A renal cyst is a fluid filled sac in the kidney. The chance finding of a cyst on the kidney is common, particularly as people are now having a lot of imaging with CT, ultrasound scan, or MRI for various reasons. Most of these cysts are of no concern, they do not represent anything sinister, and do not progress. However, unless there is a clear description of a ‘simple cyst’ on imaging, these may need to be further evaluated to make sure they don’t represent a potential problem.
There are two kinds of cystic ‘disease’ of the kidney: acquired (they develop at some point during life) and congenital (a person is born with them). Acquired cysts are by far the most common.
Acquired renal cysts
These are common and are frequently discovered by chance after a radiological test. Studies suggest that overall the rate of kidney cysts is about 10% in the population. Women are twice as likely as men to have a renal cyst. The chance of finding a cyst increases with age:
Under 50 years old - less than 10% have cysts
50-60 years - approx. 15% have cysts
60-70 years – approx. 20% have cysts
Over 70 years – approx. 30% have cysts
The vast majority of cysts are benign, but a small number may have malignant potential. A completely simple cyst on ultrasound needs no further investigation. But if there are more complex features, such as areas of calcium in the cyst, then a CT will be required to look at the cyst in more detail.
A CT scan (with contrast) allows detailed examination of the cyst. The cyst can then be classified according to the ‘Bosniak’ classification, which gives an approximate risk of malignant potential, and helps define if follow-up of the cyst, or treatment is needed. The Bosniak classification can only be made on CT.
Bosniak 1 (0% malignant)
- simple cyst, imperceptible wall, rounded
- No follow-up needed.
Bosniak 2 (0% malignant)
- minimally complex, a few thin (< 1mm) septa, thin areas of calcium,
- non-enhancing high-attenuation (thickened fluid) cystic lesions less than 3 cm are included; they should be well marginated.
- No follow-up needed
Occasionally, benign cysts can become quite large and cause discomfort, although it is rare to have to do something about these. They can be drained with a needle under ultrasound guidance, but they will almost invariably return. If treatment is needed, often the best approach is to ‘deroof’ the cyst, which means to remove some of the membrane that surrounds it. This reduces, but doesn’t completely remove, the risk of a cyst returning.
Bosniak 2F (2-10% malignant)
- minimally complex but needing follow-up.
- increased number of septa, minimally thickened or enhancing septa or wall
- thick areas of calcium
- high attenuation cyst that is more than 3 cm in diameter, mostly intrarenal (less than 25% of wall visible), with no contrast enhancement
- Need ultrasound or CT follow up
Bosniak 3 (50% are malignant, although recent studies suggest this number may be lower)
- indeterminate, thick or multiple septations, nodule in the wall, high attenuation on CT
- The lesions may need treatment
Bosniak 4 (all are considered as malignant)
- A solid mass with large cystic or necrotic component
- Treatment (nephrectomy or partial nephrectomy) is needed if the patient is fit for surgery.
If you have been diagnosed with a kidney cyst and need further information, it is a good idea to talk to your General Practitioner. If your GP decides you need a referral to a specialist urologist, Nick Brook runs urology clinics in Adelaide, South Australia, and can be contacted by using the form at the bottom of the page.
Acquired cystic kidney disease in patients with renal failure and on dialysis
Many patients with chronic kidney disease develop cysts in the kidney. Patients on dialysis are more likely to do so, and the development of these cysts is dependent on the length time a patient has been on dialysis.
Just starting dialysis – 20% of patients will have cysts
4 years on dialysis – 80% of patients will have cysts
8 years on dialysis – 90% of patients will have cysts
The vast majority of these cysts are nothing to worry about. It is thought these cysts can occasionally becoming infected and cause some symptoms, and that they can occasionally bleed. If you see blood in your urine, you should let your doctor know straight away.
Very occasionally, some people on dialysis develop a tumour in the kidney. These may or may not be associated with cysts – nobody is really sure. Overall the rate of kidney cancer is higher in patients on dialysis than the general population. Let your nephrologist know straight away if you develop blood in your urine (haematuria).
Congenital renal cystic disease
This is a very different group of renal cysts from the acquired renal cysts. The approach to these kidney conditions is very different than that for acquired renal cystic disease, outlined above. The more frequent causes of congenital (a condition that patients are born with) renal cystic disease is given below:
Polycystic kidney disease
Medullary sponge kidney
Multicystic dysplastic kidney
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