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CA: A Cancer Journal for Clinicians, Vol 37, 20-25, Copyright © 1987 by American Cancer Society

Cancer Clusters

¹ Vice President for Epidemiology and Statistics and Director of Cancer Prevention of the American Cancer Society in New York, New York.

A large number of purported clusters have been reported in the popular press and have caused widespread concern. In evaluating; such reports, one should be alert to the following:

1. If the cluster includes cancers of many different sites, it is almost certainly not a true cluster, and it cannot be attributed to a particular environmental agent. When a carcinogen is found in the air or water it is usually associated with an elevated risk of cancer of only one or two sites. The sites of cancer most commonly associated with carcinogens, although not exclusively, are lung and bladder.

2. The number of cases reported in a cancer cluster generally will be more than the number expected in a geographic area. Although the difference may be statistically significant, it could be a chance phenomenon; it does not necessarily mean that the cancers are caused by or even associated with some environmental agent.

3. Many community investigations are limited by our knowledge of the environmental agents that can cause cancer. The International Agency for Research in Cancer (IARC) has identified 23 chemicals and seven industrial processes that are causally linked to human cancer, and another 61 that are "probably" linked to cancer in humans but not proved. This is a very small percentage of all the agents that produce the fumes, particles, and dusts to which we are exposed.

4. Before it can be maintained that a cause-and-effect relationship exists, an investigation must show that the people who developed cancer had been exposed to suspected environmental agents and had greater exposure than a series of controls.

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