Bosniak classification of cystic renal masses (version 2019)
The Bosniak classification of cystic renal masses (version 2019), or usually simply Bosniak classification, version 2019, is a proposed update of the classic/current Bosniak criteria. The update hopes to improve on the classification's predictive power for malignancy and minimize the number of benign lesions undergoing treatment.
On this page:
Usage
An important note is that the Bosniak classification (version 2019) is a proposed update that has had some small validation studies with mixed results 2-4 and its use in clinical practice is not widespread (c. 2021).
Classification
The new version has separate CT and MRI criteria:
CT
Class I
- well-defined thin (≤2 mm) smooth wall
- homogeneous simple fluid (-9 to 20 HU)
- no septa or calcification
- the wall may enhance after administration of contrast
Class II
- six types all with thin (≤2 mm) smooth walls
- cystic masses with thin (≤2 mm) and few (1-3) septa
- septa and wall may enhance
- may have calcifications of any type (although if the calcification is thick and nodular consider MRI before assigning a class)
- homogeneous hyperattenuating (≥70 HU) masses on noncontrast CT
- homogeneous nonenhancing masses >20 HU at renal mass protocol CT
- may have calcifications of any type (although if the calcification is thick and nodular consider MRI before assigning a class)
- 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
- cystic masses with thin (≤2 mm) and few (1-3) septa
Class IIF
- smooth minimally thickened (3 mm) enhancing wall
- smooth minimal thickening (3 mm) of one or more enhancing septa
- many (≥4) smooth thin (≤2 mm) enhancing septa
Class III
- one or more walls or septa that are
- enhancing thick (≥4 mm width)
- enhancing irregular (displaying ≤3 mm obtusely margined convex protrusion[s])
Class IV
- one or more enhancing nodule(s)
- ≥4 mm convex protrusion with obtuse margins
- a convex protrusion of any size that has acute margins
MRI
Class I
- well-defined thin (≤2 mm) smooth wall
- homogeneous simple fluid (signal intensity similar to CSF)
- no septa or calcification
- the wall may enhance after administration of contrast
Class II
- three types all with thin (≤2 mm) smooth walls
- cystic masses with
- thin (≤2 mm) and few (1-3) enhancing septa
- any nonenhancing septa
- calcification of any type
- homogeneous masses markedly hyperintense at T2W (similar to CSF) at noncon MRI
- homogeneous masses markedly hyperintense at T1W (approximately 2.5x normal parenchymal signal intensity) at noncontrast MRI
- cystic masses with
Class IIF
- two types:
- cystic masses with
- smooth minimally thickened (3 mm) enhancing wall
- smooth minimal thickening (3 mm) of one or more enhancing septa
- many (≥4) smooth thin (≤2 mm) enhancing septa
- cystic masses that are heterogeneously hyperintense at unenhanced fat-saturated T1W imaging
- cystic masses with
Class III
- one or more walls or septa that are
- enhancing thick (≥4 mm width)
- enhancing irregular (displaying ≤3 mm obtusely margined convex protrusion[s])
Class IV
- one or more enhancing nodule(s)
- ≥4 mm convex protrusion with obtuse margins
- a convex protrusion of any size that has acute margins
Follow up
Class I
- "Benign simple renal cyst requiring no follow up"
Class II
- "Benign Bosniak II renal cyst requiring no follow up"
- "Likely benign Bosniak II renal mass requiring no follow up"
Class IIF
- "Bosniak IIF cystic renal mass. The large majority of Bosniak IIF masses are benign. When malignant, nearly all are indolent. Generally, Bosniak IIF masses are followed at imaging at 6 months and 12 months, then annually for a total of 5 years to assess for morphologic change"
Class III
- "Bosniak III cystic renal mass. Bosniak III masses have an intermediate probability of being malignant. If not already obtained, consider urology consultation"
Class IV
- "Bosniak IV cystic renal mass. The large majority of Bosniak IV masses are malignant. If not already obtained, consider urology consultation"
Practical points
- the increased specificity about thickness of walls and septa, and number of septa, is intended to increase inter-reader agreement
- the differentiation between "irregular thickening" (Bosniak III) and "nodule" (Bosniak IV) is intended to reduce the percentage of Bosniak IV masses that are benign
- this classification scheme is intended for a general patient and not for those with a renal cell carcinoma syndrome