from http://www.cancer.gov/cancertopics/factsheet/detection/tumor-markers "Tumor markers are used to help detect, diagnose, and manage some types of cancer. Although an elevated level of a tumor marker may suggest the presence of cancer, this alone is not enough to diagnose cancer. Therefore, measurements of tumor markers are usually combined with other tests, such as biopsies, to diagnose cancer.
Tumor marker levels may be measured before treatment to help doctors plan the appropriate therapy. In some types of cancer, the level of a tumor marker reflects the stage (extent) of the disease and/or the patient’s prognosis (likely outcome or course of disease). More information about staging is available in the NCI fact sheet Cancer Staging.
Tumor markers may also be measured periodically during cancer therapy. A decrease in the level of a tumor marker or a return to the marker’s normal level may indicate that the cancer is responding to treatment, whereas no change or an increase may indicate that the cancer is not responding.
Tumor markers may also be measured after treatment has ended to check for recurrence (the return of cancer)."