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Tumor Markers: Questions
and Answers
1.
What are tumor markers?
Tumor markers are substances produced by tumor cells or
by other cells of the body in response to cancer or certain
benign (noncancerous) conditions. These substances can be
found in the blood, in the urine, in the tumor tissue, or
in other tissues. Different tumor markers are found in different
types of cancer, and levels of the same tumor marker can
be altered in more than one type of cancer. In addition,
tumor marker levels are not altered in all people with cancer,
especially if the cancer is early stage. Some tumor marker
levels can also be altered in patients with noncancerous
conditions.
To date, researchers have identified more than a dozen substances
that seem to be expressed abnormally when some types of
cancer are present. Some of these substances are also found
in other conditions and diseases. Scientists have not found
markers for every type of cancer.
2. What are risk markers?
Some people have a greater chance of developing certain
types of cancer because of a change, known as a mutation
or alteration, in specific genes. The presence of such a
change is sometimes called a risk marker. Tests for risk
markers can help the doctor to estimate a person’s chance
of developing a certain cancer. Risk markers can indicate
that cancer is more likely to occur, whereas tumor markers
can indicate the presence of cancer1.
3. How are tumor markers used in cancer care?
Tumor markers are used in the detection, diagnosis, and
management of some types of cancer. Although an abnormal
tumor marker level may suggest cancer, this alone is usually
not enough to diagnose cancer. Therefore, measurements of
tumor markers are usually combined with other tests, such
as a biopsy, to diagnose cancer.
Tumor marker levels may be measured before treatment to
help doctors plan appropriate therapy. In some types of
cancer, tumor marker levels reflect the stage (extent) of
the disease. (More information about staging is available
in the National Cancer Institute (NCI) fact sheet Staging:
Questions and Answers, which can be found at http://www.cancer.gov/cancertopics/factsheet/Detection/staging
on the Internet.)
Tumor marker levels also may be used to check how a patient
is responding to treatment. A decrease or return to a normal
level may indicate that the cancer is responding to therapy,
whereas an increase may indicate that the cancer is not
responding. After treatment has ended, tumor marker levels
may be used to check for recurrence (cancer that has returned).
4. How and when are tumor markers measured?
The doctor takes a blood, urine, or tissue sample and sends
it to the laboratory, where various methods are used to
measure the level of the tumor marker.
If the tumor marker is being used to determine whether a
treatment is working or if there is recurrence, the tumor
marker levels are often measured over a period of time to
see if the levels are increasing or decreasing. Usually
these "serial measurements" are more meaningful
than a single measurement. Tumor marker levels may be checked
at the time of diagnosis; before, during, and after therapy;
and then periodically to monitor for recurrence.
5. Does the NCI have guidelines for the use of tumor
markers?
No, the NCI does not have such guidelines. However, some
organizations do have these guidelines for some types of
cancer.
The American Society of Clinical Oncology (ASCO), a nonprofit
organization that represents more than 21,500 cancer professionals
worldwide, has published clinical practice guidelines on
a variety of topics, including tumor markers for breast
and colorectal cancer. These guidelines, called Patient
Guides, are available on the ASCO Web site at http://www.plwc.org/plwc/MainConstructor/1,1744,_12-001125-00_14-00Patient+Guides-00_21-008,00.asp
on the Internet.
The National Comprehensive Cancer Network® (NCCN), which
is also a nonprofit organization, is an alliance of cancer
centers. The NCCN provides Patient Guidelines, which include
tumor marker information for several types of cancer. Most
of the guidelines are available in English and Spanish versions.
The Patient Guidelines are on the NCCN’s Web site at http://www.nccn.org/patients/patient_gls.asp
on the Internet.
The National Academy of Clinical Biochemistry (NACB) is
a professional organization dedicated to advancing the science
and practice of clinical laboratory medicine through research,
education, and professional development. The Academy publishes
Practice Guidelines and Recommendations for Use of Tumor
Markers in the Clinic, which focuses on the appropriate
use of tumor markers for specific cancers. More information
can be found on the NACB Web site at http://www.nacb.org
on the Internet.
