Cancer Clusters and Patterns of Cancer Cases

When multiple people who live or work in the same area are diagnosed with cancer around the same time, it can raise concerns about a possible cancer cluster. Public health experts look carefully at these situations to find out whether an unusual pattern of cancer is actually a cluster and whether any action is needed to help protect people and communities.

What is a cancer cluster?

A cancer cluster is a greater than expected number of related cancer cases that occurs within a group of people in a geographic area over a specific period of time.

This is the definition used by the US Centers for Disease Control and Prevention (CDC) and the National Cancer Institute (NCI). Each part is important. To be considered a cancer cluster, all criteria from the definition must be met:

• “Greater than expected number” means the number of cases is higher than what’s expected in the same or similar geographic area.
• “Related cancer cases” means the cancers are of the same or similar cancer type, or that they have a known or suspected link to the same chemical or substance in the environment.
• “Occurs within a group of people” refers to similarities shared among the people affected. For example, they could be of similar age, sex or gender, they might live or work with or near each other, or they may be of the same race or ethnic background.
• “In a geographic area” means the pattern of cancer aligns with where the people live or work, such as a specific county, town, building, or zip code.
• “Over a specific period of time” means experts determine an appropriate time frame during which to study the pattern of cancers.  

When might a cancer cluster be suspected?

A cancer cluster may be suspected when doctors and health experts recognize a pattern of cancer cases that seems to stand out and needs a closer look.

Local and regional health departments and other public health experts closely review these cases to determine whether there might be a cancer-causing agent in the environment.

Most often, when a shared exposure causes a true cancer cluster, it’s due to an exposure in a workplace rather than in communities where people live.

Is an unusual pattern of cancer the same as a cancer cluster?

Not all unusual patterns of cancer are cancer clusters. Most of the time, they do not meet all of the required criteria. And not every unusual pattern of cancer cases is examined in the same way. Each situation is different.

• Unusual patterns of cancer that meet all of the criteria of a cancer cluster are investigated. If the number of cancer cases doesn’t seem to be random, they might be looked at more closely to find out if they might have a common cause.
• If a pattern of cancer only meets some of the criteria for a cancer cluster, it may be partly or fully investigated depending on details of the cases.

Remember that cancer is a common condition. In the US, 1 in 3 people will be diagnosed with cancer during their lifetime and more than 2 million people are diagnosed with cancer every year.

Because cancer is common, cases can appear to be the same or similar just by chance. This means:

• Having several people in a relatively small area develop cancer around the same time is not necessarily unusual.
• Patterns or groups of cancer cases can happen even when nothing in the environment is determined to cause them.

Why using the right terms matters

Using the right words to describe unusual patterns of cancer helps keep people informed without causing fear and panic. But when cancer clusters are misunderstood, they can cause the spread of misinformation.

If you see news or social media posts about possible cancer clusters, remember that many times the reported cases do not turn out to be actual cancer clusters. Be wary of reports from unreliable sources or that use wording that is based on facts.

For example, reliable sources will not use terms such as:

• “Cancer outbreak.” You can’t “catch” cancer and cancer can’t spread from person to person. Because cancer is not contagious, experts don’t use the word “outbreak.”
• “Cancer hotspot.” Experts do not use the word “hotspot” because it can cause fear and confusion about a possible problem in the environment. The word can also suggest a cause for the cancers has been found. Generally, there is not just one clear cause of a pattern of cancer cases.

Learn how to find reliable sources of information in Finding Cancer Information on the Internet.

How are patterns of cancer reported?

Anyone can report a possible unusual pattern of cancer to their local health department. The reporting process can vary by county and state, but the information needed for an investigation usually includes:

• The type(s) and number of cancers involved
• Any suspected exposure(s) that might cause cancer
• The area and time period in which the cases occurred
• Specific information about each person thought to be affected
• Specific information about the cancers themselves

How are patterns of cancer and cancer clusters investigated?

When an unusual pattern of cancer is reported, the local health department reviews the initial information provided. Based on this, they decide whether an in-depth investigation is needed. If it is, the local health department might ask the state health department or federal agencies for help. The CDC is the federal agency most often involved in investigating unusual cancer patterns. Other agencies, such as the Environmental Protection Agency (EPA) and the National Institute of Environmental and Health Sciences (NIEHS), may also help investigate.

Suspected unusual patterns of cancer can cause a great deal of concern and confusion within the community or workplace where they are reported. But often, these reported situations are found not to be unusual after all. When this is the case, no further investigation is typically needed.

If enough data shows that a more in-depth investigation is needed, the process might include:

• Getting medical histories from affected people
• Testing their blood or other body fluid samples

Collecting and testing samples from the environment (air, soil, drinking water, etc.), especially if there is concern about a specific type of exposure as a possible cause.

What happens if a cancer cluster is found?

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

What to know when the answers are unclear

Unusual patterns of cancer and suspected cancer clusters can be very distressing for people affected by them. Some people might have some peace of mind if an investigation finds no clear cause for the pattern of cases. Others might find it worrisome or frustrating.

Even though many thorough investigations of potential cancer clusters don’t find an actual cluster, it’s important that communities keep reporting unusual patterns of cancer so they can be studied.

Additional resources

Along with the American Cancer Society, other sources of information and support include:

Centers for Disease Control and Prevention (CDC)
Toll-free number: 1-800-232-4636
Home page: www.cdc.gov
Cancer cluster information: www.cdc.gov/cancer-environment/about/index.html
Links to state and local health departments: www.cdc.gov/public-health-gateway/php/communications-resources/health-department-directories.html