Tumor markers.

The past decade has seen many advances in the detection, characterization, and clinical applications of tumor markers. Although cancer screening applications have been limited by low disease prevalences in asymptomatic populations, tumor markers may be of diagnostic value in specific situations. The major use of tumor markers in primary care is in monitoring disease recurrence and the response to therapy. These uses may obviate the need for second-look surgery as it has with CA 125 elevations in ovarian carcinoma. On the other hand, tumor markers may indicate a need for second-look surgery as when CEA levels are elevated in colorectal carcinoma. Despite the apparent usefulness of markers in detecting cancer recurrence, the clinician is reminded of the conditions justifying treatment with the discovery of an abnormal laboratory value as put forth by Fries and Holman: (1) the treatment should be known to be effective, (2) early treatment should be known to be more effective than therapy given after the disease is clinically apparent (or early treatment carry the risk of less toxicity), and (3) prediction of impending deterioration must be consistently accurate. When these criteria are met, the planning of therapeutic regimens on the basis of marker levels may be rationally considered. Until these criteria are met, elevated markers can only stimulate the clinician to be more vigilant in the search for response to therapy or recurrence. Exciting developments are on the horizon with respect to the use of tumor markers in radionuclide imaging, radioimmunoguided surgery, and development of drug delivery systems. Further research into the structure and function of these substances may provide further insights into the phenomena of malignant transformation, tumor invasion, metastasis, and the development of new therapeutic options. With new advances in molecular biology and with the identification of oncogenes, it may be possible in the future to detect mutant oncoproteins that are specific for early cancers and premalignant conditions. Delecting these oncoproteins may provide a basis for development of truly sensitive and specific markers that can be used to detect cancer at an early and curable stage.