Kidney or renal cysts are round, fluid filled sacs that can form in the kidneys. They may appear for no clear reason or as a sign of an underlying condition.

Simple kidney cysts can occur on one or both kidneys and range from less than 1 centimeter (cm) to the size of a golf ball to the size of a golf ball. They may be benign, or noncancerous, and may need no treatment if they do not cause symptoms. However, a person may need treatment if they cause symptoms or complications.

Medical experts can classify a kidney cyst as “complex” when it does not meet the criteria to be labeled a “simple” kidney cyst.

Some cysts result from another condition, such as polycystic kidney disease. In these cases, a doctor will address the underlying condition.

Kidney cysts are fluid-filled sacs that form in the kidneys. On an ultrasound, they appear as a round or oval mass that does not produce echoes on the scanner.

Simple kidney cysts are the most common type of kidney cysts. These cysts have a thin wall around them and develop on the surface of the kidney. It is not clear exactly what causes simple kidney cysts.

A complex kidney cyst is one that does not meet all the criteria to be classified as a simple kidney cyst.

Polycystic kidney disease is a genetic condition that can lead to numerous cysts. These can enlarge the kidneys and affect kidney function.

Acquired cystic kidney disease is not a genetic condition but can happen in people with chronic kidney disease (CKD). Over time, CKD can cause issues with filtering salt, fluid, and waste from the blood. Cysts may be more likely to develop in people who have had kidney dialysis, though it is the condition that causes the cysts, rather than the dialysis itself.

Acquired cystic kidney disease that does not cause complications may not require treatment. Large cysts that cause pain may need draining.

What is the difference between a cyst and a tumor?

There is no specific size of kidney cyst that makes it dangerous.

However, a doctor can classify a kidney cyst based on the Bosniak scale. A higher number on the Bosniak scale is linked with a greater risk of the cyst becoming cancerous. For example, a Bosniak 4 cyst is almost always cancerous.

When can a kidney cyst become cancerous?

Sometimes, kidney cysts do not cause any symptoms.

If symptoms occur, however, they can include:

  • pain on the side
  • stomach pain
  • back pain
  • fever
  • urinating more often
  • blood in the urine

Symptoms can be broad and affect other organs in the body. Be sure to give a doctor a thorough explanation of any symptoms.

Symptoms may intensify if a complication occurs, such as an infection or a rupture.

Doctors are not sure exactly why simple kidney cysts form, but they become more common with age, and they appear to affect males more often than females. A patient history of high blood pressure or smoking can also mean kidney cysts are more likely to occur.

Figures cited by the National Institute of Diabetes and Digestive and Kidney Diseases suggest simple renal cysts may affect 1 in 10 people and almost 20% of those aged 50 years or over.

Cysts can also result from:

Doctors usually detect simple kidney cysts during imaging tests for other conditions.

If a person has a kidney cyst, the doctor may recommend other diagnostic tests to find out more about the growth, such as:

  • CT, MRI, or ultrasound scans to show if cysts are present
  • blood tests to see how well the kidneys are working.
  • urine tests

These are all outpatient procedures. Most need no preparation, although a doctor may give instructions if the person needs to take any action beforehand.

Doctors use criteria known as the Bosniak classification to assess the likelihood of a cyst becoming cancerous. A cyst that scores 1 on this scale has less than 1% chance of becoming cancerous. A Bosniak 4 lesion is a solid mass that is almost always cancerous.

What to know about a mass on the kidney

In many cases, a doctor will recommend monitoring but no treatment for simple kidney cysts where symptoms are not present.

However, if troublesome symptoms arise, a person may need treatment. Symptoms may happen if a cyst blocks the flow of urine or blood through the kidneys.

The doctor may recommend procedures such as surgery, drainage, and sclerotherapy to drain the cyst. Sclerotherapy involves using ultrasound to guide a needle into the cyst, draining it, and inserting an alcohol solution to help prevent it from returning.

A large or complex cyst may need surgery to remove it. This usually takes place in a hospital under general anesthesia.

A surgeon may remove the cyst using a small tool with a light and camera on one end. This allows them to keep the incision small. Other methods may be used.

A person with an infected cyst may need antibiotic therapy before starting other treatment.

Most simple kidney cysts do not cause complications or problems, although these can occur in rare cases.

Complications of simple kidney cysts include:

  • ruptured cysts
  • infected cysts
  • blood or urine block through the ureters or kidneys

When complications occur, symptoms may intensify, and a person should seek medical attention immediately.

There are treatment options for the possible complications of kidney cysts. However, they can be life-threatening without prompt attention.

It is not possible to prevent kidney cysts.

Simple kidney cysts may be more likely to develop in older people, especially biological males.

Are there home tests to check kidney function?

The outlook for kidney cysts will vary according to the cause. Some cysts may not be cancerous, whereas others can be linked to renal cancer.

Sometimes, kidney cysts require monitoring.

Here are some questions people often ask about kidney cysts.

How serious is a cyst on the kidney?

This depends on the type of cyst. Simple cysts do not usually cause symptoms or complications. A doctor can help to evaluate and manage any cysts that become troublesome.

Does a kidney cyst need to be removed?

Most kidney cysts are simple cysts and may require treatment if they are causing symptoms or complications, such as a blockage of blood or urine. Some complex cysts generally have a higher risk of cancer than simple cysts. So, surgical treatment may be necessary.

What is the life expectancy of a person with a kidney cyst?

The life expectancy of a person with a kidney cyst will depend on what type of kidney cyst they have, among other factors. A person should discuss their individual situation with their doctor.

What foods should I avoid with kidney cysts?

No specific food can help prevent kidney cysts. A person should talk with a doctor about their own individual situation.

There are different types of renal or kidney cysts.

Simple kidney cysts are sacs of fluid that develop in the kidneys of many people as they become older. Most are harmless, but a person may need treatment if a cyst leads to symptoms or complications.

Complex cysts are less common. They have a different structure and may become cancerous. Some people also develop kidney cysts due to genetic factors or as a result of other kidney conditions.

A doctor will monitor any cysts that raise concerns, and may recommend treatment such as aspiration or surgery.