What Is a Tumor Marker? Making Sense of a Term You See in Cancer News

The headline problem

Every so often a story appears about a blood test that spots cancer. The phrase in the copy is usually "tumor marker."

It is easy to read that as a yes-or-no answer. It almost never is.

What NCI actually means by the term

NCI's definition is broader than most people expect. A tumor marker is anything present in, or made by, cancer cells, or by other cells responding to cancer or to certain benign conditions, that gives information about a cancer.

Note what that information covers: how aggressive the cancer is, what treatment it may respond to, or whether it is responding.

Traditionally these were proteins made in larger amounts by cancer cells than by normal ones. They turn up in blood, urine, stool, tumor tissue, or other fluids.

Increasingly they are genomic. NCI names tumor gene mutations, patterns of gene expression, and non-genetic changes in tumor DNA. These sit in tumors and in the fragments a tumor sheds into body fluids.

Some markers point to one cancer. Others span several.

What they get used for

NCI lists seven jobs.

• Helping to diagnose cancer, always alongside other tests.
• Indicating the type of cancer.
• Indicating the stage.
• Estimating prognosis.
• Showing which treatment may be effective.
• Showing how well a treatment is working.
• Showing whether cancer has come back.

The fifth job has its own name. A marker that says whether someone is a candidate for a targeted therapy is often called a biomarker for cancer treatment. Those are usually measured in tumor tissue. Tumors also shed cells and molecules into blood, which tests called liquid biopsies pick up. Our page on biomarker testing covers how those results steer treatment.

The last two jobs explain why the same test gets repeated. Serial measurements during treatment show whether a tumor is responding, and periodic testing afterward checks for recurrence.

Some real examples

NCI's list of markers in common use makes the variety concrete.

CA-125 is a blood test used in ovarian cancer. CA19-9 is a blood test for pancreatic, gallbladder, bile duct and gastric cancers, used to see whether treatment is working. CA15-3 and CA27.29 do that job in breast cancer, and check for recurrence.

Alpha-fetoprotein in blood is used in liver cancer, ovarian cancer and germ cell tumors, to help diagnose and to follow treatment.

The BCR-ABL fusion gene, also called the Philadelphia chromosome, is measured in blood or bone marrow in chronic myeloid leukemia. It confirms the diagnosis, predicts response to targeted therapy, and tracks the disease.

BRCA1 and BRCA2 mutations, from blood or tumor, help decide treatment in breast, ovarian, pancreatic and prostate cancer.

Very different molecules, very different jobs. Tumor marker is a category, not a test.

The sentence to remember

NCI puts it directly: having an elevated level of a tumor marker does not mean that someone has cancer.

Two things break the link in both directions. Noncancerous conditions can raise a marker. And not everyone with a given cancer has a raised level of the marker associated with it.

That is why marker measurements are combined with biopsies and imaging rather than read alone. Our page on tumor markers works through what a single result can and cannot support.

Why they are poor screening tests

The hope was obvious. If a tumor makes something measurable in blood, why not test everyone?

NCI reports what studies found. Circulating tumor markers generally do not work well for screening. They often miss people who have the disease, meaning they are not sensitive enough. Or they flag cancer in people who do not have it, meaning they are not specific enough.

Both failures cost something. A missed cancer is false reassurance. A false alarm leads to scans, biopsies and months of fear.

Where multi-cancer detection tests stand

The current effort is multi-cancer detection tests. These check several biomarkers at once in the blood of people without symptoms. Most look at DNA that tumor cells release into blood. Some also look at proteins.

NCI's assessment is careful. Many are in development, several are already being sold, and much remains to be learned about how best to use them and about their harms and benefits.

Then the question that matters. Would treating the cancers these tests find actually reduce deaths from those cancers? NCI says it will launch a clinical trial to find out. Our page on multi-cancer early detection blood tests covers what is known so far.

NCI's Early Detection Research Network is working on a related problem. It wants markers that tell aggressive early cancers apart from slow ones that would never cause symptoms. That would cut overtreatment.

When to get checked

A tumor marker is not the route in for most people. Symptoms and established screening programs are.

The US Preventive Services Task Force gives colorectal cancer screening a grade A for adults aged 50 to 75 and a grade B for those aged 45 to 49. It gives biennial screening mammography a grade B for women aged 40 to 74. It gives annual low-dose CT lung screening a grade B for adults aged 50 to 80 with a 20 pack-year smoking history who smoke now or quit within 15 years.

Outside those programs, the prompts are symptoms that persist. A lump that does not go away. Weight loss with no cause. Bleeding with no cause. A cough or hoarseness past three weeks. A change in bowel habits lasting more than a few weeks.

If a marker test has already been done, ask what it measures, what else could explain the result, and how it will be read alongside other tests.

What to keep in perspective

A result is a number in a context, not a verdict.

A rising marker during treatment is meaningful. A single value pulled out of the sequence often is not.

Marketing moves faster than evidence. A test you can buy is not the same as a test shown to help.

And no marker replaces a clinician who knows the rest of the picture.

Sources

• NCI, Tumor Markers fact sheet — https://www.cancer.gov/about-cancer/diagnosis-staging/diagnosis/tumor-markers-fact-sheet
• NCI, Tumor Marker Tests in Common Use — https://www.cancer.gov/about-cancer/diagnosis-staging/diagnosis/tumor-markers-list
• US Preventive Services Task Force, Breast Cancer: Screening — https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/breast-cancer-screening
• US Preventive Services Task Force, Colorectal Cancer: Screening — https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/colorectal-cancer-screening

How this article was prepared

An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed.

The National Cancer Information Foundation publishes Cancer Explained. This page is for learning. It is not medical advice and does not suggest a test or treatment.

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