Introduction
Current clinical practice in oncology has a growing impetus on early diagnosis, proper prognostication and (of late) screening for malignancy in asymptomatic groups. Tumor markers are assuming a growing role in all aspects of cancer care, starting from screening to follow-up after treatment. Important clinical decisions are increasingly likely to be made on the basis of these results, whether for diagnosis, screening, prediction or treatment monitoring.1
Tumor markers include a variety of substances like cell surface antigens, cytoplasmic proteins, enzymes, hormones, oncofetal antigens, receptors, oncogenes and their products. 2 There have been numerous attempts to broaden the definition to accommodate the rapidly expanding set of identified tumor markers
Table 1
Characteristics of an ideal tumor marker 3
Table 2
Methods of detection of tumor marker 4
Various guidelines Have been suggested for clinical application in various malignancies by professional bodies like American Society of Clinical Oncology, Canadian Task Force, American Association for Clinical Chemistry, etc., but only tumor marker that finds a place in any screening algorithm is prostate-specific antigen (PSA).
Recommendations for Ordering Tumor Marker Tests
It is imperative to remember that a single value or test is unreliable. It is noteworthy that in most situations, elevations of markers in nonmalignant diseases are often transient, whereas elevations associated with cancer either remain constant or continuously rise. Ordering serial testing can help detect falsely elevated levels due to transient elevation. Knowledge of the assay method is important in interpretation of either an abnormal value or a serial change in tumor marker values. 5, 6, 7 Various methods of detection have their own specific cut off values and sensitivities. 8 Thus, for any set of serial values to be meaningful, they have to come from the same assay methods and preferably from the same laboratory.
An important interfering factor to be considered before any interpretation is presence of a hook effect. This is especially true if the value of a tumor marker does not correlate to the clinical situation. Hook effect is an inherent flaw of certain methods of detection (specifically immunoassay) due to which the serum tumor marker levels may be reported to be falsely low if the concentration rises above a particular level.
Interpretation
According to guidelines published by Working Group on Tumor Marker Criteria, interpretation should take into account the therapy status of the patient
If the patient is under active treatment or has received treatment in the recent past, changes in marker levels may reflect the clinical progression of the disease. Partial remission is defined as a decrease in marker levels by at least 50%; and progressive disease, as an increase in marker levels by at least 25%, on the basis of the concept that tumor load is related to changes in serum tumor marker level
Conclusions
The use of tumor markers in clinical oncology has increased tremendously with rapid expansion of techniques of detection and identification of new markers in recent times, a trend that continues to grow as technology progresses and our understanding about our body and the disease processes increases. However, such use is not without its pitfalls; in fact, injudicious application of tumor markers is fraught with risks of mistreatment (under-treatment or over-treatment) and its consequence
Judicious application of tumor markers to clinical practice needs a thorough understanding of the basics of pathophysiology, the techniques of identification or testing, reasons (in cases of both benign and malignant tumors) for out-of-range levels of tumor markers, as well as the knowledge of evidence of their role in any given malignancy
