Kidney Cyst - When a Cyst Needs Action

The vast majority of kidney cysts - simple cysts (Bosniak I and II) - are completely benign and require no treatment. They are a very common incidental finding on ultrasound or CT scans done for other reasons, found in approximately 20–30% of adults over 50 and more than 50% of adults over 70. Simple cysts do not become cancer and do not need treatment or surveillance. However, complex cysts (Bosniak IIF, III, and IV) have features that may indicate a higher risk of malignancy and require imaging surveillance or surgical removal. The Bosniak classification - assigned from your CT scan - determines whether your cyst is a concern.

The Bosniak classification (2019 version) uses CT or MRI features to categorise renal cysts by malignancy risk. Bosniak I: simple thin-walled cyst, no septae, no calcification - 0% malignancy risk. No follow-up needed. Bosniak II: few thin septae, small calcification - approximately 5% malignancy risk. No follow-up needed. Bosniak IIF: multiple thin septae, moderate calcification, no enhancing elements - approximately 15% malignancy risk. Follow-up CT/MRI at 6 months, then 2–5 years. Bosniak III: thick or irregular septae, enhancement - approximately 50–60% malignancy risk. Discuss surgery or close surveillance. Bosniak IV: clearly enhancing soft-tissue elements - approximately 80–90% malignancy risk. Surgery recommended.

Simple Bosniak I/II cysts do not transform into cancer - they are lined by normal kidney cells and have no malignant potential. They may slowly enlarge over years but are not a cancer risk. Complex cysts (Bosniak IIF, III, IV) may contain a cystic renal cell carcinoma - a kidney cancer growing within a cystic structure. This is why the Bosniak classification matters: it identifies which cysts have features suspicious for cancer and which can be confidently left alone. A cyst that "looked simple" on ultrasound but appears complex on CT requires further characterisation.

The majority of simple kidney cysts cause no symptoms - they are found incidentally. When symptoms do occur, they include: loin pain or discomfort (usually only with large cysts >4–5cm pressing on surrounding structures), hypertension (rarely - from pressure on renal vasculature), haematuria (blood in urine - rare; when present, requires investigation). Infected cysts cause fever and loin pain (rare but require IV antibiotics and sometimes drainage). Very large cysts (>10cm) may cause a visible or palpable flank fullness. Most patients with large simple cysts remain completely asymptomatic.

Treatment is needed when: (1) Bosniak III or IV - suspected cystic renal cell carcinoma requiring surgery (partial nephrectomy if kidney-sparing is possible). (2) Symptomatic simple cyst - causing significant pain, obstruction to urinary drainage, or hypertension that does not resolve with medication. Treatment options for symptomatic benign cysts: aspiration and sclerotherapy (injection of a sclerosing agent into the cyst after aspiration - effective for simple cysts, 70–80% success); laparoscopic cyst decortication (removing the exposed cyst wall laparoscopically - most durable treatment, minimal recovery). Observation and reassurance are appropriate for asymptomatic simple cysts of any size.

A simple renal cyst is a fluid-filled sac arising from the outer cortex or medulla of the kidney - the most common type. A parapelvic cyst arises near the renal pelvis (the collecting system) and may be confused with hydronephrosis on ultrasound. Parapelvic cysts can occasionally cause obstruction of the ureter or collecting system, leading to back pressure and hydronephrosis. Unlike simple cortical cysts, significant parapelvic cysts sometimes require treatment even when "simple" in appearance, because of their proximity to the collecting system. CT urogram is the best investigation to distinguish parapelvic cysts from hydronephrosis.