Cancer Clusters

Overview

  • Cancer clusters may be suspected when people report that several family members, friends, neighbors, or coworkers have been diagnosed with the same or related cancers.
  • Some amount of clustering may occur simply by chance.
  • Epidemiologists investigate suspected cancer clusters.
  • Concerned individuals may report a suspected cancer cluster to their local health department or state cancer registry.
  • Other resources may provide additional information about cancer clusters, cancer incidence and mortality, and environmental risk factors for cancer.

Defining Disease Clusters

A disease cluster is the occurrence of a greater than expected number of cases of a particular disease within a group of people, a geographic area, or a period of time. Clusters of various diseases have concerned scientists for centuries. Some recent disease clusters include the initial cases of a rare type of pneumonia among homosexual men in the early 1980s that led to the identification of the human immunodeficiency virus and acquired immunodeficiency syndrome; the outbreak in 2003 of a respiratory illness, later identified as severe acute respiratory syndrome, caused by a previously unrecognized virus; and periodic outbreaks of food poisoning caused by eating food contaminated with bacteria.

Cancer clusters may be suspected when people report that several family members, friends, neighbors, or coworkers have been diagnosed with the same or related cancer. In the 1960s, one of the best known cancer clusters emerged, involving many cases of mesothelioma. Researchers traced the development of mesothelioma to exposure to asbestos, a fibrous mineral that was used heavily in shipbuilding during World War II and has also been used in manufacturing industrial and consumer products. Working with asbestos is the major risk factor for mesothelioma.

Facts About Cancer

Some concepts about cancer can be helpful when trying to understand suspected cancer clusters:

Facts About Cancer Clusters

Reported disease clusters of any kind, including suspected cancer clusters, are investigated by epidemiologists. Epidemiologists use their knowledge of diseases, environmental science, lifestyle factors, and biostatistics to try to determine whether a suspected cluster represents a true excess of cancer cases. 

Epidemiologists have identified certain circumstances that may lead them to suspect a potential common source or cause of cancer among people thought to be part of a cancer cluster. A suspected cancer cluster is more likely to be a true cluster, rather than a coincidence, if it involves: 

Before epidemiologists can accurately assess a suspected cancer cluster, they must determine whether the type of cancer involved is a primary cancer or a cancer that has spread from another organ. This is important to know because scientists consider only the primary cancer when they investigate a possible cancer cluster. Epidemiologists also try to establish whether the suspected exposure has the potential to cause the reported cancer, based on what is known about that cancer’s likely causes and what is known about the cancer-causing potential of the exposure. 

In addition, epidemiologists must show that the number of cancer cases which have occurred 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. They must also determine if the cancer cases could have occurred by chance. Epidemiologists often test for “statistical significance,” which is a measure of the likelihood that the observed association could simply have been due to 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, if 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 elevations. In other words, some amount of clustering within the same family or neighborhood may occur simply by chance.

Another difficulty epidemiologists face when investigating a possible cancer cluster is accurately defining the group of people who should be considered “at risk.” One of the greatest pitfalls of 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. The tendency to define the borders of a cluster on the basis of where known cases are located, rather than to first define the population and then determine if the number of cancers is excessive, creates many “clusters” that are not genuine.

Epidemiologists must also consider that a confirmed cancer cluster may not be the result of any single, external cause or hazard. A cancer cluster could be the result of chance, miscalculation of the expected number of cancer cases, or differences in the case definition between observed cases and expected cases. Moreover, because people change residence from time to time, 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 had—or if it was cancer at all.

Because a variety of factors often work together to create the appearance of a cluster where nothing abnormal is occurring, most reports of suspected cancer clusters are not shown to be true clusters. Many reported clusters do not include enough cases for epidemiologists to arrive at any conclusions. Sometimes, even when a suspected cluster has enough cases for study, a greater than expected number of cases 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. 

Genetics and Environment

Because most cancers are likely to be caused by a combination of factors related to genetics and environment, studies of suspected cancer clusters usually focus on these two issues. However, establishing significant and valid evidence that a specific genetic factor leads to an increased chance that a specific environmental exposure will result in cancer requires studies of large populations over long periods of time. Researchers are just beginning to learn about the roles heredity and environmental exposures play in carcinogenesis. Some of their discoveries are outlined below:

Genetics

Environment

Reporting Suspected Cancer Clusters

Concerned individuals may report a suspected cancer cluster to their state or local health department. State and local health departments use established criteria to investigate reports of cancer clusters. When a suspected cancer cluster is first reported, the health department gathers information about the suspected cluster and gives the inquirer general information about cancer clusters. Although health departments may use different processes, most follow a basic procedure in which increasingly specific information is obtained and analyzed in stages. Health departments are likely to request the following:

Between 75 and 80 percent of reports of suspected cancer clusters are resolved at this initial contact because concerned individuals realize that what seemed like a cancer cluster is not a true cluster. If further evaluation is needed, the health department will take the following steps to investigate a possible cancer cluster:

Most state health departments report that fewer than 5 percent of cancer cluster investigations are determined to require a comprehensive study.

Resources

The following resources may provide additional information about cancer clusters, cancer incidence and mortality, and environmental risk factors for cancer:

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