see http://en.wikipedia.org/wiki/Tumor_marker.
A tumor marker is a substance found in the blood, urine, or body tissues that can be elevated in cancer, there can also be other causes of the elevation.
CEA, or carcinoembryonic antigen, is a blood-borne protein, produced by tumors of the gastrointestinal system and by the occasional lung and breast cancer case, meaning that an elevated level does not necessarily mean a bowel cancer. However, in a patient with a history of a treated bowel cancer, a rising CEA level can be an early sign of recurring bowel cancer. This usually occurs before the site of return can be identified on imaging or examination and so many oncologists question the wisdom of doing a blood test for CEA when the end result is bad news that alarms the patient. Nevertheless, a sequence of steady low CEA readings can provide much needed reassurance to the post-operative patient. Also, a rising sequence of CEA readings should alert the physician to the need for diagnostic tests such as PET scans.
CA15-3: Elevated CA15-3, in conjunction with alkaline phosphatase, was shown to increase chances of early recurrence in breast cancer.[2]
