Tumor markers in clinical oncology

The subtle differences between normal and tumor cells are exploited in the detection and treatment of cancer. These differences are designated as tumor markers and can be either qualitative or quantitative in their nature. That means that both the structures that are produced by tumor cells as well as the structures that are produced in excessive amounts by host tissues under the influence of tumor cells can function as tumor markers. Speaking in general, the tumor markers are the specific molecules appearing in the blood or tissues and the occurrence of which is associated with cancer. According to their application, tumor markers can be roughly divided as markers in clinical oncology and markers in pathology. In this review, only tumor markers in clinical oncology are going to be discussed. Current tumor markers in clinical oncology include (i) oncofetal antigens, (ii) placental proteins, (iii) hormones, (iv) enzymes, (v) tumor-associated antigens, (vi) special serum proteins, (vii) catecholamine metabolites, and (viii) miscellaneous markers. As to the literature, an ideal tumor marker should fulfil certain criteria when using it as a test for detection of cancer disease: (1) positive results should occur in the early stages of the disease, (2) positive results should occur only in the patients with a specific type of malignancy, (3) positive results should occur in all patients with the same malignancy, (4) the measured values should correlate with the stage of the disease, (5) the measured values should correlate to the response to treatment, (6) the marker should be easy to measure. Most tumor markers available today meet several, but not all criteria. As a consequence of that, some criteria were chosen for the validation and proper selection of the most appropriate marker in a particular malignancy, and these are: (1) markers' sensitivity, (2) specificity, and (3) predictive values. Sensitivity expresses the mean probability of determining an elevated tumor marker level (over the "cut-off value") in a tumor-bearing patient. Specificity expresses the mean probability that a normal tumor marker value derives from a tumor-free individual. The predictive value shows the applicability of a tumor marker in a mixed group of patients. Many theoretical applications exist for tumor markers in clinical oncology. Clinically important utilization of markers includes (i) early detection of the tumor, (ii) differentiating benign from malignant conditions, (iii) evaluating the extent of the disease, (iv) monitoring the response of the tumor to therapy, and (v) predicting or detecting the recurrence of the tumor. Since no ideal tumor markers with adequate sensitivity and specificity currently exist, they are only exceptionally used in screening (prostate specific antigen PSA). Nevertheless, tumor markers can play a crucial role in the detection of an early disease relapse and assessment of response to therapy in selected groups of patients. In monitoring the patients for disease recurrence, tumor marker levels should be determined only when meaningful treatment is possible.