Cancer Clusters

A cancer cluster is the occurrence of more than the expected number of cancers within a group of people, a geographical area, or a period of time. Cancer clusters may be suspected when people report that several family members, friends, neighbors, or coworkers have been diagnosed with the same or related cancer(s). One of the most widely known cancer clusters occurred in the 1960s, when researchers traced the development of mesothelioma (a rare cancer of the lining of the chest and abdomen) to asbestos exposure.

Investigating disease clusters of any kind, including suspected cancer clusters, is the mission of epidemiologists (scientists who study the frequency, distribution, causes, and control of diseases in populations). With their knowledge of diseases, environmental science, and statistics, epidemiologists try to distinguish actual cancer excesses from excesses that are due only to chance. Epidemiologists generally suspect that an excessive number of cancer cases is a true cluster if it involves a large number of cases of a specific type of cancer rather than several different types; a rare type of cancer rather than common types; or an increased number of cases of a certain type of cancer in a group that is not usually affected by that type of cancer.

There are several challenges in investigating clusters. Epidemiologists must make sure that the suspected cluster they are investigating includes only primary (original) cancers and not cancers that have spread (metastasized) from another part of the body. This is important because metastatic cancers do not have the same causes as primary cancers. Epidemiologists must determine that a suspected exposure could have actually triggered new cancers based on what is known about the likely causes. Next they must show that the number of observed cancer cases is significantly greater than what they would expect. Then they must determine whether the cluster could have occurred by chance. To learn more about epidemology in small areas, visit the website From Exposure to Illness: Community health studies and environmental contamination.

At the request of concerned community members, the Environmental Health Investigations Branch (EHIB) has examined several suspected cancer clusters throughout California. EHIB approaches such investigations with serious concern, but also with caution and care to avoid the pitfalls inherent in such studies. EHIB works in conjunction with the California Cancer Registry on an ongoing basis to evaluate whether observed cancer excesses in California are truly "clusters."

Monitoring Cancer in California (Size:  298 KB)