Bosniak Classification System


Category 1

A benign simple cyst with a hairline-thin wall that does not contain septae, calcifications, or solid components. It measures water density, and does not enhance with contrast material. Bosniak 1 Bosniak 1 +C


Category 2

A benign cyst that may contain a few hairline-thin septae. Fine calcification or a short segment of slightly thickened calcification may be present in the wall or septae. Bosniak 2 Bosniak 2 +C
Uniformly high-attenuation lesions (< 3 cm) that are sharply marginated and do not enhance are included in this group. Bosniak 2 (hyperdense) Bosniak 2 (hyperdense) +C


Category 2F

These well marginated cysts may contain an increased number of hairline-thin septae, with possible minimal enhancement or thickening of the septae or wall.  The cyst may contain calcification that may be thick and nodular, but there are no enhancing soft-tissue components. Totally intrarenal nonenhancing high-attenuation renal lesions that are 3 cm or larger are also included in this category.
Bosniak 2F Bosniak 2F +C


Category 3

These lesions are indeterminate cystic masses that have thickened, irregular walls or septae in which enhancement can be seen. Bosniak 3 Bosniak 3 +C


Category 4

These lesions are clearly malignant cystic masses that not only have all the characteristics of category 3 lesions, but also contain enhancing soft-tissue components adjacent to but independent of the wall or septae. Bosniak 4 Bosniak 4 +C


For further information on the diagnosis and management of the incidental renal mass, please refer to the excellent Radiology review article on the subject by Stuart Silverman, et al.

You may also find Bosniak's original article or his comment on the 2F categorization useful as well:


All images from Silverman, S. G. et al. Radiology 2008;249:16-31.