Complex Kidney Cyst
A complex kidney cyst is a fluid-filled structure within the kidney that contains additional features such as septations, thickened walls, calcifications, or solid components. Unlike simple kidney cysts—which are common and harmless—complex cysts require careful evaluation because certain features can increase the likelihood of underlying disease or malignancy. Many are still benign, but some require follow-up or treatment.
What is it?
A complex kidney cyst is a renal cyst that shows features beyond a simple thin-walled fluid sac. These features may include internal septations, thick or nodular walls, calcifications, or small solid areas. Radiologists use established systems such as the Bosniak classification to determine the likelihood that a complex cyst is benign or requires further follow-up or treatment.
Imaging—especially contrast-enhanced CT or MRI—helps characterize the cyst and evaluate how concerning its features are. MRI is particularly useful when CT findings are unclear or when minimizing radiation exposure is preferred.
Important to Know
Most complex cysts are benign, but certain imaging characteristics increase the risk of malignancy. Cysts with thickened walls, nodules, or enhancement after contrast may need closer monitoring or surgical consultation. Many complex cysts remain stable over time, and regular follow-up imaging can help ensure safety.
Red flag symptoms include flank pain, blood in the urine, fever, or unexpected weight loss—these may indicate complications or a more concerning underlying cause. Decisions about follow-up depend on cyst appearance, growth, and clinical history.