Renal Cyst
A renal cyst is a fluid-filled sac in or on the kidney. Simple renal cysts are extremely common, particularly in older adults, and are usually harmless. Complex cysts are less common and may have features (such as thickened walls, internal structures, or contrast enhancement) that require closer evaluation to distinguish benign findings from possible kidney cancer. Cysts can also be associated with inherited conditions such as autosomal dominant polycystic kidney disease. Most renal cysts are identified incidentally on imaging done for other reasons. Evaluation combines imaging characterization (often using the Bosniak classification), kidney function testing, and, in selected cases, follow-up imaging or treatment.
What is it?
A renal cyst is a fluid-filled sac in or on the kidney. Renal cysts vary widely in their characteristics, causes, and clinical significance. The vast majority are simple cysts—thin-walled, fluid-filled sacs that are benign, cause no symptoms, and require no treatment. A smaller number are complex cysts with features such as thickened walls, internal septations, calcifications, or areas that take up contrast on imaging; these require closer evaluation because a small proportion may represent kidney cancer. Cysts can also be part of inherited conditions such as autosomal dominant polycystic kidney disease (ADPKD), in which many cysts develop in both kidneys over time and can lead to progressive kidney enlargement and, eventually, kidney failure.
Simple renal cysts are extremely common and become more frequent with age. They are identified in a significant proportion of adults on ultrasound, CT, or MRI, with prevalence increasing from roughly 5–10% in younger adults to over 25% in adults over 50 and higher rates in older age groups. Simple cysts typically appear as well-defined, round or oval, thin-walled fluid collections with the same signal characteristics as water on imaging, and they do not take up contrast material. They are almost always benign and do not require any specific treatment or ongoing follow-up.
Complex renal cysts have one or more features that distinguish them from simple cysts. These may include thickened or irregular walls, internal septations (walls dividing the cyst into compartments), calcifications, higher-than-water density on CT, or areas of enhancement (contrast uptake) that suggest the presence of solid tissue. Because a small subset of complex cysts may represent kidney cancer, complex cysts are classified using the Bosniak system, which stratifies them by imaging appearance and estimated risk of malignancy: Bosniak I (simple cyst, benign), Bosniak II (minimally complicated, benign), Bosniak IIF (indeterminate but favor benign, requiring follow-up), Bosniak III (indeterminate, moderate risk of malignancy, often surgically evaluated), and Bosniak IV (clearly malignant features, treated as cancer). The Bosniak classification has been refined over time, most recently in an updated 2019 version, and is applied primarily on contrast-enhanced CT or MRI.
Autosomal dominant polycystic kidney disease (ADPKD) is an inherited disorder in which many cysts develop in both kidneys, typically enlarging over decades and gradually replacing normal kidney tissue. It is one of the most common inherited kidney disorders, affecting roughly 1 in 400 to 1 in 1,000 people, and is caused by mutations in the PKD1 or PKD2 genes (with PKD1 mutations generally causing more severe disease with earlier onset of kidney failure) or, less commonly, other genes. ADPKD is a distinct condition from having a few simple cysts, and is diagnosed based on a combination of imaging findings, family history, and, when needed, genetic testing. It can also involve cysts in the liver, pancreas, and other organs, as well as increased risk of intracranial aneurysms, valvular heart disease, and other findings.
Autosomal recessive polycystic kidney disease (ARPKD) is a much rarer inherited disorder that typically presents in infancy or childhood and is a separate entity from ADPKD. Other cystic kidney conditions include tuberous sclerosis, von Hippel-Lindau disease, medullary cystic kidney disease, and acquired cystic kidney disease (which develops in patients with advanced chronic kidney disease, particularly those on long-term dialysis).
Most renal cysts cause no symptoms and are identified incidentally on imaging done for other reasons. When symptoms do occur, they may include flank or back pain (particularly with large cysts or when bleeding into a cyst occurs), blood in the urine, high blood pressure, urinary tract infections, and, uncommonly, symptoms from cyst rupture or infection. In ADPKD, symptoms typically develop gradually and can include flank pain, high blood pressure (often at younger ages), blood in the urine, urinary tract infections, kidney stones, and, over time, progressive kidney enlargement and reduced kidney function.
Diagnosis of a renal cyst is based on imaging. Ultrasound is often the first-line test and can identify simple cysts with high reliability. However, ultrasound has limitations for characterizing complex cysts, and CT or MRI with and without contrast is generally used for more detailed evaluation. The Bosniak classification is applied primarily on contrast-enhanced CT or MRI. In patients with kidney function limitations or contrast contraindications, MRI with specific protocols or, in selected cases, contrast-enhanced ultrasound may be used.
