Renal cyst

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: M. Khurram Afzal, MD [2]


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.

Numerous renal cysts are seen in the cystic kidney diseases, which include polycystic kidney disease and medullary sponge kidney.

Epidemiology and Demographics

Up to 27% of individuals greater than 50 years of age may have simple renal cysts that cause no symptoms.[1]


Bosniak classification

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. [2]

The Bosniak classification categorizes renal cysts into five groups.[3]

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.


Drug Side Effect

Differentiating renal cyst from other disease

Simple renal cyst must be differentiated from other diseases presenting as renal cysts such as:

For more information on differentiating simple renal cyst, click here.


  • An infected cyst may cause fever and pain.
  • A burst cyst causes severe pain in back or side.
  • A kidney cyst that obstructs the normal flow of urine may lead to swelling of the kidney (hydronephrosis).


  1. Tada S, Yamagishi J, Kobayashi H, Hata Y, Kobari T (1983). "The incidence of simple renal cyst by computed tomography". Clinical Radiology. 34 (4): 437–9. PMID 6872451. Unknown parameter |month= ignored (help)
  3. Curry NS, Cochran ST, Bissada NK (2000). "Cystic renal masses: accurate Bosniak classification requires adequate renal CT". American Journal of Roentgenology. 175 (2): 339–42. PMID 10915671. Unknown parameter |month= ignored (help)

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