Table 1 Bosniak classification of cystic renal masses, Version 2019 (5)

I

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

II

Three types, all well-defined with thin (≤ 2 mm) smooth walls:

  Cystic masses with thin (≤ 2 mm) and few (1–3) enhancing septa; any non-enhancing septa; may have calcification of any type

  Homogeneous masses markedly hyperintense at T2-weighted imaging (similar to CSF) at non-contrast MRI

  Homogeneous masses markedly hyperintense at T1-weighted imaging (approximately 2.5 > normal parenchymal signal intensity) at non-contrast MRI

IIf

Two types:

  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

  Cystic masses that are heterogeneously hyperintense at unenhanced fat-saturated T1-weighted imaging

III

One or more enhancing thick (≥ 4 mm width) or enhancing irregular (displaying ≤ 3-mm) obtusely marginated convex protrusion(s) walls or septa

IV

One or more enhancing nodule (≥ 4-mm) convex protrusion with obtuse margins or a convex protrusion of any size that has acute margins