Benign Kidney Cyst (Simple Renal Cyst)

What is a benign cyst in the kidney?

The kidneys are made of about a million tiny filtering tubes. A simple cyst is thought to begin when a tubule becomes blocked or develops a weak pouch that fills with fluid and gradually swells into a sealed sac. It has a thin, smooth wall and contains watery fluid, much like a small balloon.

Simple cysts:

• are usually single, though some people have two or three
• range from a few millimeters to several centimeters, occasionally over 10 cm (4 in)
• grow slowly, if at all, over years
• do not affect kidney function
• are not the same as polycystic kidney disease

Their frequency rises with age. They are uncommon before 40, but imaging studies find them in about a quarter to a third of people over 50 to 70, and they are slightly more common in men.

Simple cysts vs other kidney cysts

The Bosniak classification

Radiologists use the Bosniak system to grade kidney cysts seen on CT or MRI by how likely they are to be cancer. It guides what happens next.

If your report says "simple cyst" or "Bosniak I" or "II", the cyst is benign. A report that cannot classify a cyst on ultrasound (for example "complex" or "indeterminate") usually leads to a CT or MRI with contrast.

Symptoms of a kidney cyst

Most benign kidney cysts cause no symptoms. When they do, it is usually because they are large or develop a complication:

• a dull ache in the side, back or upper belly, sometimes called flank pain
• a feeling of fullness in the abdomen with very large cysts
• infection: fever, shaking chills and tender flank pain
• bleeding into the cyst or rupture: sudden severe side pain, sometimes with blood in the urine
• blockage: a cyst pressing on the kidney's drainage system can cause hydronephrosis, with pain and sometimes infection
• high blood pressure: rarely, a large cyst presses on kidney tissue and raises blood pressure

Causes and risk factors

The exact cause is unknown. The main risk factor is age. Simple cysts are not inherited and are not caused by diet, drinking or anything you did. They are also not linked to kidney stones, although both are common and can occur together.

Multiple cysts in both kidneys suggest a different condition, especially with a family history. Inherited conditions include polycystic kidney disease, von Hippel-Lindau disease and tuberous sclerosis, which raise the risk of kidney tumors.

How kidney cysts are diagnosed

• Ultrasound: often the first scan. A simple cyst looks black (fluid-filled) with a crisp thin wall and brightness behind it. A cyst that meets all these features on a good-quality ultrasound is confirmed as simple.
• CT scan or MRI with contrast: used when a cyst has any complex feature, to apply the Bosniak grading.
• Blood and urine tests: kidney function (creatinine and eGFR) and a urine test for blood or protein, mainly to check for other kidney problems.

No biopsy is needed for simple cysts. Cyst biopsy is rarely used because imaging is usually more informative.

Treatment of a benign kidney cyst

Watchful waiting

A simple cyst with no symptoms needs no treatment and no repeat scans. Major urology guidelines do not recommend routine follow-up imaging for Bosniak I or II cysts. Normal activity, exercise and diet are fine.

When treatment is needed

Treatment is considered only if a cyst causes pain, infection, blockage of urine flow or high blood pressure.

• Aspiration and sclerotherapy: a needle guided by ultrasound or CT drains the fluid, then a substance (often alcohol) is injected to scar the cyst lining so it does not refill. Drainage alone often fails because the cyst refills.
• Laparoscopic decortication (unroofing): keyhole surgery removes the outer wall of the cyst so it cannot refill. This has the best long-term success for large, painful cysts.
• Infected cysts: antibiotics, and drainage if the infection does not settle.

Complex cysts

Bosniak IIF cysts are watched with follow-up scans. Bosniak III and IV cysts are discussed with a urologist. Options include partial nephrectomy (removing only the affected part of the kidney), ablation, or careful active surveillance in older people or those with other illnesses. See kidney cancer for more.

Outlook

Simple kidney cysts are harmless in almost everyone. They do not reduce kidney function, they do not become cancer, and most people live with them without ever knowing. Complications such as rupture or infection are uncommon.

Frequently asked questions

What is a benign cyst in the kidney?

A benign kidney cyst is a thin-walled, fluid-filled sac in the kidney that is not cancer. Most are simple renal cysts, very common in people over 50, usually found by chance on a scan. They rarely cause symptoms and need no treatment.

Can a simple kidney cyst turn into cancer?

No. A truly simple cyst (Bosniak I or II) does not become cancer. Cancer risk applies to complex cysts with thick or irregular walls, enhancing septa or solid parts (Bosniak III and IV), which are a different entity. That is why radiologists grade cysts carefully.

Do kidney cysts cause back pain?

Small cysts do not. Large cysts, usually over 5 to 10 cm, can cause a dull ache in the side or back. Sudden severe pain may mean bleeding, rupture or infection in the cyst, which needs prompt medical assessment.

Does a benign kidney cyst need follow-up?

A simple Bosniak I or II cyst does not need follow-up scans. A Bosniak IIF cyst is usually rescanned over about 5 years to confirm it stays stable. Higher grades are managed by a urologist.

Can kidney cysts go away?

Simple cysts rarely disappear on their own. Most stay the same size or grow slowly. Because they are harmless, this usually does not matter. A cyst that causes symptoms can be drained or unroofed so it does not return.

Is a kidney cyst the same as polycystic kidney disease?

No. A simple cyst is one or a few harmless sacs that appear with age. Polycystic kidney disease is an inherited condition with many cysts in both kidneys that enlarge the kidneys and can cause kidney failure, high blood pressure and pain.