Cancer Clusters


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 over a period of time.

Challenges in identifying

The complex nature of cancer makes it inherently challenging to identify, interpret, and address cancer clusters.

"Cancer" is a term representing many diseases with a variety of causes. The time between exposure to a cancer-causing agent, or the existence of other risk factors, and the development of cancer can be decades; therefore, causes are hard, and in some cases impossible, to identify.

Cancer in general is common. Since 1990, about 16 million new cancer cases have been diagnosed, according to the American Cancer Society Cancer Facts and Figures 2010 . About 1,284,900 new cancer cases are expected to be diagnosed in 2002.

Cancer rates vary by age, race, gender, risk-factors, and type. We know that risk for cancer increases with age and that cancer is caused by both external factors (e.g., tobacco, chemicals, radiation, and infectious organisms) and internal factors (e.g., inherited mutations, hormones, immune conditions). Nutrition, physical inactivity, obesity, and other lifestyle factors also play a role in cancer risk and outcomes. These factors may act together or in sequence to initiate or promote cancer. Ten or more years often pass between exposures or mutations and detectable cancer.

Some racial and ethnic groups have a higher incidence of and deaths due to cancer. Such disparities 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, differences in exposures to carcinogens in the environment and the workplace, and other risk factors.


What first appears to be a cancer cluster may not be one after all. A review of the situation may show that the number of new cancer cases is in the expected range for the population and therefore that the cases do not represent a cancer cluster. Cancer cases are more likely to represent a cancer cluster if they involve (1) one type of cancer, (2) a rare type of cancer, or (3) a type of cancer in a group not usually affected by that cancer, such as a cancer in children that is normally seen in adults. However, cases of common cancers are those most often perceived and reported by the public as being part of a cancer cluster.


The investigators develop a "case" definition, a time period of concern, and the population at risk. They then calculate the expected number of cases and compare them to the observed number. A cluster is confirmed when the observed/expected ratio is greater than 1.0, and the difference is statistically significant.

Usually, a local or state health department provides the first response to a suspected cancer cluster. The local or state health department gathers information about the suspected cancer cluster (e.g., types of cancer, number of cases, addresses and occupations of those people with cancer, possible causes), develops and applies the case definition, and determines whether there is a greater-than-expected number of cases.


Confirmation of a cancer cluster does not necessarily mean that there is any single, external cause or hazard that can be addressed. A confirmed cancer cluster could be the result of any of the following:

  • chance
  • miscalculation of the expected number of cancer cases (e.g., not considering a risk factor within the population at risk)
  • differences in the case definition between observed cases and expected cases
  • known causes of cancer (e.g., smoking)
  • unknown cause(s) of cancer.

Follow-up investigations can be done, but can take years to complete and the results are generally inconclusive (e.g., usually, no cause is found).


If you suspect a cancer cluster in your community or workplace, or if you'd like information such as cancer statistics or trends in your area, first contact your local or state health department or state cancer registry. For information about how to contact your state or local health department, go to

Prevention and Early Detection

Concern about cancer and cancer clusters provides an opportunity for people to learn about how they can prevent cancer or identify it early. Sixty-five percent of public inquiries about cancer clusters involve cancers for which screening and preventive measures exist. The best steps that people can take regarding cancer are to (1) educate themselves about their personal risk and risk factors for cancer, (2) avoid these risks, and (3) take advantage of recommended cancer screenings.

Who responds to inquiries about a suspected cancer cluster?

If you suspect that there is a cancer cluster in your workplace, neighborhood, or elsewhere, first contact your local or state health department. Local or state health departments provide the first level of response to a suspected cancer cluster. These agencies have the most current local data available.

If needed, these agencies request and receive assistance from federal agencies, including CDC's National Center for Environmental Health, National Institute for Occupational Safety and Health, and National Center for Chronic Disease Prevention and Health Promotion; the Agency for Toxic Substances and Disease Registry; and the US Environmental Protection Agency; in investigating a suspected cancer cluster.

When citizens contact CDC with concerns about cancer clusters, CDC refers them to the appropriate state epidemiologist, who may, in turn, refer the inquirer to the appropriate local health official.

How are cancer clusters investigated?

Usually, a local or state health department starts by gathering information about the suspected cancer cluster (e.g., types of cancer, number of cases, addresses and occupations of those people with cancer, possible causes, and the rate of cancer in the area compared with the expected rate in the state, etc.).

If there is a need for additional evaluation, the health department may verify reported diagnoses by contacting patients and relatives and obtaining medical records.

The gathered information is then compared with census data and state cancer registry data to help determine if there is a higher-than-expected number of cancer cases among the population being evaluated. The health department may gather additional information to help decide whether or not to conduct a comprehensive epidemiological study.

What cases are more likely to be cancer clusters?

What first appears to be a cancer cluster may not be one after all. A cluster may occur when the cancer cases have a common cause or have unrelated causes that happen to occur at the same time. The number of cases may seem high, particularly among the small group of people who have something in common with the cases, such as working in the same building or living on the same street. Although the occurrence of a disease may be random, the distribution of that disease may not be uniform, and clusters of disease may arise by chance alone.

A suspected cluster is more likely to be a true cluster if it involves any of the following:

  • A large number of cases of one type of cancer, rather than several different types
  • A rare type of cancer rather than a common type
  • A number of a certain type of cancer in age groups not usually affected by that cancer.

These situations may indicate a common source or means by which cancer develops.

What challenges do investigators face regarding cancer clusters?

Many suspected clusters do not include enough cancer cases for investigators to reach any conclusions. Sometimes, even if there are enough cases, a greater-than-expected number of cases is not found. Even when a cluster is confirmed, investigators may not be able to provide an explanation because cancer is usually the result of a combination of factors that interact in ways that are not yet fully understood and cancer-causing exposures may have occurred many years before, making them impossible or difficult to document.

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