Kidney Mass: What a Renal Mass on a Scan Means

What a kidney mass means

"Mass" simply means something is taking up space where normal kidney tissue should be. Radiologists use it, or "lesion", when they cannot yet say exactly what it is. The word does not mean cancer. Kidney masses are very common because so many people now have abdominal scans, and most are found before they cause symptoms.

Radiologists first decide whether the mass is:

• cystic (filled with fluid), or
• solid (made of tissue).

That single distinction drives what happens next.

What a kidney mass could be

Cysts and the Bosniak classification

Doctors grade cystic masses with the Bosniak classification:

• Bosniak I and II: simple or minimally complex cysts. Benign, no follow-up needed.
• Bosniak IIF: slightly more complex; follow-up scans over about 5 years to confirm stability.
• Bosniak III: thick walls or septa that enhance with contrast. Roughly half are cancers, so surgery or active surveillance is discussed.
• Bosniak IV: cysts with solid enhancing parts. Most are cancers and are usually treated.

Solid masses

A solid mass that "enhances" (lights up after contrast dye) is presumed to be renal cell carcinoma until proven otherwise. Size matters: the smaller the mass, the more likely it is benign or slow-growing. Masses under 4 cm (about 1.5 inches) are called small renal masses. Many grow only a few millimeters a year.

An angiomyolipoma can often be diagnosed confidently on CT because it contains visible fat. Multiple angiomyolipomas suggest tuberous sclerosis. Multiple cysts and solid tumors in younger people suggest von Hippel-Lindau disease. Many cysts in both kidneys, enlarging the kidneys, suggest polycystic kidney disease.

Symptoms

Most kidney masses cause no symptoms. When they do:

• blood in the urine, visible or found on a urine test
• a dull ache or pain in the side or back (flank pain)
• a lump felt in the side of the belly, mainly in thin people or children
• weight loss, fever, night sweats, tiredness or anemia with advanced cancer
• high blood pressure, high calcium or a high red cell count, produced by some tumors

The classic triad of blood in urine, flank pain and a palpable mass is now uncommon because masses are found earlier.

How a kidney mass is investigated

1. Dedicated imaging: a CT scan with a renal mass protocol (scans before and after contrast) or an MRI. Contrast-enhanced ultrasound is an option for people who cannot have CT or MRI contrast.
2. Blood tests: kidney function (creatinine and eGFR), blood count and calcium.
3. Urine test for blood.
4. Chest imaging if cancer is suspected, to check for spread.
5. Biopsy: a needle biopsy is used selectively, for example when the result would change treatment, for small masses being considered for surveillance or ablation, or when lymphoma or infection is possible.

Referral to a urologist is standard for any solid or complex cystic mass.

Treatment options

• No follow-up: simple cysts and Bosniak II cysts.
• Active surveillance: repeated scans for small masses, especially in older people or those with other serious health problems. Treatment starts if the mass grows quickly.
• Thermal ablation: freezing (cryoablation) or heating (radiofrequency or microwave) a small tumor through a needle.
• Partial nephrectomy: removing the tumor while saving the rest of the kidney. This is preferred for most small cancers because it protects kidney function.
• Radical nephrectomy: removing the whole kidney for large or central tumors.
• Embolization: blocking the blood supply of a bleeding angiomyolipoma.

Read more about stages and treatment of kidney cancer.

Frequently asked questions

What does kidney mass mean?

A kidney mass means a scan has found an area in the kidney that looks different from normal kidney tissue. It may be a fluid-filled cyst, a benign tumor, a cancer or an infection. It is a description, not a diagnosis, and further imaging is usually needed to say what it is.

Is a kidney mass always cancer?

No. Most kidney masses are simple cysts, which are harmless. Of solid masses, the majority are kidney cancers, but about 20% of small solid masses turn out to be benign. Specialized CT or MRI, and sometimes a biopsy, help decide.

What is a complex kidney cyst?

A complex cyst has features beyond thin fluid, such as walls, internal divisions, calcium or solid parts. Radiologists grade them with the Bosniak system. Low grades are benign, while higher grades carry a higher chance of cancer and may need surgery or follow-up scans.

How fast do kidney masses grow?

Most small solid kidney masses grow slowly, often a few millimeters a year, which is why surveillance is safe for many people. Fast growth during surveillance is a reason to treat. Your doctor will compare scans over time to measure the growth rate.

What size of kidney mass is concerning?

Size matters, but appearance matters more. Solid enhancing masses of any size need specialist assessment. Masses larger than about 4 cm are more likely to be cancer and to be treated. Cysts are judged by their internal features rather than their size.