What is a cancer cluster?

A cancer cluster occurs when there is an unusually high number of cases of cancer within a group of people, a geographic 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).

Whenever someone reports a suspected cancer cluster, scientists use their knowledge of diseases, environmental science, lifestyle factors, and biostatistics in their investigation. It is very difficult to prove that a cluster exists. Most reported cancer clusters are not shown to be true clusters.

A true cancer cluster is likely to involve:

• Many cases of one type of cancer, rather than several different types.
• A rare cancer, rather than a common one.
• An increased number of cases of a certain cancer in an age group that is not usually affected by that type of cancer.
• Exposure to a substance that already has been proven to have the ability to cause cancer.
• A number of cases significantly greater than the number that would be expected to occur normally within a given group of people.

Suspected cancer clusters may be reported to local or state health departments. The department will investigate the report, often with help from federal agencies.

For more information on cancer clusters, call the Cancer Information Service at 1-800-4-CANCER.

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