Renal cysts are fluid-filled sacs within or on the kidney. Finding a “cyst” on a scan can be worrying, but many are harmless. The important questions are whether the cyst has a simple or complex appearance, whether it is causing symptoms, and whether it affects urine drainage or kidney function.

A large simple cyst is not automatically cancerous, and a small complex cyst is not automatically harmless. Imaging features and the clinical situation guide management.

What types of renal cysts are there?

Simple cysts have a thin, smooth wall and contain fluid. They become more common with age and usually cause no symptoms or loss of kidney function. Most need no treatment.[1]

Complex cystic masses have additional features, such as internal partitions called septa, thickened walls or tissue nodules. “Complex” describes their appearance; it does not establish a cancer diagnosis.[2]

Parapelvic cysts lie beside the renal pelvis, the funnel that collects urine before it enters the ureter. Their location can make diagnosis and treatment more difficult.[6]

Polycystic kidney disease is a separate condition, often inherited, in which multiple cysts may enlarge the kidneys and affect kidney function. Numerous cysts in both kidneys, especially with a family history, merit assessment for cystic kidney disease rather than being assumed to be ordinary simple cysts.[9]

How do renal cysts present?

Most simple cysts are discovered incidentally during an ultrasound, CT or MRI performed for another reason.

Occasionally, a cyst causes a dull ache in the flank or back, pressure or discomfort, or obstruction to urine drainage. Bleeding, infection or rupture can cause more acute symptoms. Fever with flank pain may indicate infection.[1]

Pain and blood in the urine have many possible causes, including stones and other urinary tract conditions. Finding a cyst does not prove that it caused the symptoms. Visible blood in the urine should be assessed rather than attributed to a known cyst.

What investigations are needed?

Ultrasound often identifies an uncomplicated simple cyst. If the lesion is uncertain or complex, a dedicated contrast-enhanced CT or MRI can assess its wall, septa and any solid tissue. Contrast-enhanced ultrasound is another option in centres with appropriate expertise.[3]

An important finding is enhancement: tissue becomes brighter after contrast, indicating a blood supply. Enhancement in thickened or irregular tissue or a nodule raises concern.

Urine testing and blood tests for kidney function may be appropriate. Infection symptoms may require a urine culture. Contrast choices are individualised according to kidney function, allergies and other circumstances.

Ordinary ultrasound alone may not adequately classify a complex cyst. A radiology report should ideally state which version of the Bosniak classification has been used.[2,3]

Bosniak classification: understanding the scan report

The Bosniak system groups cystic renal masses by imaging appearance. The 2019 version incorporates CT and MRI criteria. It estimates concern for malignancy; it is not a cancer stage or a substitute for specialist assessment.[2]

This table is deliberately simplified. Calcification or blood within a cyst does not by itself establish cancer. The pattern of enhancement and tissue architecture matters.[2,3]

When should cancer be a concern?

Concern increases with enhancing nodules, irregular or thick enhancing walls/septa, or progression to a more suspicious category on follow-up.

Bosniak III masses include both benign and malignant lesions. Bosniak IV masses have a high likelihood of malignancy. Published percentages vary with the classification version and which patients undergo surgery, so a single percentage should not be treated as an individual prediction.[3,4]

Absence of symptoms does not exclude cancer. Conversely, enlargement of an otherwise simple cyst does not alone prove malignant change.

Follow-up and active surveillance

For Bosniak IIF lesions, the Canadian Urological Association suggests imaging at 6–12 months during the first year, then annually if stable, for a total of five years without progression.

Its 2023 guideline also supports active surveillance as a preferred option for selected Bosniak III/IV masses measuring 2 cm or less; surveillance or surgery for those measuring 2–4 cm; and surgery generally for those larger than 4 cm. These recommendations have limited-certainty evidence and require shared decision-making.[4]

Other guidelines differ in emphasis. The EAU allows surveillance for selected Bosniak III lesions and generally manages Bosniak IV lesions as localised kidney cancer.[8] Age, overall health, kidney function, imaging features and patient preferences all matter.

Surveillance means planned scans and reassessment, with treatment considered if concern increases. Biopsy may help when there is a targetable solid component and the result would change management; sampling a mainly fluid-filled lesion often has a low diagnostic yield.[4]

When should a benign cyst be treated?

Treatment may be considered for persistent symptoms convincingly attributable to the cyst, obstruction, or complications such as infection. There is no universal size threshold requiring treatment for a confidently benign simple cyst.[1]

Before intervention, the team should establish that the lesion is benign and discuss whether treatment is likely to relieve the symptoms.

Treatment options

Interventional radiology: aspiration and sclerotherapy

An interventional radiologist can use ultrasound or CT to guide a needle or catheter into a suitable simple cyst. The fluid is drained, usually under local anaesthetic, sometimes with sedation.

