Simple Renal Cyst
A simple renal cyst is the commonest benign renal lesion — anechoic, thin-walled, with posterior acoustic enhancement and no septa or enhancing component — and the correct response to a classic one is reassurance, treating only symptomatic cysts.
The big picture
A simple renal cyst is a fluid-filled, epithelium-lined sac that increases in prevalence with age. On ultrasound it is anechoic (no internal echoes), has a thin imperceptible wall, shows posterior acoustic enhancement (through-transmission), and has no septa, calcification or solid/enhancing component. It corresponds to Bosniak I and is benign.
A cyst meeting all simple-cyst criteria is benign — reassure; only intervene if it is symptomatic, and re-characterise any cyst that is not truly simple.
Red flags
Not a simple cyst — characterise with contrast imaging and grade (Bosniak).
Consider an infected cyst — antibiotics ± drainage.
High recurrence — combine with sclerotherapy.
Symptom sorter
The common presentation.
Diagnostic algorithm
Each step answers one question. Tap to expand.
Treatment ladder
Reassure benign simple cysts; reserve intervention for symptomatic cysts and re-characterise anything atypical.
Complications
- Haemorrhage into a cyst, infection, mass effect (large cysts)
- Recurrence after aspiration; sclerotherapy/surgery complications
- Confirm truly simple before reassuring; use sclerotherapy (not aspiration alone) for symptomatic cysts
- Drain infected cysts; re-treat recurrent symptomatic cysts
Summary tables
Simple renal cyst at a glance
| Item | Detail |
|---|---|
| Ultrasound criteria | Anechoic, thin wall, posterior acoustic enhancement, no septa/enhancement |
| Bosniak | Category I (benign) |
| Default management | Reassurance, no follow-up |
| Symptomatic | Aspiration + sclerotherapy or deroofing |
| If atypical | Contrast imaging + Bosniak grading |
Memory hooks
Anechoic + thin wall + posterior enhancement + no septa = simple cyst.
Classic simple cyst → reassure, no follow-up.
Treat only if symptomatic.
Aspiration alone recurs — add sclerotherapy.
Board traps
Classic simple cyst → reassurance (no follow-up).
Atypical features → not simple; grade it.
Symptomatic cyst recurs after plain aspiration.
Clinical cases
A 58-year-old has a 3 cm anechoic, thin-walled renal lesion with posterior acoustic enhancement and no septa, found on an abdominal ultrasound for unrelated symptoms.
What is the appropriate management?
A patient has a large simple cyst causing persistent flank pain. Previous simple aspiration relieved symptoms briefly but the cyst recurred.
What is the better treatment?