Cancer Clusters

What is a cancer cluster?

Cancer clusters get a lot of attention in the media, both in the news and in movies. More than 1,000 suspected cancer clusters are reported to state health departments each year. But just what is a cancer cluster?

People may become concerned that a cancer cluster exists when there is higher than normal number of cases of cancer in a certain community. Often there’s a concern that the cancer is caused by some type of carcinogen (cancer-causing agent) that is being released into the environment.

Scientists have a specific definition of a cancer cluster. The US Centers for Disease Control and Prevention (CDC) and the National Cancer Institute both say that a cancer cluster is a “greater-than-expected number of cancer cases that occurs within a group of people in a defined geographic area over a period of time.”

In order to see if there is a greater number of cancers than expected, the number of cases seen is compared to what is typically seen in a similar group – such as a group with the same age, gender, and ethnicity. The cancers should either be all of the same type or types of cancer that are known to have the same cause.

If these cases of cancer do not seem to be random, they may need to be looked at more closely to find out if they are due to the same cause or carcinogen. Studying cancer clusters allows scientists to identify areas of increased cancer risk as well as try to figure out what is causing the increase in risk. For example, studying clusters of malignant mesothelioma led to the discovery of the link between asbestos exposure and this rare cancer.

What does a cancer cluster look like?

Cancer is a group of more than 100 different diseases. Each type of cancer has its own risk factors and causes. This is why true cancer clusters very rarely involve more than one type of cancer. For it to be considered a true cluster, it usually must have one of the following characteristics:

  • There are several cases of a rare type of cancer.
  • There are larger than expected numbers of a more common type of cancer.
  • It is a type of cancer that is not usually seen in a certain group of people (for example, children getting a cancer usually seen in adults).

If the excess cancer cases include many different types of cancer over a period of many years, it’s not likely to be a true cancer cluster. And it’s very unlikely to be caused by a single environmental factor or exposure.

It’s also important to remember that cancer is common. Millions of new cases are diagnosed every year. Nearly one-third of all people in the United States will develop cancer during their lifetimes. So, it’s fairly common for several people in a relatively small area to develop cancer around the same time.

For most well-documented cancer clusters that were found to be caused by a shared exposure, the exposure took place in the workplace, rather than in the communities where people lived. Workplace exposures may be more likely to cause disease because the level of exposure tends to be higher and last longer than in other settings. The length of exposure is important, because it usually takes many years after exposure for cancer to develop. Workplace exposures can also be easier to identify because the group of exposed people is better defined and easier to trace as compared to groups in the community. This is why the links between cancer and many cancer-causing agents (called carcinogens) are often first found in studies of workers.

Statistics can usually help figure out if a cancer cluster is strictly due to chance. But if the excess number of cases reported in a cancer cluster looks significant based on statistics, it does not mean that they are caused by something unique to that area. Some clustering of cancer cases happens by chance, but people tend to notice and report situations where rates seem to be above average.

Who investigates cancer clusters, and how is it done?

People concerned about a possible cancer cluster can report it to a local or state health department. Procedures vary by state, but most health departments will first ask for information, such as:

  • 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

After reviewing this information, most of the suspected clusters (as many as 4 of 5) are determined not to be true clusters and no further investigation is done. According to guidelines, some factors that do not support the need for further investigation include:

  • Cancer cases within family members who are blood relatives (especially cancers known to be strongly genetically linked)
  • Different types of cancers not known to be related to one another
  • A few cases of very common cancers, particularly when the people involved are of the usual age and sex for those cancers
  • Cases involving people with cancer who didn’t live in the same place at the time an exposure would have to have taken place to cause that cancer

If the health department feels that the potential cluster should be studied further, they collect more information. They will need to make sure that the cases are cancer and that they know the specific diagnosis. For example, there are several kinds of leukemia, and it is important to know what kind each person has. They also may want to see what risk factors for cancer affect the people involved. This may involve contacting patients or relatives or looking at medical records. The health department will look at the number of cases in the affected area and those nearby to see if there are really more cases than expected. Scientists in the health department may also look at reports in the medical literature to see if other clusters like this have been noted in the past.

If needed, the state or local health department may ask federal agencies for help – they tend to have more resources. The Centers for Disease Control and Prevention (CDC) is the agency most often involved. Other agencies, such as the Environmental Protection Agency (EPA) and the National Institute of Environmental and Health Sciences (NIEHS), may also help investigate.

Federal agencies may do a more in-depth investigation, including getting more thorough medical histories. These agencies may also take and test samples from the environment (air, soil, drinking water, etc.), especially if there is concern about a specific toxin. They may also test blood or other body fluid samples from both affected and non-affected people.

Suspected cancer clusters can cause a great deal of concern and confusion in a community. It’s very important that government agencies keep members of the community informed from the start of and throughout the investigation. This should include giving people a realistic idea of what may or may not be found.

What are the possible outcomes of a cluster investigation?

It may not be a true cluster.

In many cases, investigators can determine that a “cluster” of cancers isn’t a true cluster. For example, the number of cases may not really be higher than expected when other factors that could explain the increase (such as age, gender, and tobacco use) are taken into account. Or the types of cancer may not be related to each other.

It is a cancer cluster and its cause is found.

Rarely, a true cancer cluster may be confirmed and a cause is identified. At that point, steps can be taken to address it.

