Many people have kidney cysts. We often see kidney cysts on imaging studies done for unrelated reasons.
Kidney cysts can range from simple to complex cysts. As the internal contents of the cyst become more complex, we begin to be concerned about a cancer more.
There is a well established classification system called Bosniak which attempts to grade the cysts based on their appearance and risk of cancer from 1 to 4.
What is a kidney cyst?
A kidney cyst is a formation of fluid surrounded by a thin wall in the kidney. You can have one or many cysts in the kidneys.
Are renal and kidney cysts the same thing?
Yes. Renal is another way of saying kidney in medical terms.
Symptoms of kidney cysts
Kidney cysts often don’t cause symptoms. Large cysts can sometimes cause pain or urinary symptoms.
What causes kidney cysts?
It is not known with certainty.
There are some familial diseases like polycystic kidney disease which causes many cysts to form in the kidneys.
Acquired cystic kidney disease occurs in patients who have chronic kidney disease and are on dialysis.
Older age is another risk factor for developing kidney cysts.
How is a renal cyst diagnosed?
A renal cyst is diagnosed on imaging studies like ultrasound, CT and MRI. X-rays can not show cysts.
Can kidney cysts be cancerous?
Most kidney cysts are simple and can be left alone. Some cysts have internal complexity which raises the possibility of cancer. We can see how complex the cysts are on imaging studies.
Are left and right kidney cysts the same?
Yes. This makes no difference as far as risk of cancer.
Kidney cyst on Ultrasound
A simple cyst on ultrasound will have simple black appearing fluid inside, with a thin defined wall, and acoustic enhancement along the back wall (a white shadow going beyond the back wall).
A cyst can also look complex with areas of thickening, septations, nodularity along the wall, or complex looking fluid.
Ultrasound may be limited
The quality of an ultrasound is dependent on the skill and experience of the technologist who performs the test and the radiology doctor who interprets it.
The ultrasound is sometimes limited in obese patients. The sound waves have a hard time penetrating large amounts of tissue and fat, and may make a simple cyst look more concerning than it really is. A CT or MRI will often help clear up any concern.
Sometimes the radiologist may want to see the cyst by watching the technologist scan in real time.
Complex cyst on kidney
The distinction between a simple cyst and more complex cyst is critical. Simple cysts are left alone and are benign.
What are the features that make a cyst complex?
Have blood inside instead of simple fluid. Most of these cysts are not cancerous.
The thicker, more irregular and numerous the septa are the more risk the cyst has for cancer.
Nodule in cyst
Nodules which enhance are a concerning feature for cancer.
Enhancement in cyst
This is a concerning feature indicating solid components
Renal cyst with calcification
Some kidney cancers can have calcifications. This is a feature that prompts us to look closely at the contents of the cyst for any other suspicious features.
Kidney cysts are a common finding on CT done for any reason. Most of the time they are benign and do not need treatment,
Kidney cysts can be completely filled with fluid or have other more complex areas inside like thickening of the wall, nodules or septations.
Kidney cysts can range in size from barely visible to enormous, pushing adjacent structures and causing pain. In rare cases, cysts can rupture.
Some kidney cysts are more concerning because they are more complex then simply containing fluid.
A CT done without contrast injected through your vein does not completely evaluate a cyst. Since cysts are so common, many radiologists will diagnose a cyst on a CT without contrast and not require further testing unless they see something suspicious.
Kidney cysts are best evaluated on a CT with contrast injected through your vein. The radiologist will then see if parts of the cyst show complex areas or enhancement. Enhancement after contrast injected through your vein means that that part of the cyst is solid and not a cyst. This is more concerning for a cancer.
Kidney cyst on MRI
Kidney cysts are well evaluated on MRI. MRI gives us a detailed look at the internal contents of the cysts and whether it has risk for being cancerous. We can also assess the cysts for any solid areas after we inject contrast through the vein.
How do we know if a cyst is cancerous?
Cysts can also fall into a grey zone where we’re not sure if they are benign or malignant. Radiologists use a published Bosniak classification to grade the cyst based on its suspicion, with lesions graded as 1 being benign and 4 as cancerous.
Some lesions may show some features that are concerning but not clearly cancerous. These lesions may be followed (Bosniak 2F) or if more concerning (Bosniak 3) treated like a cancer with the knowledge that some will end up being benign after surgical resection.
Small kidneys cyst
Small cysts, say less then 1.5 centimeters can also be a challenge. These small cysts may not have all the classic features of a simple cyst.
The uncertainty can often be cleared up with a dedicated CT of the kidneys with thin sections going through the cysts done with and without contrast. In some cases when the cyst is not entirely simple but slightly complex, the radiologist may recommend a 6 month follow up to reevaluate the cyst.
Kidney cysts treatment
Lesions graded as 1 or 2 are left alone. Sometimes an ultrasound or MRI may be suggested for certain cysts to better evaluate the internal contents.
Cysts that fall into category 2F will be followed.
The treatment of a Bosniak 3 or 4 graded cyst is partial or complete removal of the kidney. A Urology doctor will guide the management.
How do we know if a cyst is cancer for sure?
This can only be known for sure after it is surgically removed and examined by a pathologist.
What else can look like a renal cyst in radiology?
Abscesses which are collections of pus can mimic a complex cyst.
Aneurysms and pseudoaneurysms of the renal arteries can mimic a cyst. Often aneurysm will enhance and have the same density as the adjacent arteries.
Dilated calyx can mimic a cysts. A diverticulum of a calyx can mimic a cyst.
Complex cysts can be cancerous as described above.
What type of doctor treats renal cysts?
Follow up and surgical treatment of cysts is often done by a Urologist. This is a specialist who treats conditions of the urinary tract.
Kidney cysts: summary
kidney cysts are common and often benign. Some kidney cysts are more complex and have some risk of being cancer depending on the appearance and classification grade. The more complex cysts may be closely followed or resected by a surgeon. Ultimately, the pathologist will make the determination if the cyst is cancer or not after it is resected.