|Classification and external resources|
A renal cyst is a fluid collection in the kidney. There are several types based on the Bosniak classification. The majority are benign, simple cysts that can be monitored and not intervened upon. However, some are cancerous or are suspicious for cancer and are commonly removed in a surgical procedure called nephrectomy.
Up to 27% of individuals greater than 50 years of age may have simple renal cysts that cause no symptoms.
Renal cysts are classified as either malignant or benign using the Bosniak Classification System. The system was created by Dr. Morton Bosniak, a faculty member at the NYU Langone Medical Center in New York City. 
The Bosniak classification categorizes renal cysts into five groups.
- Category 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.
- Category 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(Rich Kosak) but without enhancement.
- Category 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.
- Category III
- Indeterminate cystic masses with thickened irregular septa with enhancement.
- Category IV
- Malignant cystic masses with all the characteristics of category III lesions but also with enhancing soft tissue components independent of but adjacent to the septa.
- Tada S, Yamagishi J, Kobayashi H, Hata Y, Kobari T (July 1983). "The incidence of simple renal cyst by computed tomography". Clinical Radiology 34 (4): 437–9. PMID 6872451.
- Curry NS, Cochran ST, Bissada NK (August 2000). "Cystic renal masses: accurate Bosniak classification requires adequate renal CT". American Journal of Roentgenology 175 (2): 339–42. doi:10.2214/ajr.175.2.1750339. PMID 10915671.