Epidemiology Unit

Cancer Cell

What is a cancer cluster?

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. Even if a group of cancer cases meets the criteria to be considered a cancer cluster, there may not be a single, external cause or hazard that can be addressed.


Cancer Cluster Criteria

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


For more information, contact the Epidemiology Unit at (619) 692-8499 or send us an e-mail.