Cancer Clusters: Findings Vs Feelings

David Robinson

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In This Article

What Is a Cancer Cluster?

Cancer clusters are a frequent topic of media attention, and have come to have two different meanings, one for the public and one for scientists. While the public typically thinks of cancer clusters in terms of cancer caused by industrial pollution, scientists tend to see the issue differently. Robert N. Hoover, MD, ScD, Director of the Epidemiology and Biostatistics Program in the Division of Cancer Epidemiology of the National Cancer Institute (NCI), defines a cancer cluster as a geographic area, time period, or group of people with a greater than expected number of cases of cancer.[1] Epidemiologists -- scientists who study the causes and distribution of human diseases -- expect cancer rates to vary slightly from year to year and use statistical tests to determine whether a given rate is different enough from the average to qualify as unexpected (typically, the statistical threshold P < .05 is used). Proving that a cluster exists entails a rigorous statistical analysis, in which the number of cases observed in some area over a specified interval of time is compared with the expected number of cases for that time period and area.

Scientists can use clustering as a way to learn about associations between cancer and any external agent, not only pollutants. For example, much of the scientific literature on clusters involves the evaluation of hypotheses that attribute the spread of cancer to infectious agents. The US Centers for Disease Control and Prevention has conducted cancer cluster investigations for more than 20 years. G. Caldwell, of the Arizona Department of Health Services, reviewing this body of work, explained that "the original plan was to investigate such clusters to determine if they were a phenomenon indicative of cancer transmissibility."[2] Likewise, the 1982 edition of Schottenfeld and Fraumeni's authoritative textbook Cancer Epidemiology and Prevention begins its chapter on clustering by explaining that it is "especially directed towards examining more localized variations in the incidence of cancers in space and time that might suggest that the cancers are induced by infectious agents."[3]

The realization that smoking elevates the risk of lung cancer is described in much of the early (pre-1970) scientific literature as an analysis of clustering -- nations and areas with more residents who smoke evince higher rates of lung cancer. Such instances demonstrate that the term "cluster" can be used on a very large scale, to refer to an elevated incidence of cancer in the entire population of one nation as compared with that of another, or to similarly large groups. It is more commonly used, however, to describe elevated rates in more localized areas, such as a single town or neighborhood.

The public and the media, on the other hand, often use the term "cancer cluster" in a more general way. When members of a community believe that the number of cancer cases in their community is abnormally high, they will often seek an explanation for the "cancer cluster." The media may then report on efforts to explain the cluster, before a statistical analysis has verified that the rates of cancer in the community are actually elevated. Thus, the term "cancer cluster" is often used by the public and the media to refer to the perception of an elevated number of cancer cases, and by epidemiologists to refer to confirmation of this perception. The community perception may reflect an elevated rate of cancer, or it may not.

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