What are tumor markers
Tumor markers are also more commonly known as cancer markers or biomarkers, these are substances that are mostly protein in nature and can be present in samples such as blood, various bodily fluids or tissues and urine. Tumor markers are predominately produced in response to cancer within the human body, this could be either by normal cells or cancer cells.
There are many different types of cancer markers that have been extensively investigated and characterised. Some of these biomarkers have been found to be only specific for one type of cancer (for example: brain tumors, stomach cancers, HIV, thromboctopenia, erythropoietin, cardiovascular and heart diseases), whereas other are found in many different types of cancers (for example: multiple myeloma, hepatocellular carcinoma, gallbladder cancer and lymph nodes). It is also widely accepted through the use of many different chemistry, immunoassay based laboratory research protocols and various clinical trials that the levels of these specific tumor markers are higher or elevated when there is cancer present within the human body.
Different types of tumor markers
Listed below are some of the most common cancer biomarkers that are being widely used in helping to diagnose and treat many different tumors. The number of tumor markers that are becoming available is rapidly growing and this is instrumental in enhancing our understanding of how cancers may be functioning within the human body.
- Alpha-Fetoprotein (AFP): This is mainly produced by the fetus and therefore usually found to be increased in women that are pregnant. Alpha-fetoprotein is not present in the blood of adults (men or women that are not pregnant), however, increased levels of AFP identified may be an indication of liver, ovary or testicle cancer. There is also evidence to indicate that non-cancerous condition can also lead to elevated levels of AFP.
- CA-125: Usually ovarian cancer is mainly responsible for displaying increased levels of CA-125, however, other cancers that also show this include: liver, breast, pancreas, colon, cervix and uterus. CA-125 is the main biological marker for monitoring the progress of treatment of ovarian cancer.
- CA15-3: Increased levels of CA15-3 are linked with cancer of the breast, prostate, lung and ovary. It is an instrumental tool during the treatment of advanced breast cancer in women. Lactation and pregnancy are found to elevate the levels of CA15-3 present in the body, this is also similar for many non-cancerous conditions for example endometriosis, hepatitis, benign breast and ovarian diseases and pelvic inflammatory diseases.
- CA19-9: Elevated levels of CA19-9 are associated with advance cancer of the pancreas, stomach, bile duct and colon. Other non-cancerous conditions which are also linked with this cancer marker include: cholecystitis, pancreatitis, gallstones, and cirrhosis of the liver.
- Carcinoembroyonic Antigen (CEA): The most common tumor associated with CEA is colorectal cancer, elevated levels are normally found within the blood. Several other cancers also display elevated levels of CEA for example: stomach, lung, breast, pancreatic and ovarian. CEA are regarded as essential biomarkers for detecting a particular treatment process and for any recurrence.
- Human Chorionic Gonadotrophin (HCG): HCG is a common substance that is present during pregnancy and this is solely formed by the placenta. If there is no pregnancy, increased levels of human chorionic gonadotrophin can indicate tumors of the lung, stomach, liver, pancreas, testis and ovary. There is evidence to indicate elevated amounts of HCG can be the result of smoking cannabis or marijuana. Increased levels of HCG in the blood are also indicative of gestational trophoblastic disease (GTD) and germ cell tumors of the ovary and testicle.
- Prostate-Specific Antigen (PSA): Elevated levels of PSA within the blood system is a clear indication of prostate cancer, however, other conditions for example prostatitis and benign prostatic hyperplasia (BPH) are also known to increase the levels of PSA. The detection of PSA levels is useful marker in assessing patient response to treatment and to detect for any tumor recurrence (post treatment).
- Neuron-Specific Enolase (NSE): Is largely associated with neuroblastoma and small cell lung cancer. NSE is found in the blood and is a useful biomarker to help diagnosis and assess response to treatment.
How are tumor markers used during cancer treatment
Tumor marker can be essentially divided into two main categories: tumor tissue markers and circulating tumor markers.
(A). Tumor Tissue Markers: These are typically present in actual tumors that are usually removed from a biopsy procedure. There are three main uses for these types of tumor biomarkers: to diagnose, stage and classify a tumor; to estimate the prognosis and to select the appropriate treatment which includes a targeted therapy procedure.
One of the main functions of tumor tissue markers is to identify whether an individual is suitable for a specific targeted therapy treatment to combat the cancer. In many cases additional medical diagnosis, signs and symptoms (this may also involve a health check and physical examination by a qualified physician) in order to gather as much information as possible. Also, various other gene testing procedure may help to identify if there are any further changes in the genes that are present which could ultimately be affecting cancer growth.
Common examples of tumor tissue markers include progesterone and estrogen receptors, in these cases tests are carried out to identify whether hormone therapy will be suitable for breast cancer patients. Other examples include FGFR3 gene mutation detection, this is to determine which treatment to undertake for bladder cancer patients.
(B). Circulating Tumor Markers: These are usually present in bodily fluids, blood, urine and stool. There are many uses for these types of tumor biomarkers: identify cancer stage, prognosis estimation, detect recurrence of cancer and to assess the effectiveness of treatment. The detection of elevated levels of circulating tumor markers may be an indication of cancer being present, however this alone is not sufficient for cancer diagnosis. Since there are may non-cancerous conditions that have been discovered which can result in elevating levels of specific tumor markers. Therefore, in majority of cases the measurement of circulating tumor markers is combined with the various other tests such as imaging, biopsies and genetic testing to accurately diagnose the cancer and patient health.
During the procedure of cancer therapy, tumor markers are often only periodically analysed. A serial measurement is carried out which can display the change in the level of the cancer marker over a given time and this is often more useful that taking single point measurement. A decrease in the amount of circulating tumor marker may be an indication that the therapy is effective at treating the cancer.
Common examples of circulating tumor markers include: calcitonin which is useful in estimating the prognosis of medullary thyroid cancer, CA-125 for the analysis of ovarian cancers and beta-2 microglobulin for treatment of chronic lymphocytic leukemia, multiple myeloma and various lymphomas.
What are the limits of tumor marker testing
There are some limitations to testing of cancer biomarker and these are listed below.
- People with no history of cancer can display high levels of certain tumor markers
- Non-cancerous diseases or conditions have been discovered to elevate levels of specific tumor markers
- Over time the levels of tumor markers present within the body may change and this can affect the results obtained during testing
- Some tumor markers may remain dormant and therefore are not picked up during early phase testing. However, when these biomarkers are at elevated levels and now picked up during later testing. The cancer could be at an advanced stage and will put patient at a much higher risk
- Some cancers do not have any known tumor markers, whereas other might not be found in the blood
- Some patients do not display elevated levels of tumor markers even though their tumor is producing specific cancer biomarkers
The number of tumor markers that are being identified is continually growing at a tremendous rate. This is further enhanced by the rapid development and expansion of various novel techniques which have made it possible to identify and detect an array of new cancer biomarkers. This ultimately will lead to a better understanding of how the human body reacts when dealing with various cancers and diseases.
However, there are still many pitfalls that need to be encountered along the way. Out of the vast number of tumor markers that have already been discovered, only a small handful of these are used during actual routine clinical procedures. It is unfortunate that a greater emphasis has been devoted to understanding the genetic origin and the pathophysiology of individual tumor markers, rather than their application for routine clinical procedures.
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