Tumor markers are substances that are produced by cancer or by other cells of the body in response to cancer or certain benign (noncancerous) conditions. Most tumor markers are made by normal cells as well as by cancer cells; however, they are produced at much higher levels in cancerous conditions. These substances can be found in the blood, urine, stool, tumor tissue, or other tissues or bodily fluids of some patients with cancer. Most tumor markers are proteins. However, more recently, patterns of gene expression and changes to DNA have also begun to be used as tumor markers. Markers of the latter type are assessed in tumor tissue specifically.
Changes in any applicable tumor markers are valid methods for measuring the efficacy of chemo therapy. Other imaging methods such as CT scans or MRIs may be also, depending on they type of cancer,and assuming you had a baseline scan prior to chemo. Your oncologist is the person best suited to determine which method(s) are best suited to your particular casse.