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.
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.
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- 1. Israel GM, Bosniak MA. How I do it: evaluating renal masses. Radiology. 2005;236 (2): 441-50. doi:10.1148/radiol.2362040218 - Pubmed citation
- 2. Curry NS, Cochran ST, Bissada NK. Cystic renal masses: accurate Bosniak classification requires adequate renal CT. AJR Am J Roentgenol. 2000;175 (2): 339-42. AJR Am J Roentgenol (full text) - Pubmed citation
- 3. Warren KS, Mcfarlane J. The Bosniak classification of renal cystic masses. BJU Int. 2005;95 (7): 939-42. doi:10.1111/j.1464-410X.2005.05442.x - Pubmed citation
- 4. Park BK, Kim B, Kim SH et-al. Assessment of cystic renal masses based on Bosniak classification: comparison of CT and contrast-enhanced US. Eur J Radiol. 2007;61 (2): 310-4. doi:10.1016/j.ejrad.2006.10.004 - Pubmed citation
- 5. Israel GM, Bosniak MA. Follow-up CT of moderately complex cystic lesions of the kidney (Bosniak category IIF). AJR Am J Roentgenol. 2003;181 (3): 627-33. AJR Am J Roentgenol (full text) - Pubmed citation
- 6. Smith AD, Remer EM, Cox KL et-al. Bosniak category IIF and III cystic renal lesions: outcomes and associations. Radiology. 2012;262 (1): 152-60. doi:10.1148/radiol.11110888 - Pubmed citation
- 7. Escudier B, Eisen T, Porta C et-al. Renal cell carcinoma: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up. Ann. Oncol. 2012;23 Suppl 7 (suppl 7): vii65-71. doi:10.1093/annonc/mds227 - Pubmed citation
- 8. Muglia VF, Westphalen AC. Bosniak classification for complex renal cysts: history and critical analysis. Radiologia brasileira. 47 (6): 368-73. doi:10.1590/0100-3984.2013.1797 - Pubmed
- 9. Bosniak MA. The Bosniak renal cyst classification: 25 years later. Radiology. 262 (3): 781-5. doi:10.1148/radiol.11111595 - Pubmed
- 10. Megibow AJ, Israel GM. Morton A. Bosniak, MD. Radiology. 282 (2): 616. doi:10.1148/radiol.2016164037 - Pubmed
- 11. Silverman SG, Pedrosa I, Ellis JH, Hindman NM, Schieda N, Smith AD, Remer EM, Shinagare AB, Curci NE, Raman SS, Wells SA, Kaffenberger SD, Wang ZJ, Chandarana H, Davenport MS. Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment. (2019) Radiology. 292 (2): 475-488. doi:10.1148/radiol.2019182646 - Pubmed
- 12. Israel GM, Bosniak MA. An update of the Bosniak renal cyst classification system. (2005) Urology. 66 (3): 484-8. doi:10.1016/j.urology.2005.04.003 - Pubmed
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