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So you’ve got a kidney cyst

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Being a kidney doctor, I frequently consult patients who have discovered that they have a kidney cyst. A kidney cyst is a swelling filled with watery fluid that forms on one or both of the kidneys. Kidney cysts are round, have a thin, clear wall and range in size from microscopic to around 5 cm in diameter. They can be associated with serious conditions that lead to impaired kidney function, but usually they are what is referred to as simple kidney cysts, which do not tend to cause complications.

Kidney cysts can occur in a variety of genetic diseases in adults and children, but “simple renal cysts” are commonly observed in normal kidneys. Simple kidney cysts are more likely to develop as a person ages. Approximately one quarter of people who develop these cysts are 40 years or older and around half are 50 years or older.

As they are so common, it is difficult to consider them a disease. Symptoms are also rare and a person will not usually know they have a kidney cyst. It is often detected by accident — when a test for something else is performed.

More and more people are being diagnosed with these lesions as we use more medical imaging technology such as ultrasound, X-ray and CT scanning. In a survey of people undergoing ultrasound for evaluation of non-kidney-related problems, 15 percent of men and 7 percent of women aged 50 to 69 had renal cysts. One-third of men and 15 percent of women older than 70 had a renal cyst.

A complex kidney cyst is a cyst that has a more irregular shape or thicker walls than a simple cyst. Generally, the larger and more complex the cyst is, the greater the risk of it turning cancerous. Complex cysts are therefore carefully monitored and removed, if required.

Where do they come from?

The kidneys are comprised of tiny blood vessels that transport blood to approximately one million filtering tubes referred to as nephrons. A cyst arises when one of these tubes expands, but the exact reason why the tube expands is not yet known.

What are the symptoms?

Although kidney cysts do not usually cause symptoms, some may cause a dull pain in the back or upper abdomen if they grow large enough. Occasionally, they can become infected, in which case a person may develop a fever, as well as suffer from pain and tenderness. They may also cause severe pain in the back or side if they burst. A cyst that blocks the normal flow of urine may cause the kidney to swell, which is referred to as hydronephrosis.

How are they treated?

If a simple kidney cyst is causing symptoms or obstructing urine flow, it may need to be treated. Under ultrasound guidance, the cyst is punctured using a long needle and drained.

This procedure may be carried out on an out-patient basis and does not require an overnight stay in the hospital. In some cases, where a cyst is large enough, surgery may be required to remove it.

Can kidney cysts be cancerous?

The initial radiologic appearance of most cysts determines what further evaluation is needed.
Your doctor needs to accurately distinguish simple renal cysts from complex renal cysts. Many physicians use the Bosniak classification of renal cysts to determine follow-up.

Observation of lesions is far more common than biopsy. Traditionally biopsy requires removal and loss of the kidney. Although a biopsy can now be done with a needle through the skin using CT imaging to guide the needle. Once discouraged, these needle biopsies can now be done in very specific circumstances.

The Bosniak classification uses a complicated algorithm of CT scan characteristics such as size, density and perfusion to place cystic renal masses into one of five different categories.

Bosniak categories I and II are generally simple cysts and do not require further evaluation. Some would repeat an ultrasound at 6 to 12 months to assure stability and a correct diagnosis.

Bosniak category IIF cysts are more complex and deserve follow-up imaging to document stability. The absence of change over time supports benign disease, while progression suggests cancer. Observation can prevent many unnecessary surgeries as most cysts do remain stable.

Many recommend that all Bosniak category III lesions undergo surgical removal and evaluation, as 40 percent to 50 percent will be cancer. With this approach, there is still a significant number of unnecessary surgeries. Some recommend close follow-up with magnetic resonance imaging (MRI) to avoid this. MRI is especially useful for characterizing the inside of a cyst after ultrasound and CT scanning are still not definitive. Serial MRI examinations at three, six and 12 months are warranted in some patients.

There is little debate that category IV lesions require surgical removal of the kidney.
Approximately 85 percent to 100 percent of these are cancer.

Greater than 90 percent of those diagnosed with renal cancer still confined to the kidney are alive and disease-free five years after diagnosis. Five-year disease-free survival is 60 percent to 70 percent for those whose disease has grown outside the kidney. Survival rates are very low if the disease has spread beyond the kidney.

What do I do next?

If you have been diagnosed with a kidney cyst, discuss the test results with your doctor and plan to do repeat testing with an ultrasound, CT scan or MRI at a later date, usually in six months. This interval can be stretched out if the cyst remains stable. As always, it is best to ask for a specialist’s advice to save you from a lot of undue worry and anxiety.

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