Cancer Clusters: Investigating Unusual Patterns of Cancer and Environmental Concerns

What is a Cancer Cluster?
How does TDH handle cancer cluster concerns?
Why is establishing a link to an environmental concern unlikely?
More resources

Cancer and the Environment

Cancer affects 1 out of 2 men and 1 out of 3 women in the U.S. When we see cancer occurring so often in our families, neighborhoods, and communities, it is natural to wonder if there is something in our environment causing it. The Tennessee Department of Health (TDH) has a process to investigate community cancer concerns. They work with partners to determine if there is a higher-than-expected occurrence of cancers in an area, also known as a cancer cluster. TDH also works with partners to investigate environmental concerns in communities.

What is a Cancer Cluster and an Unusual Pattern of Cancer?

A cancer cluster is when a greater-than-expected number of people experience the same cancer, genetically related cancers, or cancers with similar contributing factors (etiologically-related cancers) in a specific geographic area over a set amount of time. An unusual pattern of cancer is similar to a cancer cluster, however the greater than expected number of cancer cases occur within a certain time period, but not in a specific geographic area. Some unuaual patterns of cancer may not meet the definition of a cancer cluster. An example of an unusual pattern of cancer might involve individuals diagnosed with cancer who are all served by the same water system. In this example, the pattern on a map may look unrelated unless further examination of their water source is done. This example may not meet the qualifications for a cancer cluster but still warrants further assessment by the health department.

Since cancer is a common disease, often it is by coincidence cancer affects many people in an area. However, TDH takes all cancer concerns seriously and has a system for investigating concerns with possible environmental causes.

How does TDH handle cancer cluster concerns?

When a community member contacts TDH about a potential cancer cluster or unusual patter on disease concern, TDH collects information from the concerned individual(s), including:

• Location or geographic area of concern (city, zip code, census tract, neighborhood, street, etc.)
• Types of cancer occurring
• Number of cases of each cancer type
• How long this cancer concern has been occurring (time frame)
• Case information
- Ages of those diagnosed with cancer
- Type of cancer
- Date of diagnosis
• Any known or suspected environmental concerns, occupational risk factors, behavioral risk factors, or other concerns in the area.

TDH staff considers the types of cancers being reported and looks to see if they are common cancers, rare cancers, or cancer types rare in certain age groups. If warranted, TDH conducts a cancer investigation to evaluate if there is a true cancer cluster while taking potential environmental concerns into account. A community cancer investigation uses information from the Tennessee Cancer Registry (TCR) and other resources to determine if the number of cancer cases is statistically significantly higher than expected.

Using resources from environmental agencies, TDH may also consider unique potential environmental exposures in the area (e.g. polluted sites and/or those undergoing hazardous waste cleanups).

There are 3 main possible outcomes from a cancer cluster investigation:

In most cases, an investigation will show the pattern of cancer, or suspected "cluster", is not unusual and that cancer rates are occurring at a rate that might be expected.
Less often, an investigation finds that cancers are occurring more often than would be expected, and in some cases represent a true cancer cluster, but no cause can be found.
Rarely, an investigation finds a cancer cluster where the cause can be determined.

Once an initial analysis is completed, the TDH will communicate findings to the public. Depending on the findings, TDH will determine next steps to further investigate environmental and health concerns.

Why is establishing a link to environmental concerns unlikely?

Cancer is very common. Cancer is not one disease but is a group of over 100 different diseases that all share the common characteristic of uncontrollable cell growth and division. Because cancer is so common, it is not unusual for communities to see multiple cases of some kind of cancer. This may increase the perception of an excess of cancer cases in the community. You can read more about cancer rates in Tennessee in the TCR's Annual Report.

Each type of cancer may have different causes (etiologies), including lifestyle factors, genetics, demographics, and environment. Often cancer is caused by a combination of these factors. Sometimes people in an area are more frequently diagnosed with a type of cancer due to factors like:
    - Inconsistent participation in cancer screening practices in the community
    - Lack of access to health care
    - Genetic susceptibility to particular cancers
    - Behavioral risks and social determinants of health (such as smoking or unstable housing)
    - In some cases, workplace exposures, or common environmental exposures such as consuming contaminated drinking awater or exposure to air pollution.

Also, cancer may take years to develop after a possible exposure. As people move in and out of an area, it is difficult to track people who were possibly exposed to an environmental risk. Additionally, not everyone who is exposed to a cancer-causing substance will develop cancer.

Resources

Tennessee Cancer Registry
www.tn.gov/health/health-program-areas/tcr.html

TN Comprehensive Cancer Control Program
www.tn.gov/health/health-program-areas/fhw/tennessee-comprehensive-cancer-control-program.html

TN Department of Health Data
https://healthdata.tn.gov/

CDC's Unusual Patterns of Cancer, the Environment, and Community Concerns
www.cdc.gov/cancer-environment/about/index.html

CDC's National Environmental Public Health Tracking Portal
https://ephtracking.cdc.gov/

ATSDR Toxicological Profiles and Substance Priority List
wwwn.cdc.gov/TSP/index.aspx

National Cancer Insitute
www.cancer.gov/about-cancer/causes-prevention

American Cancer Society
www.cancer.org/cancer/risk-prevention/understanding-cancer-risk/cancer-clusters.html

Cancer Prevention American Cancer Society
www.cancer.org/cancer/risk-prevention/understanding-cancer-risk/cancer-facts.html