Most of the well-known tumor markers are anything but specific and the evidence of cancer is therefore not always certain. This is why clinical guidelines in Germany do not recommend these markers for cancer diagnosis since a temporary increase in tumor marker levels can also occur in cases of inflammation, pregnancies or in smokers.
Tumors possess certain characteristics such as gene mutations for instance or they produce specific proteins that usually do not exist in normal tissue. These distinctive features can be utilized for diagnostic testing since tumor markers can be measured in the patient’s blood or urine for example.
"There are different ways to detect tumors," explains Prof. Wilfried Roth, Assistant Medical Director of the Institute of Pathology at the University of Heidelberg and Director of Molecular Tumor Pathology at the German Cancer Research Center (German: Deutsches Krebsforschungszentrum), DKFZ. "Tumor markers can be detected in the blood in the form of soluble proteins. The prostate-specific antigen (PSA) in prostate cancers is one example of this. However, there are also tissue-based markers or markers that can be detected in other secretions and body fluids like urine or stool."
Tumor markers can otherwise be differentiated based on their diagnostic benefits. There are biomarkers that initially indicate whether there actually is a tumor or not. Other markers are used as progression parameters to determine how effective a therapy is. Conversely, if the therapy was successful, the tumor marker can be checked during cancer follow-up care in regular intervals to exclude a disease recurrence. Another benefit is in prognostic or predictive tumor markers that are tissue-based. "A prognostic tumor marker shows how aggressive a tumor is in an individual patient. You measure the specific protein expression in the tumor tissue; the tumor is more aggressive at a higher level of expression. Predictive markers need to be distinguished from them. These are tumor markers that provide information on whether a tumor really responds to the therapy," explains Roth.
However, such markers are not available for all types of cancer and it is actually unclear whether there are markers for all types of cancer: many tumor cells do not differ enough from healthy cells and do not produce markers typical for a cancer type.