Renal / Kidney Cyst

Renal / Kidney Cyst

Cysts are abnormal, fluid-filled sacs in the body lined with a lining of cells that contains within it fluid, air, blood or other substances. Cysts can occur anywhere in the body. There are many different types of cysts.

Renal / Kidney cysts are sacs of fluid that form in the kidneys. They are usually the variety of simple cysts, characterized by a thin wall and contain water-like fluid. Renal cysts is common as people age and 50% individuals above age of 50 years have cysts in the kidney.

Renal cysts are usually often found during imaging tests, and are quite often incidental findings. These cysts may be classified by what is known as the Bosniak Classification into 4 varieties, namely the simple and complex cysts. It is important to differentiate these types of cysts as the complex varieties may at times require intervention.

Complex renal cysts may have a thicker wall or contain solid material instead of fluid. The more complex cysts may have a growth within the cyst itself.

You may require additional imaging tests to confirm these complex renal cysts and distinguish benign cysts from cancer.


Cysts occurs when a tube becomes blocked, swells up, and fills with fluid. Another possibility is that cysts start when pouches called diverticula form in weakened areas of the tubules and fill with fluid.

Renal cysts are likely to occur when you grow older.

Renal cyst on ultrasound

Renal cyst on CT scan

Pic taken from Radiopedia

The above diagram shows various renal cysts morphologies, listed in order by their potential for malignancy, using the Bosniak classification system.  

The cysts in the top row (1 and 2) do not need further evaluation or monitoring. The cysts in the bottom row (2F, 3 and 4) should be followed (the “F” in 2F means it requires “followup”) and require further evaluation and management. 

  • Type I: almost universally benign and appear as simple cysts on CT with extremely thin walls. To classify a renal cyst as type I, there must be no septa, calcifications, or solid components. These lesions do not enhance with IV contrast.
  • Type 2: almost universally benign, but unlike type I, they may contain thin septa. Thin or slightly thickened calcification may be noted within the cyst wall. Minimal but un-measurable enhancement of the septa or cyst wall may also be appreciated. High-attenuation cysts of less than 3 cm in diameter that do not enhance are considered type II.
  • Type 2F:  (the “F” is for follow-up needed) cysts are more complex that simple type II cysts, but do not meet the criteria for a type III classification. These cysts may demonstrate an increased number of septa or minimal smooth thickening of the wall or septa. The wall and/or septa may contain calcifications. No part of these lesions should appreciably enhance. Non-enhancing high-attenuation cysts that are intrarenal and larger than 3 cm are also considered IIF.
  • Type 3: indeterminate with thick, nodular multiple septa or wall with measurable enhancement.
  • Type 4: clearly malignant. They appear similar to type III cysts, but also have enhancing mural components adjacent to, but separate from the wall or septum.


Most of the times, renal cysts are without any symptoms, and are incidental findings on imaging tests. Even if they do not produce any symptoms, it is best your urologists assesses this and confirm whether it is a simple or complex cyst, whether it requires surveillance or requires an intervention.

Renal cysts may require treatment if there is infection, bleeding, or enlargement causing pain or compression of the kidney. When this occurs, symptoms may include fever, pain in the loins, upper abdominal pain, changes in urinary habits or blood in the urine (hematuria)

The tests that may be required to confirm renal cysts include ultrasound, CT scan, or at times MRI.



Your doctor will decide what the best treatment of your renal cyst is.

A simple renal cyst will not require any treatment or intervention.

Simple renal cysts are usually left alone and do not require treatment unless they are causing symptoms or harming kidney function. If treatment is required, your doctor may treat this with laparoscopy (key-hole) or open surgery to drain the cyst and take steps to prevent its recurrence.

Aspiration of the cyst and injection sclerotherapy is not done these days as it does not settle the underlying issue, and is associated with high risk of recurrence.

As for complex renal cysts, your urologist will determine if this require surveillance or an intervention to remove the cyst or part of the kidney if there is a suspicion of cancer or a high risk of cancer.

When to see your doctor

Always see your doctor to determine what the best treatment of your renal cyst is.

Although most renal cysts are simple cysts, we do often encounter complex renal cysts and cancers presenting as renal cysts. These complex cysts require surveillance or intervention.

You also need to see your urologist if you have a simple renal cyst and the following symptoms which will then require intervention:

  • Blood in your urine
  • Pain in your back or upper abdomen
  • Trouble urinating
  • Fever of 38.0°C or higher
  • Chills

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