Tumour markers

What it is

Tumour markers are measurable substances in blood, urine, or tissue that can help detect, monitor, or guide treatment for some cancers, but no single marker works as a reliable stand-alone test for all cancers. In practice, the most important point is that tumour marker results usually need to be interpreted alongside imaging, biopsy, symptoms, and clinical examination because many markers can rise in non-cancer conditions too.

A tumour marker may be made by cancer cells themselves, or by normal body tissues reacting to a cancer. Some are proteins measured in blood, such as PSA, CA-125, CEA, AFP, CA 19-9, and beta-hCG. Others are genetic or molecular markers found in tumour tissue or blood, such as HER2, EGFR, BCR-ABL, or circulating tumour DNA. Depending on the cancer type, a marker may help with one or more of these tasks: estimating risk, supporting diagnosis, choosing treatment, checking whether treatment is working, or watching for recurrence after treatment.

It is important to separate tumour markers from screening tests. A few markers have roles in selected screening settings, but most are not recommended for general population cancer screening because false positives and false negatives are common. In India, as elsewhere, oncologists usually order these tests for a specific clinical question rather than as a routine "cancer check."

Type of marker	Examples	Main clinical use
Blood protein markers	PSA, CA-125, CEA, AFP, CA 19-9, beta-hCG	Monitoring treatment, recurrence, sometimes prognosis
Tissue biomarkers	ER/PR, HER2, PD-L1	Choosing targeted or hormonal therapy
Genetic/molecular markers	EGFR, ALK, BCR-ABL, KRAS	Predicting response to specific drugs
Emerging liquid biopsy markers	ctDNA, circulating tumour cells	Selected monitoring and molecular profiling

How it works

Tumour markers work by giving indirect or direct information about a cancer. A blood marker such as CEA or CA 19-9 reflects the amount of a substance released into the bloodstream. If the cancer shrinks with treatment, the level may fall; if the cancer grows, the level may rise. This is why serial measurements, done over time in the same laboratory when possible, are often more useful than a single isolated result.

Tissue and molecular markers work differently. They identify features inside the tumour that affect behaviour or drug sensitivity. For example, HER2 overexpression in breast or gastric cancer can make a patient eligible for HER2-targeted therapy. EGFR mutations in lung cancer can guide use of EGFR inhibitors. BCR-ABL in chronic myeloid leukaemia is both a diagnostic marker and a treatment target.

A key limitation is biological overlap. Benign conditions can also raise marker levels. PSA can rise with benign prostate enlargement or prostatitis. CA-125 can rise with menstruation, endometriosis, pregnancy, or liver disease. CEA may be higher in smokers. AFP can rise in liver inflammation as well as liver cancer. Because of this, tumour markers are best seen as decision-support tools, not proof of cancer by themselves.

Evidence and uses

Tumour markers are most useful when they are linked to a specific cancer type and a specific clinical purpose. Common evidence-based uses include:

1. Monitoring response to treatment: Falling levels may suggest treatment is working; rising levels may suggest resistance or progression.
2. Checking for recurrence: In some cancers, markers can rise before symptoms appear, though this does not always improve outcomes unless effective follow-up action exists.
3. Prognosis and risk stratification: Some markers correlate with tumour burden or aggressiveness.
4. Selecting therapy: Tissue and molecular biomarkers are now central in precision oncology.
5. Supporting diagnosis in selected settings: Usually as an adjunct, not a replacement for biopsy.

Examples commonly used in practice include:

Marker	Common association	Important caution
PSA	Prostate cancer	Also rises in benign prostate conditions
CA-125	Ovarian cancer	Low specificity; many benign causes
CEA	Colorectal and some other cancers	Smoking and inflammation can affect levels
AFP	Liver cancer, germ cell tumours	Can rise in hepatitis and cirrhosis
CA 19-9	Pancreatic and biliary cancers	Not useful as a stand-alone diagnostic test
beta-hCG	Gestational trophoblastic disease, germ cell tumours	Pregnancy also raises it

For many solid tumours, expert reviews and guidelines agree that markers are more valuable for follow-up than for first diagnosis. A normal marker does not rule out cancer. A raised marker does not confirm cancer. This is one reason biopsy remains the standard method for confirming most cancers.

In modern oncology, the term "tumour marker" increasingly includes predictive biomarkers such as HER2, EGFR, ALK, ROS1, BRAF, MSI, and PD-L1. These are especially important because they can change treatment choices directly. In India, many cancer centres now routinely test such biomarkers in breast cancer, lung cancer, colorectal cancer, and blood cancers when treatment decisions depend on them.

Diagnosis / how it's measured

Tumour markers are measured from different sample types:

• Blood: the most common method for PSA, CEA, CA-125, AFP, CA 19-9, beta-hCG
• Urine: used for a smaller number of markers in selected cancers
• Tumour tissue: biopsy or surgery samples for immunohistochemistry, PCR, FISH, or sequencing
• Bone marrow or blood cells: often used in haematologic cancers

Results are usually reported as a number with a laboratory reference range, but the exact cut-off can vary by method and lab. That means patients should avoid comparing values from different laboratories without clinician input. Trend matters more than one number, especially during treatment.

Doctors usually interpret tumour marker tests with these questions in mind:

• Was the marker elevated before treatment started?
• Is the value rising, falling, or stable over time?
• Does the change match symptoms and scan findings?
• Could a non-cancer condition explain the result?

Pre-analytical and technical issues also matter. Timing of the test, recent procedures, liver or kidney disease, smoking status, pregnancy, and assay differences can all affect results. For this reason, tumour marker testing should be ordered for a clear reason and interpreted by the treating team.

When to see a clinician

See a clinician if you have a tumour marker result that is above the reference range, especially if the test was done without a clear explanation. Do not assume that a raised result means cancer, but do not ignore it either. The next step may be repeat testing, examination, imaging, or referral to a specialist.

If you already have cancer, contact your oncology team if a marker that was being followed changes significantly, or if you develop new symptoms such as unexplained weight loss, pain, bleeding, jaundice, persistent cough, or swelling. Patients should also ask why a marker is being ordered and how the result will change management.

Limitations and open questions

The biggest limitation of tumour markers is imperfect accuracy. Many lack enough sensitivity to detect early cancer and enough specificity to distinguish cancer from benign disease. This is why broad "tumour marker panels" for healthy people are generally not recommended.

Another limitation is overtesting. Marker results can create anxiety, trigger unnecessary scans, and lead to false alarms if used without a clinical question. Even in people with known cancer, an early rise in a marker does not always mean treatment should change immediately.

Research is moving toward more precise biomarkers, especially circulating tumour DNA and multi-gene tumour profiling. These approaches are promising, but evidence is still evolving for many uses, especially routine surveillance after treatment. Access, cost, standardisation, and interpretation remain important issues in India and globally.

The practical takeaway is simple: tumour markers can be very useful in the right setting, but they are only one part of cancer care and rarely give a complete answer on their own.