Centers for Disease Control and Prevention
The Centers for Disease Control and Prevention has prepared answers to questions about the nature of cancer clusters: the greater-than-expected number of cancer cases that occurs within a group of people in a geographic area over a period of time.
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, or over a period of time. A person may suspect that a cancer cluster exists when several loved ones, neighbors, or coworkers are diagnosed with cancer. However, what appears to be a cluster may actually reflect the expected number of cancer cases within the group or area. When considering the possible existence of a cancer cluster in your area, it is important to remember a few key facts: 1) cancer is a common disease, affecting about one in four people in their lifetime; 2) the term cancer refers not to a single disease, but instead to a group of related yet different diseases; 3) a cancer cluster may be due to chance alone, like the clustering of balls on a pool table; and 4) an apparent cancer cluster is more likely to be genuine if the cases consist of one type of cancer, a rare type of cancer, or a type of cancer that is not usually found in an age group.
How do I report a suspected cancer cluster or obtain information on cancer statistics or trends for my area?
Contact your local or state health department or state cancer registry. These agencies provide the first level of response and have the most current local data. Contact your local or state health department. Contact your state cancer registry.
Who responds to inquiries about a suspected cancer cluster?
Local or state health departments, along with cancer registries, provide the first level of response and have the most current local data for the area. If needed, these agencies can request assistance from federal agencies, including
- Agency for Toxic Substances and Disease Registry (ATSDR).
- U.S. Environmental Protection Agency (EPA).
When people contact CDC with concerns about a suspected cancer cluster, CDC provides information and refers them to the appropriate local or state health department or cancer registry.
How are suspected cancer clusters investigated?
State and local health departments respond to cancer cluster reports and inquiries about suspected clusters. A CDC survey revealed that most state health departments’ strategies for cluster response are based on CDC’s “Guidelines for Investigating Clusters of Health Events” with some modifications. Usually, a local or state health department starts by gathering information about the suspected cancer cluster including expected cancer rate, types of cancer, number of cases, and the age, sex, race, address, occupation, and age at diagnosis of the individuals with cancer. Information may be verified by contacting patients and relatives or by obtaining medical records. This information is then compared to census data and state cancer registry data to determine if there is a higher than expected number of cases. Most investigations do not proceed beyond evaluation of the gathered information; however the local or state health department may perform a more intensive assessment or comprehensive epidemiological study. The decision to proceed to a more intensive investigation is usually based on a set of rules developed by the health department.
What challenges regarding suspected cancer clusters do investigators face?
Cancer cluster investigations are complex and difficult for several reasons. Although any cancer case is one too many, suspected cancer clusters often do not contain enough cases for investigators to do a meaningful statistical analysis or reach a conclusion. Investigators must choose the appropriate comparison population and decide how to handle cases that move in or out of the area. Determining the cause of cancer is complicated because exposure to cancer-causing agents may have occurred many years before diagnosis. Therefore, assessing the amount and type of cancer-causing agents an individual has been exposed to is difficult. Unfortunately, cancer is often the result of a combination of agents and risk factors that interact in a way that science does not yet fully understand.
How do I find out if a suspected cancer cluster is being investigated in my area? How do I find information on an investigation in my area?
Contact your state cancer registry or your local or state health department. Contact your state cancer registry through this link or your local or state health department through this link. For information about public health assessments conducted by ATSDR, search by state at this link. For information on CDC investigations, go to this link.
What can I do to reduce my risk of developing cancer?
Adopt a healthy lifestyle that includes avoiding tobacco use, excessive alcohol consumption, and sun exposure. Increasing physical activity, maintaining a recommended body weight, eating a healthful and nutritious diet, and taking advantage of cancer screening also will reduce your risk.
For more information about preventing cancer, visit
- The National Institute of Health’s National Cancer Institute.
- American Cancer Society.
- CDC’s Cancer Prevention and Control Program.
Where can I find more information about cancer clusters?
For links to resources about cancer clusters in general, as well as information about cancer registries and publications on cancer clusters, visit this link.
For an extensive overview of cancer clusters, including facts about cancer, the environment, and heredity; methods used in investigating suspected cancer clusters; guidelines for reporting suspected cancer clusters; and a list of additional resources, visit the NCI’s Cancer Cluster Web site.
NCI has also collaborated with the National Institute for Environmental Health Sciences to publish Cancer and the Environment: What You Need to Know, What You Can Do [PDF, 615 Kb]. This booklet addresses concerns about the connection between cancer and exposure to toxic substances in the environment. It contains information about which types of substances are either known to cause or likely to cause cancer, and what can be done to reduce exposures to them. It also explains how scientists discover which substances are likely to cause cancer. The booklet provides an extensive overview of environmental causes of or risk factors for cancer including lifestyle factors such as diet and physical inactivity, certain medical drugs, hormones, radiation, viruses, bacteria, and environmental chemicals that may be present in the air, water, food, and workplace.