Unusual Patterns of Cancer

Cancer is a group of diseases in which some of the body’s cells grow uncontrollably and spread to other parts of the body. As a group, cancers are very common. Given the frequency with which cancers are diagnosed, sometimes situations arise where an unusual number of cancers are diagnosed among people in a particular location. These unusual numbers may be from chance, or they may result from the following:

• Different cancer screening practices
• Different access to health care, which may reflect other social and economic factors
• Genetic susceptibility to a particular cancer
• Behavioral risks and social determinants of health
• Occupational exposures
• Environmental exposures

Cancer Clusters

Some unusual patterns of cancer are considered a cancer cluster. A cancer cluster is defined as a greater than expected number of the same or etiologically related cancer cases that occurs within a group of people in a geographic area over a defined period of time.

This definition can be further understood as follows:

• A greater than expected number: When the number of observed cases is greater than typically observed in a similar setting.
• Of the same or etiologically related cancer cases: Cases are of the same type, are within a family of tumors (for example, Ewing’s family of tumors), or have a known or suggested link to the same specific environmental or chemical exposures. It is possible to consider multiple cancer types when such a known exposure (for example, radiation or a specific chemical) is linked to more than one cancer type or when more than one contaminant or exposure type has been identified.
• Within a group of people: The population in which the cancer cases are occurring is defined by its demographic factors (for example, race/ethnicity, age, and sex).
• In a geographic area: The geographic area may be based upon pre-existing geopolitical boundaries (for example, census tract, county, or ZIP code/ZIP code tabulation area). It may be defined according to the nature and extent of potential exposures that may cross multiple or partial boundaries. For example, air pollution from a hazardous waste incinerator which may cross multiple counties or census tracts. These geographic boundaries are used to determine the number of cancer cases as they relate to the total population in this predefined area. It is possible to create or obscure a cluster inadvertently by modifying the area of interest.
• Over a defined period of time: The time frame used to establish the beginning and end dates for analysis. The time period chosen for analysis will affect both the total cases observed and the calculation of the expected incidence of cancer in the population.

Etiology refers to causes and risk factors associated with the development of disease. For example, exposure to the sun can cause skin cancer.

It is important to note that not every unusual pattern of cancer will meet the above definition of a cancer cluster. Unusual patterns of cancer that meet some of the criteria described above and also have plausible environmental concerns still warrant further evaluation or assessment by local or state health departments.

Reducing Your Risk of Developing Cancer

In general, cancers have a variety of risk factors related to behavioral, lifestyle, occupational, and environmental risks. For many cancers, you can reduce your risk of developing cancer by adopting a healthy lifestyle that includes avoiding tobacco use, excessive alcohol consumption, and sun exposure; increasing physical activity; maintaining a recommended body weight; and eating a healthy and nutritious diet. Taking advantage of cancer screening also will reduce your risk.

A cancer cluster refers to the occurrence of a greater than expected number of cancer cases among a group of people in a defined geographic area over a specific time period. A cancer cluster may be suspected when people report that several family members, friends, neighbors, or coworkers have been diagnosed with the same or related types of cancer.

Because cancer is a relatively common disease, cases of cancer can appear to cluster even when there is no connection among them. That is, clusters of cancer can arise by chance.    

Some cancer clusters have been shown through careful investigation to be the result of a specific cancer-causing substance in the environment. However, such findings are extremely rare. One review of 576 cancer cluster investigations conducted over 20 years found that for only 72 of the apparent clusters could an increase in cancer rate be confirmed. Only three of the 72 clusters could be linked to a possible exposure, and in just one case was a clear cause identified.

Concerned individuals should contact their local or state health department to report a suspected cancer cluster or to find out if one is being investigated. Health departments provide the first response to questions about cancer clusters because they, together with state cancer registries, will have the most up-to-date data on cancer incidence in the area. The U.S. Centers for Disease Control and Prevention (CDC) website provides links to state and local health departments. These agencies may also be listed in the blue pages of government listings in telephone books.

Although potential cancer clusters are evaluated primarily by state and local health departments, states may request technical advice or assistance from several federal agencies: the CDC; the Agency for Toxic Substances and Disease Registry (ATSDR), which is part of CDC; and the Environmental Protection Agency.

Researchers at NCI and elsewhere regularly conduct surveillance studies of cancer incidence and analyze variations in those trends, including the frequency, distribution, and patterns of cancer in groups of people. These analyses can reveal patterns of cancer in specific populations and identify changes that may warrant investigation.

Several NCI resources allow users to visualize cancer trends by geographic location. For example, the NCI Cancer Atlas is a tool that allows users to create maps of cancer statistics, demographics, and risk factors. The joint NCI-CDC State Cancer Profiles website provides interactive maps to give dynamic views of cancer statistics by states and counties.