To begin investigating a reported cancer cluster, 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, DSHS 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." DSHS 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. DSHS 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 DSHS 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 DSHS 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: