Cancer Clusters (Fact Sheet)

What is a cancer cluster?

A cancer cluster is the occurrence of a greater than expected number of cancer cases among a group of people in a defined geographic area over a specific time period. A cancer cluster may be suspected when people report that several family members, friends, neighbors, or coworkers have been diagnosed with the same or related types of cancer.

Cancer clusters can help scientists identify cancer-causing substances in the environment. For example, in the early 1970s, a cluster of cases of angiosarcoma of the liver, a rare cancer, was detected among workers in a chemical plant. Further investigation showed that the workers were all exposed to vinyl chloride and that workers in other plants that used vinyl chloride also had an increased rate of angiosarcoma of the liver. Exposure to vinyl chloride is now known to be a major risk factor for angiosarcoma of the liver.

However, most suspected cancer clusters turn out, on detailed investigation, not to be true cancer clusters. That is, no cause can be identified, and the clustering of cases turns out to be a random occurrence.

Where can someone report a suspected cancer cluster or find out if one is being investigated?

Concerned individuals can contact their local or state health department to report a suspected cancer cluster or to find out if one is being investigated. Health departments provide the first response to questions about cancer clusters because they, together with state cancer registries, will have the most up-to-date data on cancer incidence in the area. If additional resources are needed to investigate a suspected cancer cluster, the state health department may request assistance from federal agencies, including the Centers for Disease Control and Prevention (CDC) and the Agency for Toxic Substances and Disease Registry (ATSDR), which is part of the CDC.

The CDC website provides links to state and local health departments. These agencies may also be listed in the blue pages of government listings in telephone books.

Although NCI does not lead investigations of individual cancer clusters, NCI researchers and staff may provide assistance to other investigative agencies as needed. In addition, scientists at NCI and researchers who are funded by NCI analyze variations in cancer trends, including the frequency, distribution, and patterns of cancer in groups of people. These analyses can detect patterns of cancer in specific populations. For example, NCI’s Cancer Mortality Maps website uses data on deaths from the National Center for Health Statistics, which is part of the CDC, and population estimates from the U.S. Census Bureau to provide dynamically generated maps that show geographic patterns of cancer death rates throughout the United States.

How are suspected cancer clusters investigated?

Health departments use established criteria to investigate reports of cancer clusters. The Centers for Disease Control and the Council of State and Territorial Epidemiologists have released updated guidelines for investigating suspected cancer clusters and responding to community concerns (1).

As a first step, the investigating agency gathers information from the person who reported the suspected cancer cluster. The investigators ask for details about the suspected cluster, such as the types of cancer and number of cases of each type, the age of the people with cancer, and the area and time period over which the cancers were diagnosed. They also ask about specific environmental hazards or concerns in the affected area.

If the review of the findings from this initial investigation suggests the need for further evaluation, investigators then compare information about cases in the suspected cluster with records in the state cancer registry and census data.

If the second step reveals a statistically significant excess of cancer cases, the third step is to determine whether an epidemiologic study can be carried out to investigate whether the cluster is associated with risk factors in the local environment. Sometimes, even if there is a clear excess of cancer cases, it is not feasible to carry out further study—for example, if the total number of cases is very small.

Finally, if an epidemiologic study is feasible, the fourth step is to determine whether the cluster of cancer cases is associated with a suspect contaminant in the environment. Even if a possible association with an environmental contaminant is found, however, further studies would be needed to confirm that the environmental contaminant did cause the cluster.

What are the challenges in investigating suspected cancer clusters?

Investigators face several challenges when determining whether a greater than expected number of cancer cases represents a cancer cluster.

Understanding the kind of cancers involved

To assess a suspected cancer cluster accurately, investigators must determine whether the type of cancer involved is a primary cancer (a cancer that is located in the original organ or tissue where the cancer started) or a cancer that has metastasized (spread) to another site in the body from the original tissue or organ where the cancer began (also called a secondary cancer). Investigators consider only the primary cancer when they investigate a suspected cancer cluster. A confirmed cancer cluster is more likely if it involves one type of cancer than if it involves multiple different cancer types. This is because most carcinogens in the environment cause only a specific cancer type rather than causing cancer in general.

Ascertaining the number of cancer cases in the suspected cluster

Many reported clusters include too few cancer cases for investigators to determine whether the number of cancer cases is statistically significantly greater than the expected number.