A cancer cluster may be found, but no cause can be identified.

Even if investigators believe that it may be a true cluster, it’s important to know that a cause is found very rarely. There are several reasons why this is the case. For example:

Random patterns can form a cluster: Even if the number of cases in an area is higher than expected, it still may not be caused by a single factor or exposure. More cancer cases in the United States are expected in large population centers or in places where the population tends to be older. But even so, for the most part, cancer cases in the United States are spread randomly across the country.

As with any random pattern, there will be more cases than expected in some spots, and fewer cases than expected in others. The areas with more cases than expected are more likely to be noticed. But many of these will be due to the “bull’s-eye effect” (which is something like drawing a target on the wall after the darts have been thrown). Suppose you took a map of the United States and started drawing circles of different sizes in different locations. You would find that some of the circles would contain more cancers than expected, and some would have less. Some of the circles with more cases might be clusters caused by a single carcinogen, but most would be due to chance.

You can see this more clearly on a smaller scale if you look at balls on a pool table. Even if the balls are randomly distributed, there are sections of the table with more and other sections with fewer balls than the average for the whole table.

Random patterns are the most common reason for a cancer cluster with no identified cause.

It is very hard to figure out which of many exposures might be the cause: With rare exceptions, scientists don’t have a way of telling what trigger (if any) may have caused cancer in any one person, whether it’s part of a cluster or not. Humans are not like lab animals – their environments are not strictly controlled. People are exposed to many natural and man-made substances during their lifetimes.

Think about how hard it would be to test for everything you’ve been exposed to, even if you knew where to start. Investigators may have a few clear leads or starting points, but they need to look at all of the possibilities. Finding the one exposure that may be the cause can be like looking for the needle in the haystack.

There is usually a long delay between exposure and cancer: In clusters where the cause is known, mostly in workplace cases, the time between exposure to the substance and the development of cancer has been anywhere from a few years to several decades. Exposures are not likely to cause cancers right away. And again, it’s not easy to study people and their environments.

Suppose a group of people live in a community that has a higher than expected number of cases of a certain type of cancer. If there is a potential cause, investigators first have to figure out when these people were exposed to it. Was it a single event or has it been ongoing? Was it 5 years ago? 10? 20? And what did the people with cancer have in common during that time? Added to this, some people will have moved into the community, while others may have moved away. Should the cases of people who moved into town in recent years be included? And can the people who moved away be found?

The boundaries of the cluster area can be hard to define: Defining the geographic cluster area is not always as clear-cut as one might think it would be. Just how big should the “bull’s eye” be? Should it include only the local neighborhood where most of the cases were found? Or should it also include the larger community, or even nearby communities? These areas may have cases that may or may not be related to the others.

Not everyone who is exposed is likely to develop cancer: To make things even more tricky, people may be more or less prone to getting cancer based on their genes. It’s unlikely that everyone exposed will develop cancer. At the same time, there may be people who were not exposed who develop the same cancer by chance.

Questions may still remain

Scientists do their best to piece the puzzle together when there is a cancer cluster, but more times than not, they don’t find a likely link. This doesn’t mean that there isn’t one; it may just be that one can’t be found with the methods scientists have at the time.

Oftentimes there really isn’t a link, for the reasons noted before, but it’s hard for everyone to feel sure of that. This may be an unsatisfactory answer for people in a community being affected, but it often is the case.

What should I do if I suspect a cancer cluster?

Concern about cancer clusters most often comes up in schools, workplaces, and in certain areas of a community. If you are concerned about a possible cancer cluster, you may want to contact your local or state health department. If you don’t know who to call in your area, the CDC has a list of web links to local and state contacts at (Also see the “Additional resources” section.)

Some of your concerns may be relieved by making a phone call. If not, the health department will probably look into doing a brief investigation and then, if needed, will do a more complete review of the situation. For more complex or urgent situations, the health department may also call in experts from the CDC or other agencies to do a more in-depth investigation.

The investigators may be able to figure out with a fair amount of certainty that there is no true cluster, or that there is a true cluster and it is probably caused by a certain exposure. But in many cases, even when it seems there is a cluster, a single cause cannot be found.

Suspected cancer clusters often greatly distress those involved. It’s very important that these situations be handled openly, promptly, and professionally. Even though thorough investigations of potential cancer clusters rarely give clear-cut answers, it’s important that communities keep reporting suspected cancer clusters, and that health organizations respond to those concerns.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

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

National Cancer Institute (NCI)
Toll-free number: 1-800-4-CANCER (1-800-422-6237)
TTY: 1-800-332-8615

Centers for Disease Control and Prevention (CDC)
Toll-free number: 1-800-232-4636
Cancer cluster information:
Links to state and local health departments:

    Provides the latest information on public health issues, including cancer clusters at the Web sites listed above

National Institute of Environmental Health Sciences (NIEHS)
Telephone: 919-541-3345

    Part of the National Institutes of Health (NIH), this group offers information on how the environment affects human health and diseases, including cancer.

*Inclusion on this list does not imply endorsement by the American Cancer Society.

No matter who you are, we can help. Contact us anytime, day or night, for information and support. Call us at 1-800-227-2345 or visit

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Last Medical Review: April 7, 2014 Last Revised: January 4, 2018





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