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Kidney Cysts

1. What are kidney cysts?

Cysts are fluid-filled collections that arise either on the surface of the kidney, or within the kidney tissue itself.

2. What problems can kidney cysts cause?

The vast majority of kidney cysts actually do not cause any symptoms, and may be observed safely over a period of time for any change in their constitution. The problems that such cysts may cause include: (1) progressive increase in size, causing aches and pain over the affected loin; (2) infection, which may present as loin pain, fever and chills; (3) progressive loss of kidney function; and (4) risk of cancerous change.

3. How do I know if my kidney cyst is likely to be cancerous?

Over the past 15 years, the management of kidney cysts has been based on their Bosniak classification score1 (named after radiologist Morton Bosniak from New York University), which stratifies the risk of cancerous change within the involved kidney cyst based on their radiographic features on computer tomography imaging with intravenous contrast (Figure 1):


Figure 1: Bosniak classification of kidney cysts, based on their radiographic features on CT imaging of the kidney

  • Bosniak 1 cyst – <1% risk of cancer, does not require follow-up
  • Bosniak 2 cyst – <5% risk of cancer, does not require follow-up
  • Bosniak 2F cyst – 5% risk of cancer, requires follow-up imaging to assess progress
  • Bosniak 3 cyst – 50% risk of cancer, should be surgically removed or biopsied
  • Bosniak 4 cyst – 80-95% risk of cancer, should be surgically removed
4. My doctor says that I have a Bosniak 3 or 4 kidney cyst, and I should have it removed. How is this surgery performed?

Given the 50-90% risk of cancerous involvement in Bosniak 3-4 kidney cysts, it is best to have these removed surgically. Nowadays, most urologists would recommend a kidney-sparing approach (known as partial nephrectomy) toward excision of such potentially cancerous cysts if these are less than 4cm in size. In this surgery, the affected segment of kidney is removed with a 1cm margin of normal tissue, preserving the rest of the unaffected kidney. Surgical removal of such Stage 1 kidney cancers, whilst sparing the rest of the unaffected kidney, has now been established as the current international standard of care advocated by the European Association of Urology and American Urological Association2,3. Nephron-sparing surgery (NSS) is now preferred to radical nephrectomy, where the entire kidney is removed. Several large-scale studies have found that patients who undergo nephron-sparing surgery, live longer and have a significantly lower risk of developing hypertension, ischaemic heart disease or strokes over the long-term compared to patients who had their entire kidney removed4.

NSS may be performed through three approaches: (1) traditional open surgery, which involves a 15-20cm incision in the abdomen or flank; (2) laparoscopic surgery, which is performed through small incisions in the abdomen; or (3) robotic surgery, using the da Vinci® surgical robot to remove the tumour and sew up the resulting defect in the affected kidney. Minimally invasive surgery, with or without robotic instrumentation, offers many advantages over traditional open surgery – small incisions result in significantly less pain, much quicker recovery, shorter hospital stays and earlier return to daily activities. For open surgery, many patients complain of chronic pain or numbness over the large incision (Figure 2).


Figure 2(a): Patient with conventional open kidney surgery scar


Figure 2(b): Patient with minimally invasive nephron-sparing surgery scar.


5. My doctor says that I have several cysts in both my kidneys. Does this mean that my kidneys will start to fail in future?

The presence of multiple cysts in both kidneys may be due to certain genetic conditions such as (1) autosomal dominant polycystic kidney disease (usually diagnosed in adulthood); (2) autosomal recessive polycystic kidney disease (usually diagnosed in infancy and childhood); and (3) medullary sponge kidney. In such conditions, healthy kidney tissue is gradually replaced with non-functioning cystic collections, resulting in progressive loss of kidney excretory function (Figure 3). Such patients should have their kidney function closely monitored and optimized by their nephrologists, although end-stage kidney failure requiring eventual dialysis or kidney transplant is a very real possibility.


Figure 3. Diagram of a polycystic kidney, where healthy kidney tissue is replaced by non-functional fluid collections throughout the kidney.


6. My kidney cyst(s) have been getting progressively larger and more painful. What treatment options are available?

There are several approaches to treating symptomatic kidney cysts1,5. Percutaneous aspiration with sclerotherapy is a popular first approach, wherein a fine needle is introduced into the cyst under ultrasound imaging guidance, the fluid is aspirated or sucked out, following which sclerosant chemicals (eg. 95% ethanol, 1–3% sodium tetradecyl sulphate, 50% acetic acid, or 10% ethanolamine) are injected into the cyst to cause inflammation and scarring to prevent re-accumulation of fluid within the cyst.

Laparoscopic deroofing of such cysts (also known as decortication or marsupialization) remains the gold standard with the lowest incidence of cyst recurrence. In this minimally invasive procedure, the kidney cyst(s) are dissected out using keyhole instruments, and the cyst wall excised after draining all the fluid within the cyst.

If the cysts are chronically infected and/ or have resulted in near complete loss of function of the affected kidney, urologists would recommend surgical removal of the affected kidney if the other kidney remains in good function. This is known as simple nephrectomy, and avoids the problems of recurrent infections and future cancerous change of the affected non-functioning kidney.

7. How frequently should I return for follow-up of my kidney cyst(s)?

You should consult your urologist on the most appropriate follow-up regime for your kidney cyst(s). Most surveillance protocols recommend follow-up with ultrasound imaging of the kidneys at 6 to 12-monthly intervals for Bosniak 2 or 2F cysts.


1. McGuire BB, Fitzpatrick JM. The diagnosis and management of complex renal cysts. Current Opinion of Urology 2010; 20: 349-354.

2. Novick AC, Campbell SC, Belledegrun A et al. AUA Guidelines for the management of the clinical Stage 1 renal mass. American Urological Association 2010.

3. Go AS, Chertow GM, Fan D, and McCulloch CE: Chronic kidney disease and the risks of death, cardiovascular events, and hospitalization. N Eng J Med 2004; 351: 1296.

4. Sprenkle PC, Power N, Ghoneim T et al. Comparison of open and minimally invasive partial nephrectomy for renal tumours 4-7cm. European Urology 2012; 61(3): 593-599.

5. Agarwal MM, Hemal AK. Surgical management of renal cystic disease. Current Urology Reports 2011:12: 3-10.

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