How are tumor markers classified?
Tumor markers can be classified into two groups: Cancer-specific markers and
tissue-specific markers.
Cancer-specific markers
Cancer-specific markers are related to the presence of certain cancers (the tumor
itself produces the biomarker). Because there is a large overlap between the many
different tumor types and the markers produced, these markers might not be specific
enough to make a correct diagnosis.
They can, however, be useful in the follow-up of treated patients to track the
progress of the disease or response to treatment. A few examples of these markers
are CEA, CA19-9, and CA125 (see the section on "Examples of Tumor Markers" for
more information).
Tissue-specific markers
Tissue-specific markers are associated with specific tissues, i.e., the body makes the
biomarker in response to the presence of cancer. Generally speaking, these
substances are usually not exclusive to the tumor, but may also be elevated
in the presence of cancer. But unlike the cancer-specific markers, elevated levels point to a specific tissue having a
problem. An example is PSA where it can be elevated when the prostate tissue has problems - sometimes this may be cancer, sometimes not.
How tumor markers are identified
Many tumor markers are proteins. Therefore, many tumor markers are identified
through tests that evaluate specific proteins in the body associated with cancer or
non-cancerous conditions.
Proteins are identified through the help of a branch of science called proteomics,
which is the study of all proteins in a cell, tissue, or organism, including the shape,
function, and patterns of expression.
Cancer proteomics, or the study of proteins associated with cancer, enables mapping
of proteins involved when normal cellular pathways support malignant growth. In
cancerous tissue, some of the proteins critical for normal communication are
damaged, inactive, overactive, or missing entirely.
The full set of proteins disrupting cellular communications may vary from one cancer
type to another, and possibly from one patient to another within a cancer type.
The challenge for cancer detection and diagnosis is to locate the renegade genes and
proteins (the defective, and dominating molecules) that disrupt communication in
once normal cells.
