Renal Cysts — Simple & Complex | Kidney Surgery Sydney

What is a kidney cyst?

A renal cyst is a fluid-filled sac in or on the kidney. They are extremely common with age — many are found incidentally on scans done for unrelated reasons. The important question for most patients is not "do I have a cyst?" but "what type of cyst, and does it need to be watched or treated?"

Simple vs complex cysts

Simple cysts are thin-walled, round, filled with clear fluid, and have no solid parts. They are almost always benign and usually need no treatment at all.
Complex cysts have one or more features that might — sometimes — suggest cancer: thickened or irregular walls, internal septations, calcifications, or solid (enhancing) areas.

The Bosniak classification

Radiologists grade complex cysts using the Bosniak system. In plain English:

Bosniak categories

Bosniak I — simple cyst. Benign. No follow-up required.
Bosniak II — minimally complex (thin septations or fine calcifications). Benign. No follow-up required.
Bosniak IIF — mildly more features than II. Benign but warrants imaging follow-up ("F" for follow-up).
Bosniak III — indeterminate. About half turn out to be cancer on surgical pathology. Usually treated or biopsied.
Bosniak IV — clearly cystic and has solid enhancing components. Treated as cancer until proven otherwise.

Symptoms — or the lack of them

Most kidney cysts cause no symptoms at all. Large cysts can occasionally cause a dull flank ache or early satiety. Very rarely, a cyst can bleed or become infected, which presents with pain or fever.

When a cyst needs treatment

Indications for treatment include:

Bosniak III or IV cysts — discussed as potential cancer and usually managed with surgery or biopsy.
A symptomatic simple cyst large enough to cause pain or pressure.
An infected or haemorrhagic cyst that is not settling.

Treatment options include keyhole deroofing (for benign symptomatic cysts) or partial nephrectomy (for cystic tumours). Dr Kooner will explain the right approach after reviewing the imaging.

Polycystic kidney disease (brief note)

Autosomal dominant polycystic kidney disease (ADPKD) is a separate, inherited condition in which both kidneys contain many cysts. Primary care is led by a nephrologist. Urological input may be requested for stones, bleeding, pain, or consideration of transplant. See the Kidney Transplant Information page.

Frequently asked questions

Does a kidney cyst mean I have cancer?

Usually not. The large majority of kidney cysts (Bosniak I and II) are benign. Only a small subset (III and IV) raise concern and are evaluated further.

Can a simple cyst turn into cancer?

A truly simple Bosniak I cyst does not evolve into cancer. Cysts that are already complex are followed or treated based on their features.

What is surveillance follow-up?

Periodic imaging (usually CT or MRI, sometimes ultrasound) at 6–12 month intervals for a period, looking for any change in size or appearance. If a cyst remains stable, surveillance can be relaxed.

Should I drain the cyst?

Simple drainage alone (aspiration) is not usually recommended because the cyst fluid reaccumulates. For symptomatic cysts, laparoscopic or robotic deroofing has a much higher long-term success rate.

Renal (Kidney) Cysts