Definition

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. In the U.S., 1 in 3 people will develop cancer in his or her lifetime. According to the American Cancer Society's Cancer Facts and Figures, about 1,372,910 new cancer cases are expected to be diagnosed in 2005. 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.

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Characteristics

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.

Identification

21st Century Understanding Cancer Toolkit: Cancer Clusters, Carcinogenesis, Cancer and the Environment, Studies of Suspected Clusters, Interaction of Environmental Factors and Genes ebook

Cancer Clusters (Cancer Etiology, Diagnosis and Treatments)

When people contact CDC with concerns about a suspected cancer cluster, CDC provides information and refers them to the appropriate local or state health department or cancer registry.

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. Information may be verified by contacting patients and relatives or by obtaining medical records. This information is then compared to census data and state cancer registry data. Most investigations do not proceed beyond evaluation of the gathered information; however the local or state health department may perform a more intensive assessment or comprehensive epidemiological study. The decision to proceed to a more intensive investigation is usually based on a set of rules developed by the health department.

Interpretation

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).

Cancer cluster investigations are complex and difficult for several reasons. Although any cancer case is one too many, suspected cancer clusters often do not contain enough cases for investigators to do a meaningful statistical analysis or reach a conclusion. Investigators must choose the appropriate comparison population and decide how to handle cases that move in or out of the area. Determining the cause of cancer is complicated because exposure to cancer-causing agents may have occurred many years before diagnosis. Therefore, assessing the amount and type of cancer-causing agents an individual has been exposed to is difficult. Unfortunately, cancer is often the result of a combination of agents and risk factors that interact in a way that science does not yet fully understand.

Reporting a suspected cluster

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.

Information about how to contact your state or local health department.
State cancer registry contact information.

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.

Adopt a healthy lifestyle that includes avoiding tobacco use, excessive alcohol consumption, and sun exposure. Increasing physical activity, maintaining a recommended body weight, eating a healthful and nutritious diet, and taking advantage of cancer screening also will reduce your risk. The following Web sites provide information on cancer in general, risk factors, and recommended screenings:

American Cancer Society
CDC's Cancer Prevention and Control Program
The National Institutes of Health's National Cancer Institute (NCI)