Tumor 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 tumor markers.

Some tumor markers can help the doctor diagnose certain cancers. And tumor 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 tumor marker level goes up in the future, it could mean that the cancer has come back.

Tumor markers include:

• Cancer antigen 15-3 (CA 153). This is a marker for breast cancer.
• Cancer antigen 125 (CA 125). This is a marker for ovarian cancer.
• Carcinoembryonic antigen (CEA). This is a marker for gastrointestinal tract cancer, among others.
• Prostate-specific antigen (PSA). This is a marker for prostate cancer.