Aspiration alone can provide temporary relief, but the cyst may refill. Sclerotherapy adds an agent to damage the fluid-producing lining and reduce recurrence. The team must exclude communication with the urine collecting system before using a sclerosant.

A systematic review of 57 studies reported treatment success in approximately 88% of patients receiving aspiration with sclerotherapy, defined as symptom resolution together with more than 50% cyst-size reduction. Techniques and study quality varied; this is not a guarantee for an individual patient.[5]

Risks include pain, bleeding, infection, leakage of the agent, injury to nearby structures and recurrence. Repeat treatment may be needed.

Laparoscopic cyst deroofing

Deroofing, also called decortication, removes part of the cyst wall so it cannot readily reform as a closed sac. The removed tissue is usually sent for pathology.

Performed through small incisions under general anaesthesia, it may suit large, accessible symptomatic cysts or cysts that recur after percutaneous treatment.

A comparative meta-analysis found lower treatment-failure rates with laparoscopic deroofing than aspiration with sclerotherapy, while percutaneous treatment generally involved shorter procedures, shorter hospital stays and fewer complications.[7]

Surgical risks include bleeding, infection, injury to nearby organs or the collecting system, urine leakage, persistent pain and recurrence. Recovery depends on the procedure and the patient’s health.

Robotic-assisted deroofing

Robotic assistance is another way to perform minimally invasive cyst surgery. It may help with dissection in selected cases, but the robot is operated by the surgeon. Its availability does not mean it is necessary or offers a better result for every cyst. Location, complexity, expertise and cost should guide the choice.[7]

If the cyst is suspicious for cancer

Simple drainage or deroofing is not an appropriate cancer operation. Management may involve surveillance, partial nephrectomy to remove the lesion while preserving kidney tissue, or occasionally removal of the whole kidney. Ablation may be considered in selected patients, although evidence for complex cystic lesions is limited.[4,8]

I no longer perform nephrectomies. If partial or radical nephrectomy is indicated, I will refer you to a colleague who performs this surgery.

Parapelvic cysts: a special situation

Parapelvic cysts are beside the kidney’s central urine collecting system and usually do not communicate with it. They can mimic hydronephrosis, a dilated collecting system, on ultrasound. Contrast imaging, sometimes including delayed images, can distinguish the two and show whether a cyst is compressing urine drainage.[6]

Many asymptomatic benign parapelvic cysts can be observed. Treatment may be considered when a cyst causes pain or demonstrable obstruction.

Options include carefully planned laparoscopic or robotic deroofing, or ureteroscopic internal drainage in selected cases. With ureteroscopy, a telescope passes through the urethra, bladder and ureter into the kidney; a laser opens the cyst into the collecting system. A temporary ureteric stent may be required. Published studies support feasibility, but long-term comparative evidence remains limited.[6,10]

Sclerotherapy is generally avoided for parapelvic cysts because leakage or inflammation near the collecting system can cause scarring and obstruction. Any proposed percutaneous approach requires specialist assessment; routine simple-cyst treatment should not be applied automatically.[6]

Their proximity to blood vessels and the collecting system also increases the technical difficulty of surgery. Risks include bleeding, urine leakage, infection, narrowing and recurrence.

When to seek prompt medical attention

Seek urgent assessment for severe flank pain with fever or rigors, significant bleeding or clots in the urine, difficulty passing urine, or pain accompanied by faintness or marked illness. Persistent visible blood in the urine also requires medical review.

References

1. National Institute of Diabetes and Digestive and Kidney Diseases. Simple kidney cysts.
2. Silverman SG, et al. Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment. Radiology. 2019;292:475–488. Article.
3. European Association of Urology. Renal Cell Carcinoma Guidelines: Diagnostic evaluation.
4. Canadian Urological Association. 2023 update: Management of cystic renal lesions. Canadian Urological Association Journal. Guideline.
5. Brown D, et al. Radiologically guided percutaneous aspiration and sclerotherapy of symptomatic simple renal cysts: a systematic review of outcomes. Abdominal Radiology. 2021. PubMed.
6. Ureteroscopic management of asymptomatic and symptomatic simple parapelvic renal cysts. BMC Urology. 2015. Article.
7. Comparison of aspiration with sclerotherapy and laparoscopic deroofing for the treatment of symptomatic simple renal cysts: a systematic review and meta-analysis. Updates in Surgery. 2021. PubMed.
8. European Association of Urology. Renal Cell Carcinoma Guidelines: Epidemiology, aetiology and pathology and Disease management.
9. NIDDK. What is polycystic kidney disease?.
10. Transurethral flexible ureteroscopic incision and drainage with holmium laser in the treatment of parapelvic renal cysts. International Brazilian Journal of Urology. 2022. Article.

General patient education. Individual management depends on imaging findings, symptoms, kidney function and discussion with the treating team. Reviewed October 2026.