Small kidney mass found by chance: follow-up scans

What the report means

A small renal mass is a solid area in the kidney, 4 cm or smaller, that takes up contrast dye on a scan. (Sebastià 2020) It is not the same as a kidney cyst. Under the Bosniak 2019 system, a mass in which about a quarter or more is tissue that takes up dye is no longer called a cystic mass. (Bosniak 2019) If your report says cyst, see kidney cyst found by chance.

Words a report may use (simplified)
Report wording	What it describes
Small renal mass (SRM)	A solid kidney area 4 cm or smaller that takes up contrast dye (Sebastià 2020)
Enhancing	Takes up contrast dye, a sign of solid tissue rather than fluid (Sebastià 2020)
Indeterminate	The scan could not tell what it is; another type of scan may be suggested (Sebastià 2020)
cT1a	A stage label for a kidney tumour 4 cm or smaller, used in guidelines (Silvestri 2025)

How often they are benign or slow growing

Found by chance. The American Cancer Society says kidney cancers are often found by accident on imaging done for another reason, usually at an early stage. (ACS)
Benign. Reviews say about 20 to 30 in 100 masses under 4 cm are benign. (Silvestri 2025) One review says about a quarter are benign and another quarter are slow growing types with limited ability to spread. (Sebastià 2020)
Growth. A pooled analysis of 18 series (880 people) watched with repeat scans found an average growth of about 0.3 cm a year. About 1 in 4 masses did not grow, and 18 people (about 2 in 100) had spread during follow-up. (Smaldone 2012)

These are results from groups of people, not a forecast for one person.

What comes next

Another scan to describe it. A review notes that a mass that is unclear on CT may be looked at again with a contrast ultrasound or MRI. (Sebastià 2020)
Referral. Your doctor decides whether and when to refer you to a kidney specialist (urologist).
Active surveillance. This means watching the mass with repeat scans and acting only if it changes. Reviews describe it as an option for selected people, often older people or those with other health problems, and some studies include younger people too. (Silvestri 2025)
Repeat scans. In one large study, scans were every six months for two years and then yearly. (Silvestri 2025) A review says the same scan type should be used each time, and scans can be spaced out, or stopped, after 5 years with no growth. (Sebastià 2020)
What counts as a change. Reviews describe growth of more than 0.5 cm a year, a size over 4 cm or signs of spread as reasons to look again at the plan. (Sebastià 2020)(Silvestri 2025)

Your doctor decides which scans fit your report, your health and your history.

Who it applies to

Adults told that a CT, MRI or ultrasound done for another reason shows a small solid area in a kidney. If you notice symptoms, the advice below applies whatever the scan showed.

MedlinePlus, kidney cancer: (MedlinePlus)

See your health care provider if you notice: blood in your urine, a lump in your abdomen, weight loss for no reason, pain in your side that does not go away, loss of appetite.

Routine age-based screening

A scan done for another reason is not a screening test, and there is no routine kidney cancer screening for people at average risk. (ACS) Your other screenings stay on their usual schedule unless your doctor says otherwise.

Kidney cancer risk. MedlinePlus says kidney cancer becomes more likely with age, and lists smoking and certain genetic conditions among the risk factors. (MedlinePlus)
Blood pressure. USPSTF recommends blood pressure screening for all adults 18 and older. (USPSTF blood pressure) See blood pressure screening.
Kidney function. Kidney function tests (eGFR and urine albumin) are a separate question from a mass on a scan. See kidney disease screening and kidney test results.
All screenings by age. See screenings by age and gender.

Where they differ:

Who can be watched. A 2025 review of guidelines reports that the European (EAU), ESMO and Canadian (CUA) guidelines consider surveillance for kidney tumours up to 4 cm, while the American Urological Association limits it to masses under 2 cm. All agree it suits frail people and those with other health problems. (Silvestri 2025)
How often to scan. The same review reports that the American, Canadian and Latin American groups suggest scans every 3 to 6 months in the first year and then every 6 to 12 months, while the European and ESMO guidelines set no schedule. (Silvestri 2025)
When to stop. One review says scans can be spaced to every 2 to 3 years, or stopped, after 5 years with no growth. (Sebastià 2020)

Your doctor decides what your result means for you.

Questions people ask

What is a small renal mass?

Reviews use the term for a solid area in the kidney that takes up contrast dye on a scan and is 4 cm or smaller. It is different from a simple fluid-filled cyst. Most of these masses are found on scans done for another reason. (Sebastià 2020) (Silvestri 2025)

Is a small kidney mass usually cancer?

Not always. Reviews say about 20 to 30 in 100 masses under 4 cm turn out to be benign, and many of those that are cancer grow slowly and rarely spread. Only further tests can tell what a mass is. Your doctor decides. (Silvestri 2025) (Sebastià 2020)

How fast do small kidney masses grow?

Usually slowly. A pooled analysis of people watched with repeat scans found an average growth of about 0.3 cm a year, and about 1 in 4 masses did not grow at all. (Smaldone 2012)

What is active surveillance?

It means watching the mass with repeat scans instead of treating it straight away, and acting only if it grows or changes. Reviews describe it as an option for selected people, often older people or those with other health problems. (Silvestri 2025) (Sebastià 2020)

Is there a screening test for kidney cancer?

No. The American Cancer Society says there are no recommended screening tests for kidney cancer for people at average risk. Your other age-based screenings stay on their usual schedule unless your doctor says otherwise. (ACS)