Busting Cancer Clusters: Realities Often Differ From Perceptions
Authors: S. Benowitz
Journal: JNCI Journal of the National Cancer Institute (2008)

C ancer clusters often have all the necessary ingredients for great detective stories and human drama, and that fact has not been lost on either Hollywood or the news media. Woburn, Mass., Hinkley, Calif., and Toms River, N.J., are all examples of small towns thrust into the public spotlight because of a reported cluster of cancer cases (and potentially related environmental conditions) that drew both local and national headlines. The Woburn story spawned a bestselling novel and a movie , A Civil Action . The movie Erin Brockovich tackled the story of a cluster of childhood cancers in a small desert town in California. Cancer clusters come in two basic types: occupational and environmental, with variations on both themes. Occupational clusters are easier to spot because they are confi ned to one population with similar exposures to potential contaminants. Examples include asbestos and mesothelioma, vinyl chloride monomer and liver cancer. Community — and potentially environmentally caused — clusters are harder to unravel because of the diffi culty in determining and measuring the myriad variables involved, such as age, potential exposure, and length of time living in an area. They are incredibly diffi cult to prove and even harder to trace to specifi c causes. Epidemiologists like to distinguish between perceived and observed clusters. It is one thing for some residents to think that their community has had an abnormally high number of cancers over a given period and another to actually meet the Centers for Disease Control and Prevention’s (CDC) defi nition of “a greater than expected number of cancer cases that occurs within a group of people, in a geographic area or over a period of time.” But clusters mean different things to different people. When residents begin to see several cancers in a small community, or even in their families, they tend to want to fi nd a common cause. Sometimes they forget how common cancer can be. Or they fail to take into account that cancer is a hundred different diseases with different causes. And of course, few think about random chance. According to veteran “cluster buster” Tim Aldrich, Ph.D., associate professor of epidemiology at East Tennessee State University in Johnson City, the cancers that are involved in a cluster have to be rare diseases to be detected. “Common cancers such as breast, lung, colon, and prostate have too much background noise to tell if an environmental exposure is the culprit,” he said. “If, as some say, 3% – 5% of cancers are environmental, and 1% – 2% of perceived clusters really are clusters, then you’re dealing with a very uncommon event. You have to have a rarefi ed distribution to see these things jump up.” “There have only been a few reported cancer clusters that have proven to be real clusters,” said Melissa Bondy , Ph.D., professor of epidemiology at the University of Texas M. D. Anderson Cancer Center in Houston. “People get alarmed when they hear about cancers at various sites in an area. There have been some that epidemiologists have been able to untangle, but most cancer clusters have not been well documented. They usually don’t pan out to be anything.”