Bosniak classification system of renal cystic masses (version 2005)

Last revised by Tariq Walizai on 21 Aug 2024

The Bosniak classification system of renal cystic masses, now known as Bosniak v2005, divides renal cystic masses into five categories based on imaging characteristics on contrast-enhanced CT, and helps predict a risk of malignancy and suggests either follow-up or treatment. An updated classification, Bosniak v2019, has been published, and is gradually replacing v2005 in mainstream usage.

Radiologists and urologists widely use the Bosniak classification for assessing renal cysts 3 and was last updated in 2005 12. An updated classification, Bosniak v2019, has been published, including separate CT and MRI-based classification 11.

The Bosniak classification version referred to in international guidelines is variable, with the 2022 European and Canadian guidelines referencing v2019 25,26 and the 2021 American guidelines referencing v2005 27,28.

Although practised by some, using ultrasonography to characterise the Bosniak classification remains controversial. Originally, it was thought ultrasound was inadequate for the task as it could not show neovascularisation (cf. contrast-enhanced CT/MRI) ref. However, newer studies looking at contrast-enhanced ultrasound suggest this impediment is no longer true ref. 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, not Arabic ones, 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% 17

  • benign cyst - "minimally complex"

    • few hairlines thin <1 mm septa or thin calcifications (thickness not measurable)

    • perceived enhancement

    • non-enhancing high-attenuation (due to proteinaceous or haemorrhagic contents) renal lesions <3 cm

    • generally well marginated

    • work-up: none

    • percentage malignant: ~0-6% 17,18

  • 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: ~5-26% 6,19-21

  •  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 23,24

    • percentage malignant: ~55-72% 6,17,19,22

  • 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: ~91-100% 19,22

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, introducing the 2F category in 1993, and revisions in 1997, 2005 and 2019 9,10,14-16.

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