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Bosniak 2019 Renal Cyst Calculator [CT and MRI]

Stratify Risk of Renal Cysts on Imaging

The Bosniak classification system is widely used for the characterization of renal cysts detected on imaging studies such as computed tomography (CT) and magnetic resonance imaging (MRI). This system was originally proposed by Dr. Bosniak in 1986 and was subsequently updated in 1993 and 2005. The latest update to this system, known as the Bosniak 2019 renal cyst classification, was published by Dr. Davenport in 2019.

The Bosniak classification system has become a widely accepted and essential tool for radiologists in the evaluation of renal cystic lesions. In 2019, Dr. Matthew Davenport and his team proposed an updated version of the Bosniak classification system for renal cysts on CT and MRI imaging. The latest Bosniak 2019 Renal Cyst Classification provides a more nuanced and precise approach to characterizing renal cystic lesions and helps to guide clinical management decisions, including surveillance versus intervention. The updated classification system incorporates imaging features such as the thickness of the cyst wall, septa, and calcifications to provide a more comprehensive assessment of cystic renal lesions.

This calculator is based on the original Radiology paper on the new classification by Dr. Davenport/Silverman et al. and a Radiographics paper by Dr. Schieda et al. This is a work in progress, so feedback and suggestions are welcome.

Bosniak 2019 Renal Cyst Classification on CT and MRI

ClassImaging Features on CT/MRI
IAll well-defined with thin (<=2 mm) smooth walls:
– Homogeneous simple fluid (-9 to 20 HU) similar.
– No septa or calcifications
Wall may enhance
IIAll well-defined with thin (<=2 mm) smooth walls:
CT
– Cystic masses with thin (<=2 mm) and few (1-3) septa; septa and wall may enhance; may have calcification of any type
– Homogeneous hyperattenuating (>=70 HU) masses at non-contrast CT
– Homogeneous nonenhancing masses > 20 HU at renal mass protocol CT, may have calcification of any type
– Homogeneous masses -9 to 20 HU at noncontrast CT.
– Homogeneous masses 21 to 30 HU at portal venous phase CT.
– Homogeneous low attenuation masses that are too small to characterize.
MRI
– Homogeneous masses markedly hyperintense at T2-weighted imaging (similar to CSF) at noncontrast MRI
– Homogeneous masses markedly hyperintense at T1-weighted imaging (approx 2.5 x normal parenchymal signal intensity) at noncontrast MRI
IIFCT / MRI
– Cystic masses with a smooth minimally thickened (3 mm) enhancing wall, or smooth minimal thickening (3 mm) of one or more enhancing septa, or many (>=4) smooth thin (<=2 mm) enhancing septa.
MRI
– Cystic masses that are heterogeneously hyperintense at unenhanced fat-saturated T1-weighted imaging.
IIIOne or more enhancing thick (>=4 mm width) or enhancing irregular (displaying <=3 mm obtusely marginated convex protrusions) walls or septa
IVOne or more enhancing nodules (>=4 mm convex protrusion with obtuse margins, or a convex protrusion of any size that has acute margins)
Bosniak 2019 Renal Cyst Classification

Management Recommendations for Bosniak Cysts based on CT and MRI:

Bosniak ClassRecommendations as per 2019 GuidelinesMalignancy Rates
IBenign simple renal cyst requiring no follow-up. 0
IILikely a benign renal mass requiring no follow-up.<1%
IIFThe large majority of Bosniak IIF masses are benign. When malignant, nearly all are indolent. Generally, Bosniak IIF masses are followed by imaging at 6 months and 12 months, then annually for a total of 5 years to assess for morphologic change.0-38%
IIIBosniak III masses have an intermediate probability of being malignant. If not already obtained, consider seeking a urology consultation.~50%
IVThe largest majority of Bosniak IV masses are malignant. If not already obtained, consider seeking a urology consultation.~90%
Management of Renal Cysts based on their Bosniak Class and risk of malignancy in renal cysts [1]

Here is a quick animated tutorial for understanding the Bosniak 2019 Classification

Bosniak 2019 Tutorial

Explore more radiology calculators on our website here:

Disclaimer: The author makes no claims of the accuracy of the information contained herein; this information is for educational purposes only and is not a substitute for clinical judgment.

References:

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