Cancer Clusters

by National Cancer Institute

Key Points

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.

Epidemiologists (scientists who study the frequency, distribution, causes, and control of diseases in populations) investigate suspected cancer clusters.

Some amount of clustering may occur simply by chance.

A suspected cancer cluster may be reported to a state or 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 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 (HIV) and acquired immunodeficiency syndrome (AIDS); the outbreak in 2003 of a respiratory illness, later identified as severe acute respiratory syndrome (SARS), 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(s). In the 1960s, one of the best known cancer clusters emerged, involving many cases of mesothelioma (a rare cancer of the lining of the chest and abdomen). 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 (something that may increase the chance of developing a disease) for mesothelioma.

Facts About Cancer

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

Cancer is the uncontrolled growth and spread of abnormal cells anywhere in the body. However, cancer is not just one disease; it is actually an umbrella term for at least 100 different but related diseases.

Each type of cancer has certain known and/or suspected risk factors associated with it.

Cancer is not caused by injuries, nor is it contagious. It cannot be passed from one person to another like a cold or flu virus.

Cancer is almost always caused by a combination of factors that interact in ways that are not yet fully understood.

Carcinogenesis (the process by which normal cells are transformed into cancer cells) involves a series of changes within cells that usually occur over many years. More than 10 years can go by between the exposure to a carcinogen (any substance that causes cancer) and a diagnosis of cancer, which makes it difficult to pinpoint the cause of that cancer.

Cancer is more likely to occur as people get older; because people are living longer, more cases of cancer can be expected in the future. This increased life expectancy may create the impression that cancer is becoming much more common, even though an increase in the number of cases of cancer is related in large part to the growing number of elderly people in the population.

Some racial and ethnic groups have higher rates of cancer than other racial and ethnic groups. Such differences may be due to multiple factors, such as late stage of disease at diagnosis, barriers to health care access, history of other diseases, biologic and genetic differences, health behaviors, and other risk factors.

Cancer, in general, is common. More than 17 million new cases of cancer have been diagnosed since 1990.

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.

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