearly malignant cystic mass
  • Bosniak III criteria + enhancing soft tissue components adjacent to but independent of wall or septum
  • treatment: partial or total nephrectomy
  • percentage malignant: ~100% ^ref

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.