6. Can tumor markers be used as a screening test
for cancer?
Screening tests are a way of detecting cancer early, before
there are any symptoms. For a screening test to be helpful,
it should have high sensitivity and specificity. Sensitivity
refers to the test’s ability to identify people who have
the disease. Specificity refers to the test’s ability to
identify people who do not have the disease. Most tumor
markers are not sensitive or specific enough to be used
for cancer screening.
Even commonly used tests may not be completely sensitive
or specific. For example, prostate-specific antigen (PSA)
levels are often used to screen men for prostate cancer,
but this is controversial. It is not yet known if early
detection using PSA screening actually saves lives. Elevated
PSA levels can be caused by prostate cancer or benign conditions,
and most men with elevated PSA levels turn out not to have
prostate cancer. Moreover, it is not clear if the benefits
of PSA screening outweigh the risks of follow-up diagnostic
tests and cancer treatments. (More information about PSA
screening is available in the NCI fact sheet The Prostate-Specific
Antigen (PSA) Test: Questions and Answers, which can be
found at http://www.cancer.gov/cancertopics/factsheet/Detection/PSA
on the Internet.)
Another tumor marker, CA 125, is sometimes used to screen
women who have an increased risk for ovarian cancer. Scientists
are studying whether measurement of CA 125, along with other
tests and exams, is useful to find ovarian cancer before
symptoms develop. So far, CA 125 measurement is not sensitive
or specific enough to be used to screen all women for ovarian
cancer. Mostly, CA 125 is used to monitor response to treatment
and check for recurrence in women with ovarian cancer.
7. What research is being done in this field?
Scientists continue to study tumor markers and their possible
role in the early detection and diagnosis of cancer. The
NCI is currently conducting the Prostate, Lung, Colorectal,
and Ovarian Cancer screening trial, or PLCO trial, to determine
if certain screening tests reduce the number of deaths from
these cancers. Along with other screening tools, PLCO researchers
are studying the use of PSA to screen for prostate cancer
and CA 125 to screen for ovarian cancer. Final results from
this study are expected in several years.
Cancer researchers are turning to proteomics (the study
of protein shape, function, and patterns of expression)
in hopes of developing better cancer screening and treatment
options. Proteomics technology is being used to search for
proteins that may serve as markers of disease in its early
stages, or predict the effectiveness of treatment or the
chance of the disease returning after treatment has ended.
More information about proteomics can be found in Questions
and Answers: Proteomics and Cancer, which is available at
http://www.cancer.gov/newscenter/pressreleases/proteomicsQandA
on the Internet.
Scientists are also evaluating patterns of gene expression
(the step required to translate what is in the genes to
proteins) for their ability to predict a patient’s prognosis
(likely outcome or course of disease) or response to therapy.
NCI’s Early Detection Research Network is developing a number
of genomic- and proteomic-based biomarkers, some of which
are being validated. More information about this program
can be found at http://www3.cancer.gov/prevention/cbrg/edrn/
on the Internet.
Information about clinical trials is available from the
NCI’s Cancer Information Service (see below) or on the clinical
trials page of the NCI’s Web site at http://www.cancer.gov/clinical_trials
on the Internet. In addition, PDQ®, the NCI’s comprehensive
cancer information database, contains cancer information
summaries that can be found at http://www.cancer.gov/cancerinfo/pdq/
on the Internet.
Selected Reference
1. Bigbee W, Herberman RB. Tumor markers and immunodiagnosis.
In: Bast RC Jr., Kufe DW, Pollock RE, et al., editors. Cancer
Medicine. 6th ed. Hamilton, Ontario, Canada: BC Decker Inc.,
2003.
http://www.cancer.gov/cancertopics
/factsheet/Detection/tumor-markers
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