Bosniak classification of cystic renal masses version 2019

Non-enhancing hairline-think wall; water attenuation, no septa, no calcification, no solid components

Bosniak I

Few thin septa with perceived (not measurable) enhancement

Bosniak II

Few think septa without perceived enhancement and fine cacification in the septa

Bosniak II:

Few think septa without perceived enhancement perceived (not measurable) enhancement, and a short segment of slightly thickened calcification in the wall

Bosniak II

Few thin septa with perceived enhancement perceived (not measurable) enhancement, and a short segment of slightly thickened calcification in the septa

Bosniak II

Homogeneously high-attenuating masses less than or equal to 3 cm that are sharply marginated and do NOT enhance

Bosniak II

Minimally thickened septa with perceived (not measurable) enhancement, with thick calcification

Bosniak IIF

Minimally thickened septa with perceived (not measurable) enhancement, with nodular calcification

bosniak IIF

More than a few septa, without enhancement, with thick calcification

Bosniak IIF

More than a few septa, without enhancement, with nodular calcifications

Bosniak IIF

Intrarenal non-enhancing hyperattenuating renal masses >3cm

Bosniak IIF

Thickened walls with measurable enhancement

Bosniak III

Irregular walls with measurable enhancement

Bosniak III

Thickened septa with measurable enhancement

Bosniak III

Bosniak IV

Soft tissue components (i.e. nodule or nodules) with measurable enhancement

Hairline-thin wall; water attenuation, no septa, calcification or solid component; nonenhancing

Bosniak I

Bosniak II (Two types)

Two types:

1. Few thin septa, with or without perceived (not measurable) enhancement; fine calcification or a short segment of slightly thickened calcification in the wall or septa.

2. Homogeneously high attenuating masses less than or equal to 3 cm that are sharply marginated and do not enhance.

Bosniak IIF (2 types)

Two types:

1. Minimally thickened or more than a few thin septa with or without perceived (not measurable) enhancement that may have thick or nodular calcification

2. Intrarenal nonenhancing hyperattenuating renal masses >3cm

Thickened or irregular walls or septa with measurable enhancement

Bosniak III

Soft-tissue components (i.e. nodule, or nodules) with measurable enhancement

Bosniak IV

What does reclassifying a bosniak II to III mean?

Strongly associated with malignancy

CT: Bosniak I

Well defined, thin (equal to or <2mm) smooth wall; homogenous simple fluid (-9 to 20 HU); no septa or calcification; the wall MAY enhance

MR: Bosniak I

Well-defined, thin (equal to or <2mm) smooth wall; homogenous simple fluid (signal intensity similar to CSF), no septa or calcifications, the wall MAY enhance

CT: Bosniak II (6 types)

6 types, all well-defined with thin (equal to or <2mm) smooth walls:

1. Cystic masses with thin (equal to or <2mm) and few (1-3) septa; septa and wall MAY enhance; may have calcification of any type

2. Homogenous hyperattenuatuing (equal to or >70HU) masses at noncontrast CT

3. Homogenous nonenhancing masses >20HU at renal mass protocol CT; MAY have calcification of any type

4. Homogenous masses -9 to 20HU at noncontrast CT.

5. Homogenous masses 21to 30 HU at portal venous phase CT

6. Homogenous low-attenuation masses that are too small to characterise

MRI: Bosniak II (3 types)

3 types: All well defined with thin (equal to or <2 mm) smooth walls:

1. Cystic masses with thin (equal to or <2 mm) and few (1-3) enhancing septa; any nonenhancing septa; may have calcification of any type

2. Homogenous masses markedly hyperintense at T2-weighted imaging (similar to CSF) at noncontrast MRI

3. Homogenous masses markedly hyperintense at T1-weighted imaging (approximately x2,5 normal parenchymal signal intensity) at noncontrast MRI

CT: Bosniak IIF

Cystic masses with a smooth minimally thickened (3mm) enhancing wall or smooth minimal thickening (3mm) of one or more enhancing septa, or many (equal to or >4) smooth thin (equal to or <2 mm) enhancing septa

MR: Bosniak II (2 types)

Two types:

1. Cystic masses with a smooth minimally thickened (3mm) enhancing wall or smooth minimal thickening (3 mm) of one or more enhancing septa, or many (equal to or >4) smooth thin (equal to or <2 mm) enhancing septa

2. Cystic masses that are heterogenously hyperintense at unenhanced fat-saturated T1 weighted imaging

CT: Bosniak III

1 or more enhancing thich (equal to or 4mm with) or enhancing irregular (displaying equal to or <3mm obtusely margined convex protrusion(s)) walls or septa

MR: Bosniak III

1 or more enhancing thick (equal to or >4 mm width) pr enhancing irregular (displaying equal to or <3mm obtusely margined convex protrusion(s)) walls or septa

What cystic lesions need to be excluded to use bosniak classification

The Bosniak classfication is intended for cystic renal masses after:

1. Infectious

2. Inflammatory

3. Vascular etiologies

4. Necrotic solid masses

Are excluded

If a cystic mass has features described in more than one Bosniak class, what do we do?

The highest Bosniak class as assigned

CT: Bosniak VI

1 or more enhancing nodule(s) (equal to or 4 mm convex protrusion with obtuse margins; or a convex protrusion of any size that has acute margines)

MR: bosniak VI

1 or more enhancing nodule(s) (equal to or 4 mm convex protrusion with obtuse margins; or a convex protrusion of any size that has acute margines)

A mass that has unusual combination of features (undefined, not fitting any class)

May warrant inclusion into Bosniak IIF

Ultrasound and Bosniak classification

Other than for diagnosis of Bosniak I simple cysts, the of US with or without contrast material in assigning Bosniak class is uncertain.

What renal masses on CT are best visualised on MRI prior to the assignment of a Bosniak class? And why?

Renal masses that at CT have abundant thick or nodular calcifications; are hyperattenuating, homogenous, nonenhancing, AND larger than 3 cm.

(OR)

Are heterogenous (incl. but not limited to many (4 or more) nonenhancing septa or 3 mm or larger nonenhancing septa or wall)

Might be best visualised on MRI prior to the assignment of a Bosniak class to determine if there are occult enhancing elements that might affect classification.