A Kidney Mass Found on a Scan: What It Can Mean and What Happens Next

What it can mean

A kidney mass is any spot on the kidney that looks different from normal kidney tissue. Radiologists describe whether it is a fluid-filled cyst or solid, whether it takes up contrast dye (enhances), and whether it contains fat. These features, along with size, guide what happens next.

Common and usually benign

Simple kidney cyst: Fluid-filled cysts are very common, especially with age. Simple cysts, often reported as Bosniak 1 or 2, are benign and usually need no treatment or follow-up.
Angiomyolipoma: A benign tumor made partly of fat, which can often be recognized on CT or MRI. Small ones are usually just watched; larger ones may be treated because of bleeding risk.
Oncocytoma and other benign tumors: Some solid masses turn out to be benign tumors such as oncocytoma. They can look similar to cancer on imaging, which is one reason a biopsy is sometimes considered.

Less common

Complex cysts: Cysts with thick walls, internal walls (septa) or solid areas are graded on the Bosniak scale. Higher grades are more likely to be cancer and are managed like solid masses.
Infection or inflammation: A kidney infection or abscess can look like a mass. Symptoms such as fever and recent infection usually point toward this.
Imaging artifacts: Sometimes a spot seen on one scan is not confirmed on a dedicated kidney scan with contrast.

What the work-up checks for

Renal cell carcinoma: The most common kidney cancer. Many small renal cell carcinomas grow slowly, and the risk they pose depends on size, appearance and type.
Urothelial cancer of the kidney: Less often, a mass in the central drainage area of the kidney is a urothelial cancer, which is evaluated and treated differently.
Other, rarer causes: Lymphoma or a cancer that spread from elsewhere can occasionally appear as a kidney mass. A biopsy may be recommended when these are suspected.

Solid kidney masses are more likely than simple cysts to be cancer, but not all of them are. The AUA guideline notes that roughly 15 to 20 percent of surgically removed kidney tumors under 4 cm turn out to be benign, and that many small cancers carry a low risk. That is why treatment options range from monitoring to surgery.

The usual work-up

What usually happens next

01A dedicated kidney scanIf the mass was found on an ultrasound or a scan without contrast, the next step is usually a multiphase tomography (CT) or magnetic resonance imaging (MRI) scan, taken before and after contrast, to characterize it properly.
02Blood and urine testsBlood tests check kidney function and blood counts, and a urinalysis looks for blood or protein. Kidney function matters for every treatment decision, because preserving it is a priority.
03Chest imaging, if cancer is suspectedA chest X-ray or chest CT checks whether anything has spread to the lungs. For most small masses this is a routine precaution.
04A conversation with a urologistA urologist reviews the scans with you and explains the options in light of the mass, your kidney function, your other health conditions and your preferences. Other specialists are involved when needed.
05Biopsy, in selected casesA needle biopsy may help when the result would change the plan, for example when deciding between monitoring and treatment, or when lymphoma, infection or spread from another cancer is possible. It is not needed for everyone.
06Genetic counseling, when it appliesGenetic counseling is recommended for people 46 or younger with a kidney cancer, those with masses in both kidneys or several masses, and those with a family history that suggests an inherited syndrome.
07Choosing a planOptions may include active surveillance with repeat imaging, partial nephrectomy (removing only the tumor), radical nephrectomy (removing the kidney), or ablation (destroying the tumor with heat or cold). The right choice depends on the mass and on you.

The order and the tests vary from person to person. Your care team may skip steps or add others.

Guidelines say · AUA Renal Mass and Localized Renal Cancer Guideline, 2021
Recommends high quality, multiphase cross-sectional imaging to characterize a solid or complex cystic kidney mass, including whether it enhances and whether it contains fat, along with blood tests, a urinalysis and chest imaging when cancer is suspected.

Guidelines say · AUA Renal Mass and Localized Renal Cancer Guideline, 2021
Counseling should be led by a urologist and include the low cancer risk of many small kidney masses. Biopsy should be considered whenever it may change management, and nephron-sparing approaches are prioritized when appropriate.

Do not wait

When to seek care urgently

A kidney mass found on a scan is rarely an emergency. Seek care the same day, or go to an emergency department, if you have any of the following:

Sudden, severe pain in your side, back or abdomen, especially with dizziness or fainting, which can mean bleeding from the mass.
Visible blood in the urine with clots, or you cannot pass urine.
Fever and chills with pain in your side.
Sudden shortness of breath, chest pain, or new swelling in one leg.

Bring to your visit

Questions to ask

01Is this a cyst or a solid mass, and does it enhance with contrast?
02If it is a cyst, what is its Bosniak category?
03How large is it, and has it changed compared with any earlier scans?
04Do I need another scan to characterize it better?
05Would a biopsy change the plan in my case?
06Is active surveillance reasonable for me, and how often would I need imaging?
07If treatment is recommended, can the rest of the kidney be preserved?
08How will each option affect my kidney function over time?

Common questions

Frequently asked questions

Does a kidney mass mean I have cancer?

Not necessarily. Simple cysts are very common and benign, and a meaningful share of small solid masses also turn out to be benign. A dedicated scan with contrast is the first step to sorting this out.

What is the Bosniak classification?

It is a scale radiologists use to describe kidney cysts. Simple cysts sit at the low end and are benign. Higher categories have thicker walls, more internal structure or solid parts, and are more likely to be cancer.

Why would anyone watch a kidney mass rather than remove it?

Many small kidney masses grow slowly, and some are benign. For some people, especially those with other health conditions or small masses, regular imaging may be safer than treatment, with treatment offered if the mass grows or changes.

Do I need a biopsy?

Not always. A biopsy is most useful when its result would change the plan. Some people proceed straight to treatment, and others are monitored regardless of the biopsy result. Your urologist can explain whether it would help in your case.

Can I keep my kidney?

Often, yes. When it is technically and medically appropriate, removing only the tumor and preserving the rest of the kidney (partial nephrectomy) is generally preferred, because kidney function matters over the long term.

Was it caused by something I did?

Most kidney masses have no identifiable cause. Smoking, obesity and high blood pressure are associated with kidney cancer, and some people have an inherited tendency, which is why genetic counseling is recommended in certain situations.