About Cancer Clusters


A cancer cluster is defined as a greater-than-expected number of cancer cases that occurs within a group of people in a geographic area over a period of time.

To be a cancer cluster, a group of cancer cases must meet the following criteria. Until all of these parameters are met, the group of cancer cases is often referred to as a suspected cancer cluster.

  • A greater than expected number:
    A greater than expected number is when the observed number of cases is higher than one would typically observe in a similar setting (in a group with similar population, age, race, or gender). This may involve comparison with rates for comparable groups of people over a much larger geographic area – e.g., an entire state.
  • Of cancer cases:
    All of the cases must involve the same type of cancer, or types of cancer scientifically proven to have the same cause.
  • That occurs within a group of people:
    The population in which the cancers are occurring is carefully defined by factors such as race/ethnicity, age, and gender, for purposes of calculating cancer rates.
  • In a geographic area:
    Both the number of cancer cases included in the cluster and calculation of the expected number of cases can depend on how we define the geographic area where the cluster occurred. The boundaries must be defined carefully. It is possible to “create” or “obscure” a cluster by selection of a specific area.
  • Over a period of time:
    The number of cases included in the cluster – and calculation of the expected number of cases – will depend on how we define the time period over which the cases occurred.


Confirmation of a cancer cluster does not necessarily mean that there is any single, external cause or hazard that can be addressed. A confirmed cancer cluster could be the result of any of the following:

  • chance
  • miscalculation of the expected number of cancer cases (e.g., not considering a risk factor within the population at risk)
  • differences in the case definition between observed cases and expected cases
  • known causes of cancer (e.g., smoking)
  • unknown cause(s) of cancer.

Follow-up investigations can be done, but can take years to complete and the results are generally inconclusive (i.e., usually, no cause is found).

Complex nature of cancer

The complex nature of cancer makes it inherently challenging to identify, interpret, and address cancer clusters.

Cancer is a term describing different diseases that share a similar characteristic: uncontrollable cell growth and division. As a group, cancers are very common. Cancers are the second leading cause of death in the United States, exceeded only by diseases of the heart and circulatory system. One of every four deaths in the United States is due to some form of cancer. It has been estimated that in 2010 alone, over 1.5 million Americans will be diagnosed with a cancer, and more than half a million will lose their lives as a result of cancer.

Reporting a suspected cluster

If you suspect a cancer cluster in your community or workplace, or if you’d like information such as cancer statistics or trends in your area, first contact your local or state health department or state cancer registry. A local or state health department provides the first response to a suspected cancer cluster. The local or state health department gathers information about the suspected cancer cluster (e.g., types of cancer, number of cases, addresses and occupations of those people with cancer, possible causes), develops and applies the case definition, and determines whether there is a greater-than-expected number of cases. For information about how to contact your state or local health department, go to https://www.cdc.gov/nceh/clusters/statelocal.htm. For state cancer registry contact information, go to https://nccd.cdc.gov/dcpc_Programs/index.aspx#/3.

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Page last reviewed: May 14, 2019