A cortical cyst is a fluid-filled sac that forms in the outer layer (cortex) of the kidney. These are the most common type of kidney mass found on imaging, and the vast majority are completely benign. They become increasingly common with age, showing up in about 23% of people in their 50s and roughly 33% of people over 60. If you’ve just had an ultrasound or CT scan that mentions a cortical cyst, the finding is almost always harmless and requires no treatment.
Where Cortical Cysts Form
Your kidneys have two main layers: an outer cortex and an inner medulla. The cortex contains the filtering units that clean your blood. Cortical cysts develop specifically in this outer layer, which is why imaging reports use the term “cortical” to pinpoint the location. They can appear in one kidney or both, and you can have a single cyst or several.
These cysts are distinct from polycystic kidney disease (PKD), a genetic condition where hundreds of cysts grow throughout both kidneys over time, eventually enlarging them and impairing their function. A simple cortical cyst doesn’t change kidney size or affect other organs. Most people who have one or two cortical cysts have entirely normal kidney function.
How Cortical Cysts Develop
Simple cortical cysts form when a tiny tube inside the kidney becomes partially blocked or weakened. The kidney’s filtering system is made up of thousands of microscopic tubes called tubules. When part of a tubular wall weakens, or when pressure inside a segment of the tube rises due to a small obstruction, the wall can balloon outward and gradually fill with fluid. Think of it like a weak spot on a garden hose that slowly bulges when the water pressure increases.
The likelihood of developing these cysts rises with age, nearly doubling every decade. Men develop them more frequently than women. High blood pressure is also associated with cyst formation, and larger or more numerous cysts correlate with a greater risk of hypertension, though the direction of that relationship isn’t fully settled. Kidney ischemia (reduced blood flow to kidney tissue) and changes in how the kidney handles certain minerals may also play a role.
Symptoms and Complications
Most cortical cysts cause no symptoms at all. They’re typically discovered incidentally when imaging is done for an unrelated reason. When a cyst does grow large enough to cause problems, the most common symptoms are a dull ache in the back or side, discomfort in the upper abdomen, or a sense of fullness.
Complications are uncommon but possible. A cyst can become infected, causing fever and localized pain. Less often, a cyst can rupture, which typically causes sudden, sharp pain in the back or side and sometimes blood in the urine. In rare cases, a large cyst positioned near the kidney’s drainage system can partially block urine flow, leading to swelling of the kidney.
How Doctors Classify Cysts
When a cyst shows up on imaging, radiologists use a grading system called the Bosniak classification to sort it into one of five categories. This system determines whether the cyst is clearly harmless, needs monitoring, or warrants further investigation.
- Bosniak I: A simple cyst with thin walls, no internal dividers (septa), no calcium deposits, and no solid components. Malignancy risk is 0%. No follow-up needed.
- Bosniak II: A minimally complex cyst that may have a few thin internal dividers or fine calcium deposits, but nothing that enhances (lights up) on contrast imaging. Malignancy risk is 0% to 6%. No follow-up needed.
- Bosniak IIF: A cyst with slightly thickened walls or multiple thin dividers that needs periodic imaging to watch for changes. The “F” stands for follow-up. About 13% of these cysts show progression on imaging over roughly 18 months, and when they do progress, the malignancy rate is high (around 85%).
- Bosniak III: An indeterminate cyst with irregularly thickened walls, measurable enhancement on contrast imaging, and thick calcium deposits. About 50% of these turn out to be malignant.
- Bosniak IV: A cyst with clearly enhancing solid nodules inside it. These are treated as malignant.
The overwhelming majority of cortical cysts fall into categories I and II. If your imaging report describes a “simple cortical cyst” or a “Bosniak I cyst,” there is essentially zero chance it is cancerous.
How Cysts Are Detected
Ultrasound is typically the first imaging tool that picks up a cortical cyst. On ultrasound, a simple cyst appears as a dark (fluid-filled), well-defined circle with a bright signal behind it. When all three of these features are present, the cyst is confidently classified as simple and benign.
If the ultrasound shows anything unusual, like internal dividers, thickened walls, or solid areas, a CT scan with contrast dye is the next step. The CT allows radiologists to measure whether parts of the cyst are “enhancing,” meaning they take up contrast dye the way living tissue does. Enhancement is the key feature that separates a cyst with a small amount of debris from one that may contain abnormal tissue growth. MRI can serve a similar role when CT isn’t appropriate.
When Treatment Is Needed
Simple cortical cysts (Bosniak I and II) require no treatment and no follow-up imaging. They can be safely ignored. Treatment only comes into play in two situations: the cyst is causing symptoms, or imaging suggests it could be something more serious (Bosniak III or IV).
For symptomatic cysts, two main approaches exist. The less invasive option is aspiration with sclerotherapy: a needle drains the fluid, and a solution is injected to collapse the cyst walls. This is quicker and cheaper but has a higher recurrence rate. The more definitive option is laparoscopic decortication, a minimally invasive surgery where the cyst wall is removed. Both procedures relieve symptoms at similar rates, but the surgical approach is less likely to require retreatment. The choice between them typically depends on cyst size, location, and individual circumstances.
For Bosniak IIF cysts, the standard approach is periodic imaging, often at 6 and 12 months initially, then annually. If the cyst remains stable over several years, monitoring can be spaced out. If it progresses to a higher category, surgical removal is considered. Bosniak III and IV cysts are generally managed surgically because of their significant malignancy risk.
Cortical Cysts vs. Other Kidney Conditions
A simple cortical cyst is not the same as polycystic kidney disease. PKD is an inherited condition diagnosed through imaging and family history, involving numerous cysts that progressively enlarge the kidneys and can lead to kidney failure. A person with one or a handful of simple cysts does not have PKD.
Cortical cysts also differ from acquired cystic kidney disease (ACKD), which develops in people who already have chronic kidney disease or are on dialysis. In ACKD, multiple cysts form in kidneys that are already damaged, but the kidneys remain normal-sized, unlike in PKD.
If your imaging report mentions a “simple cortical cyst” or “Bosniak category I cyst,” it is describing one of the most common and benign findings in kidney imaging. For most people, no action is needed beyond whatever prompted the scan in the first place.