Renal cysts are fluid-filled sacs found in the kidney. They can be classified as either simple or complex.
Simple cysts have a well-defined outline and homogeneous features. They are very common in older patients, with prevalence of up to 50% in those over 50yrs.
Simple cysts are thought to develop from the renal tubule epithelium in response to previous ischaemia, however their exact pathophysiology is still not fully understood.
Complex cysts have more complicated structures, including thick walls, septations, calcification, or heterogeneous enhancement on imaging.
They can be classified using the Bosniak classification (discussed below). All complex cysts have a risk of malignancy and this risk increases with its increasing complexity.
Risk factors for the development of renal cysts include increasing age, smoking, hypertension, and male gender.
Genetic conditions that can cause renal cysts include polycystic kidney disease, tuberous sclerosis, and Von Hippel-Lindau disease.
Polycystic Kidney Disease
Autosomal dominant polycystic kidney disease (ADPKD) is an inherited condition caused by mutations in the PKD1 or PKD2 genes, resulting in multiple renal cysts forming in affected individuals.
The condition is also associated with Berry aneurysm formation (leading to subarachnoid haemorrhage), mitral valve disease, and liver cysts. Patients will eventually develop end-stage renal failure and may require either dialysis or a renal transplant.
Autosomal recessive polycystic kidney disease (ARPKD) is an even rarer inherited disorder, usually diagnosed in-utero. Around 60% of neonates with this condition will not survive their first month of life.
Renal cysts are most likely to be found incidentally on abdominal imaging, as they are usually asymptomatic, especially if simple.
Clinical features however can include flank pain (if a cyst ruptures or becomes infected) or haematuria. Patients with polycystic kidney disease can present with uncontrolled hypertension or even with a flank mass.
The main differential for any renal cyst is cystic renal cell carcinoma and hence should be evaluated accordingly with imaging as required.
The definitive diagnosis for a renal cyst is through CT or MRI imaging, with pre- and post-enhancement scans with IV contrast. Bosniak scoring can then be used to further characterise the cyst/mass (see below).
Ultrasound often picks up incidental findings of renal cysts (Fig. 2), however in such cases, will require further imaging with CT or MRI. Whilst the main advantage of ultrasound is that it remains radiation-free, the modality remains user-dependant, time-consuming, and does not provide conclusive assessment.
A patient’s serum U&Es should also be checked in known cases, ensuring there is not impact to the patient’s renal function. This should be monitored regularly in patients who have a known genetic risk for renal cyst formation as well.
*For Bosniak scores of IIF to IV, the accuracy of CT is limited and MRI is more sensitive and specific, therefore should be undertaken in such cases. MRI should also be used as the initial choice of investigation in young patients or in patients where radiation exposure is of concern.
Bosniak Scoring System
The Bosniak scoring system is the currently accepted method for classifying renal cysts. It uses contrast-enhanced computed tomography scan data to classify cysts into five stages.
Whilst stage I is a simple renal cyst, all other stages are complex cysts with abnormal morphology. The risk of malignancy increases with increasing stage.
|Cyst type||Malignancy risk||
|Complex||5%||CT scan at 3, 6, & 12 months|
|Complex||50 – 70%||Surveillance or Surgical|
|Complex||90 – 100%||Surgical|
Table 1 – The Bosniak Scoring System
Asymptomatic simple cysts do not normally need any further follow-up or treatment.
Symptomatic simple renal cysts can be managed initially with simple analgesia, however needle aspiration or cyst deroofing* may be warranted if significantly impacting the patient.
Complex cysts are managed depending on their Bosniak Stage and this may involve continued surveillance or surgical intervention, including consideration for nephrectomy.
*Cyst deroofing, usually performed laparoscopically, involves aspirating the cyst and excising part of the wall to discourage cyst recurrence
Complications are rare, however can include infection, haemorrhage, and rupture. Complications in simple cysts can make them very hard to differentiate from complex cysts on subsequent imaging.
Prognosis depends on the complexity of the renal cysts and their risk of malignancy.
- Renal cysts are fluid-filled sacs found in the kidney, classified as either simple or complex
- Complex cysts have an associated risk of malignancy
- Definitive diagnosis is made via CT or MRI imaging
- The Bosniak scoring system is the currently accepted method for classifying complexity of renal cysts and associated risk of malignancy
- Most renal cysts require no intervention