Kidney cysts: simple vs complex

Kidney cysts are fluid-filled sacs that can form within or on the surface of the kidney, and are extremely common, particularly with increasing age. Most are found incidentally on scans done for unrelated reasons and cause no problems.

Simple cysts have thin, smooth walls, contain clear fluid and show no concerning features on imaging, and are almost always harmless. Complex cysts have thicker or irregular walls, internal septations, calcification or solid components, which raise the possibility of cancer and usually need further imaging or follow-up.

Simple cysts develop as a normal part of ageing in most people, without any clear underlying cause. Multiple or unusually numerous cysts can sometimes indicate an underlying condition such as polycystic kidney disease, which has a genetic basis.

Simple kidney cysts are almost always benign and don't require treatment. Complex cysts carry a variable risk of being cancerous depending on their imaging features, which is why they're classified using a standard grading system (the Bosniak classification) to guide follow-up or intervention.

Simple, isolated cysts are not usually inherited. However, numerous cysts affecting both kidneys, especially with a family history, can suggest polycystic kidney disease, which is a genetic condition and may need separate assessment.

Simple cysts generally need no treatment, only occasional monitoring if large or symptomatic. Complex cysts with higher-risk imaging features often require surgical removal or closer surveillance imaging, decided based on their specific classification.

Simple cysts usually need no routine follow-up once confirmed as simple. Complex cysts are typically monitored with repeat imaging (ultrasound, CT or MRI) at intervals determined by their specific risk category, with some requiring surgical assessment.

They become increasingly common with age, and simple kidney cysts are found in a substantial proportion of people over 50 on routine imaging. Most people with simple cysts are completely unaware of them and experience no symptoms.

Classification depends on specific imaging features assessed by a radiologist, including wall thickness, presence of septations, calcification and any solid enhancing components, using the Bosniak scoring system. Your urologist will explain your specific classification and what follow-up, if any, is recommended.

If imaging shows a complex cyst or any features causing concern, urology or renal specialist review is advisable to plan appropriate follow-up. Private specialist consultations can typically be arranged within about two weeks, considerably faster than standard NHS routine referral waits.