When ADPKD is suspected, the diagnosis is based on imaging criteria (using age-specific thresholds for the number of cysts in each kidney), family history, and, in some cases, genetic testing. Genetic testing may also be useful for family planning and for evaluation of at-risk family members.
Blood tests for kidney function (creatinine and eGFR), urine tests for protein and blood, and blood pressure measurement are standard components of evaluation. Additional testing may be appropriate depending on the specific findings and clinical context.
Important to Know
Management of renal cysts depends heavily on the specific type of cyst, its imaging characteristics, whether symptoms or complications are present, and whether an underlying inherited condition is identified. Care is typically coordinated by primary care clinicians, nephrologists, urologists, and, when needed, geneticists, radiologists, and other specialists.
For simple renal cysts (Bosniak I) identified incidentally with no symptoms, no specific treatment is required, and no follow-up imaging is typically needed. Reassurance about the benign nature of the finding is often sufficient. Simple cysts do not need to be drained, monitored, or otherwise addressed in the absence of symptoms or complications.
For Bosniak II cysts, the imaging appearance is minimally complex but still considered benign, and typically no specific follow-up is needed.
For Bosniak IIF cysts, follow-up imaging (typically CT or MRI at intervals such as 6, 12, and 24 months, and then periodically) is recommended to confirm stability. Most Bosniak IIF cysts remain stable, and only a small proportion progress to a higher Bosniak category over time.
For Bosniak III cysts, further evaluation and management options include surgical treatment (partial nephrectomy or, less commonly, radical nephrectomy), active surveillance in selected patients (particularly older patients or those with significant comorbidities), or, occasionally, image-guided biopsy. The decision is individualized based on patient age, health status, risk factors, and preferences.
For Bosniak IV cysts, which have imaging features clearly consistent with malignancy, treatment typically follows kidney cancer principles and often involves partial or radical nephrectomy or, in selected patients, thermal ablation techniques.
For large symptomatic simple cysts causing significant pain or other complications, treatment options include cyst aspiration with sclerotherapy (drainage followed by injection of an agent to prevent recurrence) or, less commonly, surgical decortication (removal of part of the cyst wall). These procedures are considered when symptoms clearly relate to the cyst.
For patients with ADPKD, management includes several components. Blood pressure control is one of the most important aspects, with ACE inhibitors or ARBs commonly used. Tolvaptan, a medication that can slow the progression of ADPKD in appropriately selected adults with rapidly progressive disease, has become a standard option since its approval. Careful attention to hydration, avoidance of medications that can harm the kidneys, management of pain, treatment of infections and kidney stones, and periodic assessment of kidney function are also important. Screening for intracranial aneurysms is considered in patients with a family history of aneurysms or specific risk factors, following current guidelines. In advanced disease, preparation for kidney replacement therapy (dialysis or transplant) becomes important. Genetic counseling is valuable for family planning and for family members who may benefit from evaluation.
For patients with cysts associated with tuberous sclerosis, von Hippel-Lindau disease, or other syndromes, management is coordinated with the broader care of the underlying condition.
For patients with acquired cystic kidney disease (which develops in patients with advanced chronic kidney disease, particularly on long-term dialysis), periodic imaging may be considered because of an increased risk of kidney cancer, though specific surveillance protocols vary.
Care during pregnancy in patients with renal cysts—including ADPKD—requires attention to blood pressure control, kidney function monitoring, and, in ADPKD, awareness of the increased risk of certain pregnancy complications. Coordination with obstetrics, nephrology, and, when appropriate, maternal-fetal medicine is important.
Care is typically coordinated by primary care clinicians and, when needed, nephrologists, urologists, and other specialists. Imaging and laboratory findings are interpreted alongside the patient’s symptoms, examination, family history, and broader clinical context rather than in isolation.
Patient education plays an important role. Understanding that simple renal cysts are common and usually harmless, the meaning of complex cyst classifications, the rationale for recommended monitoring or treatment, the implications of inherited conditions when present, and warning signs of complications all contribute to appropriate care.
Red flag symptoms include severe flank or back pain, high fever with signs of severe urinary tract infection or possible kidney infection (chills, back pain, and nausea), significant decrease in urine output, significant blood in the urine, severe abdominal pain, sudden severe headache (which may suggest a ruptured intracranial aneurysm in ADPKD patients), symptoms of severe electrolyte abnormalities, or rapid clinical deterioration. These warrant prompt or urgent medical evaluation, as they may indicate cyst rupture or infection, obstruction, aneurysm rupture (in ADPKD), or other serious problems.