Written by Dr. Melissa Heuer and Dr. Jaime Landman
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 an approximately 35% 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 70 years old and you do not have a simple cyst, you are 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 as part of the contents of the cyst. A hyperdense kidney cyst is not suspicious for kidney cancer and is just another type of simple kidney cyst.
The vast majority of simple kidney cysts are simply identified on radiographic testing that has been performed for another reason. Simple kidney cysts almost never have 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 (area in the back where the lower ribs meet the spine) pain, but this is quite unusual. Other symptoms caused by larger cysts include early satiety (feeling full with little food due to the cyst pushing on the stomach) and difficulty breathing if the large kidney cyst makes it difficult to inflate the lungs. 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). While many larger cysts do recur after aspiration and sclerosis, aspiration also helps to confirm that the cyst is indeed causing the symptoms as cyst related symptoms typically disappear or decrease when the cyst fluid is removed. 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 are greater than 8 centimeters or have failed aspiration and sclerosis by a radiologist. The procedure is very effective, results in little pain 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 have irregularities within the walls of the cyst and/or within the contents of the cyst. 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 a strong indicator that the cyst harbors a kidney cancer within it, and these cysts often require intervention by a urologist for treatment. Most complex kidney cysts with enhancing components are managed by surgical removal or by percutaneous or laparoscopic ablation.
Complex kidney cysts have a risk ranging from 13% to 90% for being kidney cancer. The more irregularities a complex renal cyst has and the larger it grows, the more potential it has or could become a kidney cancer. Often, complex kidney cysts that do not show any enhancement on radiologic studies may be treated with active surveillance to see if the cyst changes in any way over time.
The risk that a complex kidney cyst is, or may become, a kidney cancer depends on its appearance on radiographic imaging (CT scan or MRI). 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.
||A simple benign cyst with a hairline thin wall that does not contain septa, calcifications, or solid components. It measures as water density and does not enhance with contrast material.
||0% to < 2%
||A cyst that might contain a few hairline thin septa. Fine calcifications might be present in the wall or septa. Uniformly high-attenuation (hyperdense) lesions of <3 cm that are well marginated and do not enhance.
||These cysts might contain more hairline thin septa. No measurable enhancement of a hairline thin septum or wall can be seen. There can be minimal thickening of the septa or wall. The cyst might contain calcifications that are nodular and thick but there is no contrast enhancement. No enhancing soft tissue elements are seen. Totally intrarenal non-enhancing high-attenuation renal lesions equal to or greater than 3 cm are also included in this category. These lesions are generally well marginated.
||14.3% to 24%
||These lesions are indeterminate cystic masses that have thickened irregular walls or septa in which enhancement can be measured.
||All of the criteria of category III along with enhancing soft-tissue components adjacent to, but independent of the wall or septum. These cysts are usually malignant cystic masses.
The Bosniak classification above, is designed to help your doctor predict the chances that your complex renal cyst is associated with a kidney cancer. This predictive ability is very important in helping you work with your urologist to determine the best treatment strategy for you. The risk that your kidney cyst harbors a kidney cancer must be balanced with your overall health to so that a proper treatment strategy can be created.
Bosniak category I complex kidney cysts have a less than 2% chance of being associated with a kidney cancer. Bosniak category II complex kidney cysts have an approximately 14% chance of being associated with kidney cancer. Bosniak category IIF lesions are complex kidney cysts that have more irregularities and are typically larger than category II cysts. Also, category IIF cysts do not have measurable enhancement on imaging and therefore do not qualify as category III complex kidney cysts. The “F” in the IIF designation is meant to suggest “Follow” or observe this cyst rather than perform a procedure such as partial nephrectomy or kidney cryoablation to treat the cyst. However, the category IIF cysts have a higher rate of malignancy which is up to 24%, and therefore it is important to discuss the merits of treatment versus observation with your urologist. Bosniak catagory III and category IV complex kidney cysts have a 50% and 90.1% chance respectively of being kidney cancer and are often recommended for surgical removal or treatment with cyst ablation.
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 entire kidney depending on the particular clinical circumstances.