Benign & Cystic Tumors of the Kidney



Diagnosis & Evaluation


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A cyst is simply a round fluid-filled mass (much like a water balloon). They can be found in almost every part of the body. The kidney is one of the most common sites for cysts. These are infact so common that about 50% of adults who have a kidney ultrasound will be found to have at least one kidney (or renal) cyst. Nearly all cysts are perfectly benign, of absolutely no threat, and need not be treated or followed. However, rarely a kidney cancer can appear "cystic", although it will have characteristics on imaging that make it obvious it is not a "simple cyst". On the other hand, some perfectly benign cysts can have atypical features. Your urologist will discuss with you the chance of whether your particular cyst is benign or not and discuss with you management options.

There are several other "cystic diseases" of the kidneys, including so-called "polycystic kidney disease" (PCKD). this is usually an inheritable condition of progressive production of many cysts in the kidneys through adult life. The kidneys can literally become replaced with innumerable cysts that lead eventually to kidney failure. There is usually a family history of PCKD, and/or the presence of MANY renal cysts at dianosis, not just 2 or 3 in a kidney.

Cystic tumors may be benign (such as "multilocular cystic nephroma") or malignant. Your urologist will discuss the details of this in you case if he suspects either of these relatively rare conditions.




What causes benign simple cysts to occur is still unknown. There is nothing that can be done to prevent them, nor need there be as they are extremely common and non-threatening.


Diagnosis & Evaluation


An ultrasound usually first demonstrates the presence of one or more renal cysts. If the radiologist is satisfied that the cyst(s) satisfies the criteria of a simple cyst, nothing more is necessary. If there is any doubt, then depending on certain factors it may be recommended that a CT scan be done to decide if the cyst in question is indeed benign or not. Sometimes we will simply follow with another ultrasound in a few months to reassess.


Ultrasound and CT are complementary. Ultrasound is very good at establishing the presence of fluid - which is critical in differentiating simple benign cysts from small solid renal masses (some of which are cancerous). CT is not as good for small lesions, but good in defining solid tumors as well as extent of tumor.




Simple cysts require no treatment and usually no followup is necessary. When appropriate imaging is performed and the presence of a simple cyst is confirmed, cancer is not a concern. No matter what their size, simple cysts almost never cause any problems - if you have a simple cyst (especially if it is less than 10 cm) and any kind of symptoms, another cause should be sought. Rarely, problems can arise and include:


  1. Symptoms: pain, fullness, bloating. Cysts less than 10 cm are almost never symptomatic. If you have a small cyst and have pain, there is likely another cause.
  2. Bleeding: bleeding can rarely occur within cysts. If not caused by trauma, further evaluation to check for cancer will be arranged.
  3. Hypertension: compression of the kidney parenchyma can rarely cause problems with blood pressure. Because run-of-the mill hypertension (so called essential hypertension) is so common, if you have a cyst and high blood pressure, the cyst is probably not the cause.


PCKD is best followed by a nephrologist for prevention and delay of renal failure as well as genetic counselling. A cystic tumor where cancer cannot be ruled out may require surgical removal.


In some cases, cysts can be serious. A commonly used classification of cysts is the Bosniak classification system. It is quite good at the extremes where either a benign lesion or malignancy can be specified with good certainty, but in the middle ground, follow-up is sometimes advisable where the diagnosis is uncertain.


Class Description Follow-up

Benign simple cyst


Thin wall (no internal echoes, sharpley defined walls, round or ovoid). No enhancement on CT


Benign minimally complicated cyst


The vast majority of these are non-cancerous. Out of the tens of thousands of Bosniak II renal cysts, there have only been a few case reports of malignancy developing in these cysts. They always have a thin wall. May have any of the following:

  • Thin sepation, no more than a few
  • Thin calcification of the wall
  • 'Hyperdense': high hounsfield units on CT, but no enhancement (can confirm fluid with ultrasound); <3 cm

Usually None

Sometimes repeat imaging


Indeterminant Cyst

Some of these can be cancerous. Some of the components may enhance when contrast is given during CT. May have any of the following

  • Thicker wall
  • More than a few septations
  • Irregular calcification of the wall

Sometimes repeat imaging

Usually remove cyst or kidney.


Malignant (cancerous cyst)


May have any of the following:

  • Enhances on CT
  • Nodularity
  • Solid components
Usually removal of the cyst or kidney


Guideline Recommendations for Renal Cysts

The Canadian Urological Association 2019 Guidline recommendations are as follows.


Bosniak I (simple) renal cysts are common. These are almost never symptomatic regardless of size. The natural history of cysts is to grow over time and therefore growth should not necessarily be considered a sign of malignancy. Most importantly, transformation to a more complex cyst is very rare. Follow-up is not necessary or recommended. In the rare cases where the cyst is symptomatic then consideration can be given to aspiration and decompression with or without sclerotherapy or cyst decortication.


Bosniak II renal cysts are slightly more complex. Management is essentially the same as for Bosniak I renal cyst.


Bosniak IIF renal cysts is an intermediate class of cysts which cannot clearly be placed into category II or III. Criteria include increased number of thin or slightly thickened septations but without soft tissue nodularity, calcification, large hyperdense cysts greater than 3 cm in any cyst that cannot clearly be placed into category 3. It is felt that somewhere between 10 and 30% of such cysts or ultimately found to harbor malignancy. Correcting for selection bias, the risk may be as low as 8%. Current recommendations are serial imaging for at least 5 years at a schedule as specified below.


Bosniak III renal cysts have an approximately 50% probability of harboring malignancy and surgical excision is generally recommended. As per small renal masses, kidney sparing surgery can be considered if technically feasible and active surveillance is an option in select cases.


Bosniak IV renal cysts have an 80-90% probability of harboring malignancy, typically cystic renal cell carcinoma. Excision is the treatment of choice though as most of low metastatic potential conservative management can be considered.




Angiomyolipoma (AML) is a non-cancerous (benign) tumor of the kidney. They are comprised of 3 components

  1. Angio - blood vessels
  2. Myo - muscle
  3. Lipoma - fat


Definitive diagnosis can often be made with CT scan as fat has a very specific appearance on CT and other types of fat containing tumors are exceedingly rare. There are instances where the amount of fat is too small to detect with CT scan and a definitive diagnosis cannot be made.


The primary risk associated with AML is bleeding. The blood vessels are thin walled and can rarely spontaneously rupture. The risk of bleeding increases with the size of the tumor but is very rare with tumors less than 4-5 cm in size. Tumors that are larger can be considered for angioembolization - a radiologic procedure to cut off the blood supply to the tumor.


AML has been associated with tuberous sclerosis. In most cases, tuberouse sclerosus associated AML are multiple and occur in both kidneys. Tuberous sclerosus requires multiple clinical features to make a diagnosis. More information can be found with these links: TS-associated skin conditions, updated diagnostic criteria.



On the Web

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Medline Plus Produced by the US National Institutes of Health with information on virtually every health topic and extensive list of links The patient information site of the American Urological Association.