Renal Cysts: Cause for Concern?

A cyst is a thin-walled, cell-lined collection of fluid.  They can occur in nearly every tissue type and many places in the body including the brain, lung, nerve tissue and skin. Renal cysts are incredibly common, affecting up to 50% of 50-year olds in the United States and increasing in prevalence with age.[1]  Renal cysts represent upwards of 70% of the asymptomatic, benign renal masses discovered incidentally.[2]  Risk factors for cyst formation include:

• increasing age
• male gender
• hypertension
• chronic kidney disease (renal insufficiency) [3]

The main concern of patients and physicians is that renal cysts are cancerous or will develop into a cancer.  In an excellent study of the natural history of renal cysts, with increasing time cysts are demonstrated to grow slowly (average 1.9mm/year), increase in number and only rarely develop into cancers.[4]  Predicting which cysts are potentially malignant is a tough task.

The Bosniak Classification System

Bosniak I

In 1986, Dr. Bosniak, radiologist at New York University in New York City, defined the characteristics of simple and complex renal cysts to reduce unnecessary surgery on benign lesions.  Thus the Bosniak System was created, categorizing each renal cyst into four (and then five in 1993) categories that predict malignant behavior.[5] The single-most important imaging characteristic that predicts malignancy is enhancement (brightening) on contrast-based CT imaging.

Bosniak II: hyperdense cyst (right)

Bosniak I

Risk of Malignancy: 0-1.7%

Non-enhancing
Benign simple cyst
Water density
Hairline thin cyst wall
No septa, calcifications, or solid components

Bosniak II

Bosniak IIF

Risk of Malignancy: 0-18.5%

Non-enhancing

Benign cyst
May have a few thin septa
May have fine calcification in wall or septa
May be hyperdense (uniformly high attenutation lesions) if <3cm
Perceived enhancement may be present

Bosniak IIF

Bosniak III

Bosniak IV

Risk of Malignancy: 5-18.5%

Non-enhancing

May have a few thin septa
May have minimal wall thickening in wall or septa
May have thin or nodular calcifications
Perceived enhancement may be present
May be hyperdense (uniformly high attenutation lesions) if ≥3cm
Require follow-up studies to prove benignity

Bosniak III

Risk of Malignancy: 33-82%

Indeterminate cystic mass
Thickened and/or irregular walls/septa
Measurable enhancement
Surgical lesion

Bosniak IV

Risk of Malignancy: 92-93%

Clearly malignant cystic masses

Enhancing soft tissue components

Surgical lesion

Management of Renal Cysts

Most Bosniak I and II cysts do not require follow-up or treatment due to the low rate of documented progression to malignancy.  There is an association between renal cysts, chronic kidney disease and renal cell carcinoma; therefore it is reasonable to follow patients with Bosniak I and II lesions with serial ultrasounds and labs for renal function (creatinine, urinalysis) on an annual or biannual basis.  

Bosniak IIF cysts, however, should have repeat imaging on a regular basis to ensure they do not progress to malignant disease.  Progression of a cyst is defined by an increase in the Bosniak Classification Score.  In studies of Bosniak IIF cysts, 13-15% progress to Bosniak III lesions while under observation and 50-100% of the resected cysts were renal cancers.[6-8]

Bosniak III and IV cysts warrant consideration of surgical treatment.  Treatments may include partial nephrectomy (removal of the cyst only), radical nephrectomy (removal of the entire kidney) or ablative therapies.  While it is generally believed that most complex renal cysts are low-grade renal cell carcinomas, there is a significant proportion that are high-grade tumors.  

Summary

• Renal cysts are the most common benign lesion in the kidney.  
• The risk of malignancy increases with increasing complexity of the cyst.
• Bosniak I, II and IIF cysts have low-risk of malignancy and should be observed.
• Bosniak III and IV cysts have a higher risk of malignancy and therefore warrant surgical treatment.

  

This blog entry was written by Debasish Sundi, MD, Senior Assistant Resident at the Brady Urological Institute at Johns Hopkins University.  Dr. Sundi is pursuing a career in urological oncology.








[1] Torres VE, Grantham JJ. Cystic diseases of the kidney. In: Brenner BM, ed. Brenner & Rector’s The Kidney. Vol. 2. 8th ed. Philadelphia: Saunders Elsevier; 2008: 1451–1453.
[2] Terada N, Ichioka K: The natural history of simple renal cysts. J Urol 2002; 167: 21-2
[3] Terada N, Arai Y: Risk factors for renal cysts. BJU Int 2004; 93: 1300-1302
[4] Terada N, Arai Y: The 10-year natural history of simple renal cysts. Urology 2008; 71: 7-11discussion 11–12
[5] Israel GM, Bosniak MA: An update of the Bosniak renal cyst classification system. Urology 2005; 66: 484-488
[6] O'Malley RL, Godoy G, Hecht EM, Stifelman MD, Taneja SS.Bosniak category IIF designation and surgery for complex renal cysts.  J Urol. 2009 Sep;182(3):1091-5. doi: 0.1016/j.juro.2009.05.046. Epub 2009 Jul 18.
[7] Smith AD, Remer EM, Cox KL, Lieber ML, Allen BC, Shah SN, Herts BR.Bosniak category IIF and III cystic renal lesions: outcomes and associations.  Radiology. 2012 Jan;262(1):152-60. doi: 10.1148/radiol.11110888. Epub 2011 Nov 21.
[8] El-Mokadem I, Budak M, Pillai S, Lang S, Doull R, Goodman C, Nabi G.  Progression, interobserver agreement, and malignancy rate in complex renal cysts ( ≥ Bosniak category IIF).  Urol Oncol. 2014 Jan;32(1):24.e21-7. doi: 10.1016/j.urolonc.2012.08.018. Epub 2013 Feb 6.