The prognosis of such patients remains poor. Decreasing tumor marker levels to concentrations associated with the absence of cancer, and maintaining such low tumor marker levels over time, indicates that the cancer remains in remission. In contrast, a measurable rise from previously achieved low levels signals renewed tumor growth. Such a reversal in tumor marker levels can be one of the earliest indications of cancer recurrence, many months before the appearance of other clinical signs and symptoms. Detection of such increases allows for repeating or changing treatment regimens while the recurrent cancer remains within the very early phases.
Tumor markers are therefore powerful tools for guiding the course of cancer therapies.
Hypothetical examples of possible time course profiles of tumor marker concentrations are shown in the following figure. Such differences may be related to assay format, calibration, antibody specificity, and how the epitopes on the tumor marker are presented in an individual patient sample. Interpretation of serial results obtained with different assays calls for caution. Discontinuity in a series of results caused by switching from one assay to another must be recognized as an assay-related artifact and not as a change in cancer status. The uncertainty can be best avoided by obtaining a series of matched results with both assays before switching from one assay to the other.