One of the best ways to diagnose cancer early, aid in its prognosis, or predict therapeutic response, is to use serum or tissue biomarkers. Cancer biomarkers can be DNA, mRNA, proteins, metabolites, or processes such as apoptosis, angiogenesis or proliferation. The markers are produced either by the tumor itself or by other tissues, in response to the presence of cancer or other associated conditions, such as inflammation. Such biomarkers can be found in a variety of fluids, tissues and cell lines.
The National Cancer Institute (NCI) definition of biomarker is: "A biological molecule found in blood, other body fluids, or tissues that is a sign of a normal or abnormal process, or of a condition or disease. A biomarker may be used to see how well the body responds to a treatment for a disease or condition. Also called molecular marker and signature molecule." Naturally, a cancer biomarker pertains to any biomarker that fits the aforementioned definition but only for cancer, and no other disease. An ideal cancer biomarker should be able to be measured easily, reliably and cost-effectively by use of an assay with high analytical sensitivity and specificity. One of the most well-known cancer biomarkers is Prostate-specific antigen, or PSA, and elevated levels of PSA in men tend to signal for prostate cancer. Actually, cancer biomarkers are more than just a signal of the disease, and have many roles in the fight against cancer.
Receiver operating characteristic (ROC) curve
A graphical representation of the relationship between sensitivity and specificity. This curve is used to evaluate the efficacy of a tumor marker at various cut-off points. An ideal graph is the one giving the maximum area under the curve (AUC). In the given example, the red curve represents a useless test (AUC = 0.5). The green curve represents a useful (AUC <1.00) but not perfect (AUC = 1.00) test.
Diagnostic (screening) biomarker
A marker that is used to detect and identify a given type of cancer in an individual. These markers are expected to have high specificity and sensitivity; for example, the presence of Bence–Jones protein in urine remains one of the strongest diagnostic indicators of multiple myeloma.
This type of marker is used once the disease status has been established. These biomarkers are expected to predict the probable course of the disease including its recurrence, and they therefore have an important influence on the aggressiveness of therapy. For example, in testicular teratoma, human chorionic gonadotropin and alfa-fetoprotein levels can discriminate two groups with different survival rates.
Stratification (predictive) biomarker
This type of marker serves to predict the response to a drug before treatment is started. This marker classifies individuals as likely responders or nonresponders to a particular treatment. These biomarkers mainly arise from array-type experiments that make it possible to predict clinical outcome from the molecular characteristics of a patient's tumor.
The proportion of control (normal) individuals who test negative for the biomarker.
The proportion of individuals with confirmed disease who test positive for the biomarker.