Bosniak classification of renal cysts

Bosniak classification of renal cystic masses has five cathegories. It was devised by Dr. Morton A. Bosniak to separate lesions that require surgery (Bosniak III and IV) from those that can be safely followed-up or left alone.

Category Description
Bosniak I Benign simple cyst with thin wall without septa, calcifications, or solid components. It does not enhance with contrast, and has a density equal to that of water.
Bosniak II Benign cyst with a few thin septa, which may contain fine calcifications or a small segment of mildly thickened calcification. This includes homogenous, high-attenuation lesions less than 3 cm with sharp margins but without enhancement.
Bosniak IIF Well marginated cysts with a number of thin septa, with or without mild enhancement or thickening of septa. Calcifications may be present; these may be thick and nodular. There are no enhancing soft tissue components. This also includes nonenhancing high-attenuation lesions that are completely contained within the kidney and are 3 cm or larger.
Bosniak III Indeterminate cystic masses with thickened irregular septa with enhancement
Bosniak IV Malignant cystic masses with all the characteristics of category Bosniak III but also with enhancing soft tissue components independent of but adjacent to the septa.


1. Bosniak MA. The current radiological approach to renal cysts. Radiology. 1986 Jan;158(1):1–10.
2. Aronson S, Frazier HA, Baluch JD, Hartman DS, Christenson PJ. Cystic renal masses: usefulness of the Bosniak classification. Urol Radiol. 1991;13(2):83–90.
3. Curry NS, Cochran ST, Bissada NK. Cystic renal masses: accurate Bosniak classification requires adequate renal CT. AJR Am J Roentgenol. 2000 Aug;175(2):339–42