Renal cyst

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Renal cyst
Classification and external resources
ICD-10 N28
ICD-9-CM 593.2
eMedicine med/453831

A renal cyst or kidney cyst, is a fluid collection in or on 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.


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

Bosniak classification[edit]

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 New York University 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 but without enhancement. Hyperdense cysts must be exophytic with at least 75 percent of its wall outside the kidney to allow for appropriate assessment of margins, otherwise they are categorized as IIF.[4]

Category IIF

Up to 5 percent of these cysts are malignant and as such they require follow-up imaging, though there is no consensus recommendation on the appropriate interval of follow up. 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 centimetres (1.2 inches) or larger.

Category III

Indeterminate cystic masses with thickened irregular septa with enhancement. 50 percent of these lesions are ultimately found to be malignant.

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. 100 percent of these lesions are malignant.

Peripelvic versus parapelvic cysts[edit]

Parapelvic cysts originate from around the kidney at the adjacent renal parenchyma, and plunge into the renal sinus. Peripelvic cysts are contained entirely within the renal sinus, possibly related to dilated lymphatic channels. When viewed on CT in absence of contrast, they can mimic hydronephrosis.[5] If symptomatic, they can be laparoscopically decorticated - removal of the outer layer or cortex.[6]


  1. ^ 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. 
  2. ^
  3. ^ 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. 
  4. ^ De Miranda, C. M.; Maranhão, C. P.; Dos Santos, C. J.; Padilha, I. G.; De Farias Lde, P; Da Rocha, M. S. (2014). "Bosniak classification of renal cystic lesions according to multidetector computed tomography findings". Radiologia Brasileira. 47 (2): 115–21. doi:10.1590/S0100-39842014000200015. PMC 4337166Freely accessible. PMID 25741060. 
  5. ^ Zinn, H. L.; Becker, J. A. (1997). "Peripelvic cysts simulating hydronephrosis". Abdominal Imaging. 22 (3): 346–7. PMID 9107666. 
  6. ^ Shiraishi, K; Eguchi, S; Mohri, J; Kamiryo, Y (2006). "Laparoscopic decortication of symptomatic simple renal cysts: 10-year experience from one institution". BJU International. 98 (2): 405–8. doi:10.1111/j.1464-410X.2006.06249.x. PMID 16879687. 

External links[edit]