Cancer Clusters: Fact or Fiction?

Woburn Trial Resources:Cancer-related trial transcripts
Key Issues in the Woburn Trial: State and Federal Health Studies
Learning Activities: Activity #2 - Cancer
Resources for Educators: Faculty Notes for Student Activity #2 - Cancer

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What is a Cancer Cluster?

A disease cluster occurs when a higher than expected number of people become ill with the same disease in a given geographic area, a group of people, or a period of time. When this disease is the same form of cancer, it is called a "cancer cluster". When epidemiologists suspect a true cancer cluster, rather than a coincidence, it involves:

• A large number of cases of a specific type of cancer, rather than several different types;
• A rare type of cancer, rather than common types; or
• An increased number of cases of a certain type of cancer in an age group that is not usually affected by that type of cancer.

What Data are Needed to Confirm a Cancer Cluster?

To decide if a cancer grouping is a true cluster an epidemiologist must find answers to the following questions:

• Is the cancer is the primary (original) cancer or a cancer that spread from another organ (metastasis)? Only original cancers are considered in the investigation of cancer clusters.
• Is the number of cancer cases is significantly greater than the expected number of cases, given the age, gender, and racial distribution of the group of people at risk of developing the disease? One of the difficulties in making this assessment is accurately defining the group of people who should be considered "at risk." One of the problems when defining clusters is the tendency to extend the geographic borders of the cluster to include additional cases of the suspected disease as they are discovered.
• Could the cancer cases have occurred by chance? Some amount of clustering within the same family or neighborhood may occur simply by chance. In common practice, a statistically significant finding means that there is a 5 percent or less chance that the observed number of cases could have happened by chance. For instance, one examines the number of cancer cases in 100 neighborhoods, and cancer cases are occurring randomly, one should expect to find about five neighborhoods with statistically significant number of cancer cases.

Difficulties in Identifying Cancer Clusters

Difficulties in confirming cancer clusters include:

• Difficulties in obtaining data. People change residences and it can be difficult for epidemiologists to identify previous exposures and find the records that are needed to determine the kind of cancer a person contracted, or if it was cancer at all. In addition, cancer may take years to develop after exposure to a cancer causing substance.
• Many reported clusters do not include enough cases for epidemiologists to arrive at any definitive conclusions. Sometimes, even when a suspected cluster has enough cases for study, a greater than expected number of cases (a true statistical excess) cannot be demonstrated.
• Other times, epidemiologists find a true excess of cases, but they cannot find an explanation for it. For example, a suspected carcinogen may cause cancer only under certain circumstances, making its impact difficult to detect.
• Confirming the cause of a cancer cluster is particularly difficult considering all of the risk factors involved in cancer

Who Decides if a Cancer Grouping is a Cancer Cluster?

State epidemiologists typically decide if a grouping is a cancer cluster.

For more information on cancer clusters see:

• National Cancer Institute, Cancer Cluster Factsheet
• American Cancer Society

Example Cancer Clusters

One of the best known cancer clusters emerged in the 1960s involving mesothelioma (a rare cancer of the lining of the chest and abdomen). Researchers traced the development of mesothelioma to exposure to asbestos, which was used in shipbuilding during World War II and has also been used in manufacturing industrial and consumer products. For an abstract, see A survey of cancer and occupation in young and middle aged men. I. Cancers of the respiratory tract., Coggon D, Pannett B, Osmond C, Acheson ED., Br J Ind Med. 1986 May;43(5):332-8.

Other Suspected Cancer Cluster Information:

• Cancer Cluster Information Families against cancer keeps a list of information on suspected cancer clusters.
• Summary of environmental data: McFarland childhood cancer cluster investigation (1991)
• Tom's River Cancer Cluster Settlement
• Fallon, Utah Newspaper Articles