Bosniak classification system of renal cystic masses

Dr Henry Knipe and Assoc 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.

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.

  • 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
  • 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
  • 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
  •  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
  • 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

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.

Article information

rID: 1006
System: Urogenital
Synonyms or Alternate Spellings:
  • Current Bosniak classification (CBC)
  • Bosniak classification system of renal cysts
  • Bosniak renal cyst classification system
  • Bosniak renal cyst classification
  • Bosniak classification system of renal cystic lesions

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Cases and figures

  • Figure 1
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  • Case 1: Bosniak grade 1
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  • Case 2: Bosniak grade 1
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  • Case 3: Bosniak grade 2 - hemorrhagic cyst
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  • Case 4: Bosniak grade 2
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  • Case 5: Bosniak grade 2
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  • Case 6: Bosniak grade 2
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  • Case 7: Bosniak grade 3 - with enhancing mural nodule
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  • Case 8: Bosniak grade 4
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  • Case 9: Bosniak grade 4
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  • Case 10: Bosniak grade 4
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  • Case 11: Bosniak type 4 cyst on a background of APKD
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  • Case 12: Bosniak grade 4
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  • Case 13: Bosniak grade 2 (CEUS)
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  • Case 14: Bosniak grade IIF (CEUS)
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  • Case 15: Bosniak category 4
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  • Case 16: Bosniak category 3
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  • Case 17: Bosniak category 3
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  • Case 18: Bosniak category 3
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