Renal Cysts

Renal cysts are simple or complex, and are one of the most common incidental findings on CT — renal lesions (mostly cysts) turn up on 14% of CT scans, and incidence rises with age. Most cysts are benign.

• Simple cysts increase in frequency with age and are a normal variant
• Complex cysts carry an increased malignancy risk — some require repeat imaging or biopsy

If a cyst doesn’t appear simple on ultrasound, the next step is contrast-enhanced CT (CECT) — helpful whenever there’s uncertainty about how a non-contrast CT should be classified.

Bosniak classification (version 2019)

The Bosniak classification characterises renal cysts, predicts malignancy risk and guides management. Radiological features are based on contrast-enhanced CT unless otherwise specified.

Category	Features	Malignancy risk	Management
I (simple)	Thin, smooth wall (≤2mm), might enhance, no septa, no calcification, no solid component	0%	No follow-up
II	A few thin septa with fine calcifications, <3cm	Benign	No follow-up
IIF	More complex — may have nodules, no contrast enhancement; includes totally intrarenal non-enhancing high-attenuation lesions >3cm	Requires follow-up imaging	Active surveillance (initially 6-monthly, extending up to 12-monthly, for a total of 5 years); referral to urologist if symptomatic, unclear diagnosis, or metastasis is a concern
III	≥1 thick (≥4mm) or irregular enhancing wall/septa, without nodular enhancement	50%	Referral to urologist for consideration of surgery, or active surveillance in selected cases (urologist/MDT decision)
IV	≥1 enhancing nodule (focal enhancing convex protrusion of any size with acute margins, or ≥4mm)	90–100%	Referral to urologist for consideration of surgery

Bosniak IIF cysts carry a 5–10% risk of malignancy, and 12% of them progress to Bosniak III/IV during follow-up — this is why they get active surveillance rather than being dismissed as benign or referred immediately. A patient with a stable Bosniak IIF cyst can be surveilled in general practice, with re-referral to a urologist triggered by an increase in size, new symptoms, or concern for metastatic disease.

Referral urgency for Bosniak III/IV cysts: category 2 (seen within 90 days) if the lesion is <4cm; category 1 (seen within 30 days) if the lesion is >4cm or there is concern of metastatic disease.