Tumour markers are substances made in excess in the body when cancer or a benign (harmless) condition is present. Tests done on blood or other body fluids can find tumour markers.

Some tumour markers can help the doctor diagnose certain cancers. And tumour markers often help the doctor track a person's response to treatment. For example, a woman with ovarian cancer may have a high CA 125 level when she is first diagnosed. After treatment, her levels of CA 125 should fall. Then if her tumour marker level goes up in the future, it could mean that the cancer has come back.

Some tumour markers help doctors choose the most effective treatment. For example, a person who has non-small cell lung cancer may have a tumour sample checked for the KRAS gene mutation to see if a certain kind of targeted therapy will work.

Tumour markers include:

• ALK gene rearrangements. This is a marker for non-small cell lung cancer and anaplastic large cell lymphoma.
• Cancer antigen 125 (CA 125). This is a marker for ovarian cancer.
• Cancer antigen 15-3 (CA 153). This is a marker for breast cancer.
• Carcinoembryonic antigen (CEA). This is a marker for breast cancer and colorectal cancer.
• Epidermal growth factor receptor (EGFR) mutation. This is a marker for non-small cell lung cancer.
• KRAS mutation. This is a marker for colorectal cancer and non-small cell lung cancer.
• Prostate-specific antigen (PSA). This is a marker for prostate cancer.