Fact-sheet: Atypical renal cyst
Updated on 11/30/2021 at 4:33 PM
View all RADEOS cases associated with this fact-sheet
Classification
2019 Bosniak classification:
Bosniak calculator: https://bosniak-calculator.herokuapp.com/
Figures from https://doi.org/10.1148/radiol.2019182646
Illustrations:
Determining the number of septa using the 2019 Bosniak classification of cystic renal masses. Example of a Bosniak II cyst (top) and a Bosniak IIF cystic mass (bottom) classified based on the number of thin septa (≤2 mm). A septum is defined as a linear or curvilinear structure connecting two surfaces. Each differently colored line indicates a single septum (two at top, five at bottom).
A smooth, thin wall (≤2 mm) is a feature of a Bosniak I cyst, a smooth, thin wall and septa (≤2 mm) are features of a Bosniak II cyst, a smooth, minimally thickened wall or septum (3 mm) is a feature of a Bosniak IIF mass, and a thickened (≥4 mm), enhancing wall or septum is a feature of a Bosniak III mass. The wall in the bottom image is 1 mm thick.
Examples of convex protrusions in 30-mm Bosniak III and Bosniak IV cystic masses (measurements are to scale). Prominent convex protrusions arising from a wall or septum are either nodules (of any size if they have acute margins with the wall or septa, or ≥4 mm if they have obtuse margins with the wall or septa [a Bosniak IV feature]) or irregular thickening (≤3 mm if they have obtuse margins with the wall or septa, a Bosniak III feature). Size measurements are obtained perpendicular to the originating wall or septum. If the convex protrusion(s) are present on both sides of a wall or septum, the cumulative perpendicular distance is used and excludes the thickness of the underlying wall or septum. Orange features have acute margins and blue features have obtuse margins. The Bosniak III features are examples of focal irregular thickening. The Bosniak IV features are examples of nodules.
2005 Bosniak classification (Obsolete, superseded by the 2019 Bosniak classification)
Class 1: benign cyst: no follow-up
- Fluid-filled formation
- Thin, regular wall
- No enhancement after injection
Class 2: Atypical cyst: no follow-up
- Thin septum
- Fine calcification
- Hypodense, homogeneous formation, without enhancement after injection < 3 cm
Class 2F: follow-up
- Very slight wall thickening
- Thick calcification
- Hyperdense, homogeneous formation without enhancement ≥ 3 cm
Class 3: indeterminate cystic lesion: surgery or follow-up
- Thick or irregular, enhancing septum
- Thick or irregular, enhancing wall
Class 4: necrotic or cystic tumor: surgery
- Enhancement outside the septa and walls