Kidney cyst

Kidney cysts are frequent incidental findings when CT or ultrasonography is performed. We distinguish simple cysts from complex cysts based on their radiological features according to the Bosniak classification. Most kidney cysts are simple.

Simple kidney cysts

Simple kidney cysts are benign thin-walled and fluid filled structures. There are no septa, calcifications, or solid components. These are Bosniak I cysts.

More than 25% of adults have simple kidney cysts. Most of them are harmless and require no follow-up or intervention. Intervention is only indicated if the cyst is at high risk for complications, or if it causes symptoms.

Complex kidney cysts

Cysts with varying degrees of septae, calcifications, enhancing (with CT contrast) or solid components are called complex. Bosniak II cysts have the least amount of these, while Bosniak IV have the most. Bosniak III and IV have >40 and >80% risk of malignancy and should be followed with imaging for progression or removed.