Facts About Cancer Clusters

Reported disease clusters of any kind, including suspected cancer clusters, are investigated by epidemiologists (scientists who study the frequency, distribution, causes, and control of diseases in populations).

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 one or more of the following factors:

     

  • A large number of cases of one type of cancer, rather than several different types.
  • A rare type of cancer, rather than common types.
  • 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.
    •  
    Before epidemiologists can assess a suspected cancer cluster accurately, they must determine whether the type of cancer involved is a primary (original) cancer or a cancer that has metastasized (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 about the cancer-causing potential of the exposure. After developing a case definition (the guidelines that determine whether the cases being investigated are related to the cluster). 
     
    Epidemiologists must identify the time period of concern and the population at risk. They then calculate the expected number of cases and compare that number with the observed number of cases. Epidemiologists must show that the number of cancer cases that 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.
     
    Epidemiologists must also determine if the cancer cases could have occurred by chance. They often test for "statistical significance," which is a mathematical measure of the difference between groups. The difference is said to be statistically significant if it is greater than what would be expected to happen by chance alone.
     
    In common practice, a statistically significant finding means that the probability that the observed number of cases could have happened by chance alone is 5 percent or less. For instance, if one examines the number of cancer cases in 100 neighborhoods, and cancer cases are occurring by chance alone, one should expect to find about five neighborhoods with a statistically significant elevation in the number of cancer cases.
     
    In other words, some amount of clustering within the same family or neighborhood may occur simply by chance. Accurately defining the group of people who should be considered "at risk" is important when investigating a possible cancer cluster. One of the greatest problems in defining clusters is the tendency to expand 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 geographic area and then determine if the number of cancers is excessive, creates many "clusters" that are not real.
     
    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, an error in the calculation of the expected number of cancer cases, or differences in the case definition between observed and expected cases.
     
    Moreover, because people change where they live from time to time, it can be difficult for epidemiologists to identify previous exposures and find the medical 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.
     
     

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