About Cancer Clusters
Definition
A cancer cluster is defined as a greater-than-expected number
of cancer cases that occurs within a group of people in a geographic
area over a period of time.
Challenges in identifying
The complex nature of cancer makes it inherently challenging to
identify, interpret, and address cancer clusters.
"Cancer" is a term representing many diseases with a variety of
causes. The time between exposure to a cancer-causing agent, or the
existence of other risk factors, and the development of cancer can be
decades; therefore, causes are hard, and in some cases impossible, to
identify.
Cancer in general is common. Since 1990, about 18 million new cancer
cases have been diagnosed, according to the American Cancer Society (Cancer
Facts and Figures 2004). About 1,368,030 new cancer cases are
expected to be diagnosed in 2004.
Cancer rates vary by age, race, gender, risk-factors, and type. We
know that risk for cancer increases with age and that cancer is caused
by both external factors (e.g., tobacco, chemicals, radiation, and
infectious organisms) and internal factors (e.g., inherited mutations,
hormones, immune conditions). Nutrition, physical inactivity, obesity,
and other lifestyle factors also play a role in cancer risk and
outcomes. These factors may act together or in sequence to initiate or
promote cancer. Ten or more years often pass between exposures or
mutations and detectable cancer.
Some racial and ethnic groups have a higher incidence of and deaths
due to cancer. Such disparities may be due to multiple factors, such as
late stage of disease at diagnosis, barriers to health care access,
history of other diseases, biologic and genetic differences, health
behaviors, differences in exposures to carcinogens in the environment
and the workplace, and other risk factors.
Characteristics
What first appears to be a cancer cluster may not be one after all. A
review of the situation may show that the number of new cancer cases is
in the expected range for the population and therefore that the cases do
not represent a cancer cluster. Cancer cases are more likely to
represent a cancer cluster if they involve (1) one type of cancer, (2) a
rare type of cancer, or (3) a type of cancer in a group not usually
affected by that cancer, such as a cancer in children that is normally
seen in adults. However, cases of common cancers are those most often
perceived and reported by the public as being part of a cancer cluster.
Identification
The investigators develop a "case" definition, a time period of
concern, and the population at risk. They then calculate the expected
number of cases and compare them to the observed number. A cluster is
confirmed when the observed/expected ratio is greater than 1.0, and the
difference is statistically significant.
Usually, a local or state health department provides the first
response to a suspected cancer cluster. The local or state health
department gathers information about the suspected cancer cluster (e.g.,
types of cancer, number of cases, addresses and occupations of those
people with cancer, possible causes), develops and applies the case
definition, and determines whether there is a greater-than-expected
number of cases.
Interpretation
Confirmation of a cancer cluster does not necessarily mean that there
is any single, external cause or hazard that can be addressed. A
confirmed cancer cluster could be the result of any of the following:
- chance
- miscalculation of the expected number of cancer cases (e.g., not
considering a risk factor within the population at risk)
- differences in the case definition between observed cases and
expected cases
- known causes of cancer (e.g., smoking)
- unknown cause(s) of cancer.
Follow-up investigations can be done, but can take years to complete
and the results are generally inconclusive (e.g., usually, no cause is
found).
Reporting
If you suspect a cancer cluster in your community or workplace,
or if you'd like information such as cancer statistics or trends in your
area, first contact your local or state health department or state
cancer registry. For information about how to contact your state or
local health department, go to
http://www.cdc.gov/other.htm#states. For state cancer registry
contact information, go to
http://www.cdc.gov/cancer/npcr/statecon.htm#list.
Prevention and Early Detection
Concern about cancer and cancer clusters provides an opportunity for
people to learn about how they can prevent cancer or identify it early.
Sixty-five percent of public inquiries about cancer clusters involve
cancers for which screening and preventive measures exist. The best
steps that people can take regarding cancer are to (1) educate
themselves about their personal risk and risk factors for cancer, (2)
avoid these risks, and (3) take advantage of recommended cancer
screenings.
The following Web sites provide information on cancer in general,
risk factors, and recommended screenings:
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