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To begin investigating
a reported cancer cluster, the Texas Cancer Registry
(TCR) of the Department of State Health Services (DSHS) follows
a protocol based on recommendations made by the Centers for
Disease Control and Prevention (CDC). When a suspected cancer
cluster is first reported, the TCR gathers information about
the suspected cluster and provides the caller with general
information about cancer clusters. Reports of suspected cancer
clusters are sometimes resolved at this initial contact stage
because concerned individuals realize that what seemed like
a cancer cluster is not a true cluster.
If a potential
cluster is indeed observed, it must be carefully evaluated
to see if it is "real." The TCR epidemiologists
begin by addressing the question: "Is the number of
cancer cases that occurred in this population, within this
defined period of time, greater than would normally be expected?"
To answer this question, the number of cancer cases observed
in the community must be compared with the number expected
for that population. The fact that cancer is so common means
that many clusters will be explainable solely on the basis
of chance. Statistical testing is used to determine if the
community rates are significantly higher than the state
rates.
If the rates are higher and chance can be statistically
ruled out, a cancer cluster may
exist. In that event, investigators must evaluate whether
it might be because of factors known to be related to that
type of cancer, or unknown factors. TCR epidemiologists
review the most current cancer information available and
consult with the environmental and risk assessment programs
within DSHS to make this determination
If the rates
are evaluated and the initial evidence is compelling,
the TCR will proceed to the final stage of an investigation
and recommend a comprehensive study of the proposed potential
disease-exposure relationship. The primary purpose of such
a study would be to pursue the epidemiologic and public
health issues that the cluster generated - not necessarily
to investigate the specific cluster. Most state health departments,
including Texas, have reported fewer than five percent of
cancer cluster investigations reach the final stage of actually
conducting the comprehensive study
Cancer cluster
investigations take time and effort; they cannot
be done in an hour or a day. They require population data,
cancer registry information and statistical analyses. As
data on the incidence of cancer in Texas become more complete,
the TCR and other cancer researchers can more quickly and
accurately determine if a cluster exists in a community.
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For additional information on
cancer cluster investigations see:
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