Bosniak classification system of renal cystic masses

Last revised by Dr Daniel J Bell on 20 Apr 2022

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 (2022) have widespread validation.

Although practised by some, the use of ultrasonography to characterize the Bosniak classification remains controversial. Originally, it was felt that ultrasound was not adequate for the task as it was incapable of showing neovascularization (cf. contrast-enhanced CT/MRI), however newer studies looking at contrast-enhanced ultrasound, suggest that this impediment is no longer true. There is also evidence that ultrasound has a higher sensitivity for intralesional septa than either CT or MRI 8,13.

The "official" Bosniak classification uses Roman numerals, and not Arabic numerals, for each category. The use of the term "grade", "stage", "group", "type" or similar for each category is technically incorrect. Version 2019 has switched from "category" to "class" 11.

  • 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

The Bosniak classification is named after Morton A Bosniak (1929-2016), who was professor emeritus in radiology at New York University (NYU) Langone School of Medicine. It was first published in 1986, with the introduction of the 2F category in 1993, and revisions in 1997, 2005 and 2019 9,10,14-16.

Cases and figures

  • Figure 1
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  • Case 1: Bosniak category 1
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  • Case 2: Bosniak category 1
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  • Case 3: Bosniak category 2 - hemorrhagic cyst
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  • Case 4: Bosniak category 2
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  • Case 5: Bosniak category 2
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  • Case 6: Bosniak category 2
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  • Case 7: Bosniak category 3 - with enhancing mural nodule
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  • Case 8: Bosniak category 4
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  • Case 9: Bosniak category 4
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  • Case 10: Bosniak category 4
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  • Case 11: Bosniak category 4 with APKD
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  • Case 12: Bosniak category 4
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  • Case 13: Bosniak category 2 (CEUS)
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  • Case 14: Bosniak category 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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  • Case 19: Bosniak category 4
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