Bosniak Classification v2019 (Cystic Renal Mass, CT/MRI)

The 2019 Bosniak revision applies only to cystic renal masses (less than about 25% enhancing tissue) evaluated with adequate technique. The highest applicable feature governs: an enhancing nodule (a convex protrusion of 4 mm or more with obtuse margins, or any convex protrusion with acute margins) is class IV; enhancing thick (4 mm or more) or irregular walls or septa are class III; a smooth minimally thickened (3 mm) enhancing wall or septa, many (four or more) thin enhancing septa, or - on MRI - a heterogeneously T1-hyperintense nonenhancing mass is class IIF and merits surveillance. Class II includes several benign, thin-walled types: cysts with a few thin septa (any calcification allowed), homogeneous masses of 70 HU or more at unenhanced CT, homogeneous nonenhancing masses above 20 HU at renal mass protocol CT, homogeneous masses of -9 to 20 HU at unenhanced CT or 21-30 HU at portal venous phase CT, homogeneous low-attenuation masses too small to characterize, and - on MRI - homogeneous markedly T2- or T1-hyperintense masses at noncontrast imaging. Class I is the simple cyst: thin smooth wall, simple fluid, no septa and no calcification. Predominantly solid masses, fat-containing masses, heavily calcified masses, and incompletely characterized lesions fall outside the system and need dedicated renal mass protocol imaging.