There are two significant categories of kidney cysts that exist. The most common type of kidney cyst is the “simple kidney cyst.” The simple kidney cyst is very common and has no risk of becoming a kidney cancer. The other type of cyst is called a “complex cyst.” The term “complex cyst” refers to a spectrum of cysts that have different characteristics which may make them suspicious for kidney cancer.

A “simple kidney cyst” is a finding that has a very clear definition depending on the radiologic imaging test (eg. ultrasound, CT scan, etc.) that is used to identify it. The simple kidney cyst is a spherical space somewhere in the kidney that is filled with liquid. The lining or wall of the cyst is very thin and has no irregularities in it. Inside the simple kidney cyst there is nothing but fluid, which when removed is usually yellow or clear. A simple kidney cyst has no risk of becoming a kidney cancer and is therefore nothing to worry about. Indeed, by the time a person is 50 years old, there is a 50% chance that he or she will have a simple cyst. As we get older the chances of having a simple cyst increase and the cysts increase in size as well. If you are over 50 years old and you do not have a simple cyst, you are therefore in the minority of people. Another type of simple kidney cyst is called a hyperdense cyst. A hyperdense cyst a simple kidney cyst that has blood within the walls of the cyst. A hyperdense kidney cyst is not suspicious for cancer and is just another type of simple kidney cyst.

The vast majority of simple kidney cyst are simply identified on a radiographic test that has been performed for another reason. Simple kidney cysts have no clinical implications. Usually, simple kidney cysts do not result in any symptoms and cause no harm to the body. On rare occasions, a cyst may cause symptoms such as flank pain, but this is quite unusual. If a simple kidney cyst is believed to cause pain or another symptom, it can usually be treated by a radiologist who can perform aspiration and sclerosis of the simple kidney cyst under radiographic guidance with ultrasound or CT scanning. Aspiration is defined as placing a needle through the skin to suck out the fluid in the cyst with a needle. Sclerosis is then performed. Sclerosis means that the radiologist will inject some material to “sclerose” (scar down) the space in the cyst. Aspiration and sclerosis of simple kidney cysts that are associated with symptoms is usually effective although in some cases the cyst may recur (re-fill with fluid). Other than observation of the cyst (if it is not terribly bothersome), aspiration and sclerosis by a radiologist is the least invasive way to treat a simple kidney cyst that is causing symptoms.

Rarely, a simple renal cyst that is causing symptoms will require laparoscopic cyst decortication. In this procedure, small incisions are made and the body spaces are gently inflated with gas. The cyst can be identified and the wall of the cyst cut out to both get rid of the cyst and prevent it from re-forming. Usually, laparoscopic cyst decortication is reserved for simple kidney cysts that have failed aspiration and sclerosis by a radiologist. The Procedure is very effective and usually people only require an overnight stay in the hospital.

A “complex kidney cyst” is different from a simple kidney cyst as the cyst may be irregular in its outer shape. To be defined as a complex kidney cyst, the cyst must have some type of irregularities inside of it. The irregularities come in different varieties. “Septations” are walls within the cyst. These walls may be very fine and thin or quite thick and coarse. Having many or thick walled septations may suggest that the cyst is more likely to be associated with a kidney cancer. Cysts may also be “calcified.” This means that the mineral calcium is visualized inside of the cyst. There may be very little calcium, or the calcifications may be quite thick. A complex kidney cyst may also have tissue inside of it that “enhances.” Enhancement means that a part of the complex kidney cyst gets a blood supply, which can be demonstrated by giving contrast material into a vein while radiologic testing is in progress. Solid enhancing material inside of a complex kidney cyst is suspicious for kidney cancer and may require intervention by a Urologist for treatment.

Many complex kidney cysts may have a low risk for being or becoming a kidney cancer. However, there is some risk that a complex cyst is a kidney cancer and complex kidney cysts do require evaluation by an experienced Urologist who may consult with a radiologist. Often, complex kidney cysts may be treated with active surveillance to see if the cyst changes in any way over time.

The risk that a complex kidney cysts is, or may become, cancer depends on its appearance. A system to grade kidney cysts by their appearance on CAT scan has been developed, which help doctors to predict which complex kidney cysts are more likely to have kidney cancer inside. This system is known as the Bosniak classification. The Bosniak classification provides specific definitions to classify cysts by the risk of kidney cancer.

Bosniak Categories of Complex Kidney Cysts

The Bosniak classification, which is above, is designed to help your doctor predict the chances that your complex renal cyst is associated with a kidney cancer. Bosniak category I complex kidney cysts have a less than 2% chance of being associated with a kidney cancer. Bosniak category II and III complex kidney cysts are associated with an approximately 18% and 33% chance of being associated with kidney cancer respectively. The majority (92%) of Bosniak IV complex kidney cysts are associated with kidney cancers.

By using the Bosniak system people with complex renal cysts can work with an experienced Urologist to establish a treatment plan. The treatment plan will incorporate all factors including patient age and health as well as the risk of kidney cancer associated with the complex kidney cyst. The plan will include one of the strategies that are typically used for kidney cancers and may range from active surveillance to removal of the kidney depending on the particular clinical circumstances.

Copyright 2006 Kidney Cancer Institute