A Statement from the American Cancer Society Website on Cancer Clusters

Cancer clusters have received a good deal of attention in the media in recent years, both in the news and in Hollywood movies. More than one thousand suspected cancer clusters are reported to state health departments each year. But what exactly is a cancer cluster?

Many people tend to think of a cancer cluster as a higher than normal number of cases of cancer in a defined community. Cancer clusters are usually thought to be caused by some type of environmental pollutant.

Scientists, however, usually define a cancer cluster a little differently, as a greater than expected number of cancer cases in a defined geographic area or group of people over a certain period of time. A cancer cluster almost always involves only one type of cancer.

What are the characteristics of a true cancer cluster?

There are some important points to know about cancer clusters. First, cancer is actually a group of more than one hundred 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. True clusters usually involve one of the following:

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

If the cluster includes cancers of many different sites or over a period of many years, it is very unlikely to be a true cluster caused by a single environmental agent.

It's also important to remember that cancer is a common disease. Nearly half of all men and a little over one third of all women in the United States will develop cancer during their lifetimes. It is also a leading cause of death; about one out of every five deaths in the United States results from cancer. It isn't necessarily uncommon for several people in a relatively small area to develop cancer around the same time.

Most well-documented cancer clusters have not been found in the community. Rather, they have been seen in the workplace, where exposures to certain chemicals or other factors tend to be higher and over longer periods of time. Also, the group of people being exposed is much better defined in workplace groups. In fact, the links between cancer and many cancer-causing agents (carcinogens) were first figured out from studies of workers in certain jobs.

Lung, skin, and bladder cancers are the types of cancer most commonly linked with high level exposure to workplace carcinogens are. Other cancers such as leukemia, lymphoma, testicular, and brain cancer occasionally occur in clusters as well.

Statistics can usually give an idea of a particular cancer cluster being strictly due to chance. But even statistics can't tell the whole story. The excess number of cases reported in a cancer cluster may look significant based on statistics, but this does not necessarily mean that the cancers are caused by something unique to that area. Some clustering of cancer cases can still happen by chance (see below), but people tend to notice and report situations where rates seem to be above average.

Who investigates cancer clusters, and how is this done?

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

• the type(s) and number of cancers involved 
• any suspected exposure(s) that might be causing these cases 
• 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

At this point, as many as four out of five suspected clusters are found NOT to be true clusters. For example, the group of people may have different types of cancers, or the number of cases isn't greater than what would be expected, etc.

If the health department feels that the potential cluster should be studied further, more information will be collected. Attempts may be made to contact patients or relatives to check information. Sometimes investigators will ask to see medical records. The health department may do a more in-depth statistical analysis on the number of cases in the affected area and surrounding regions. Scientists in the health department may also look at reports in the medical literature to see if similar clusters have occurred elsewhere in the past.

If needed, the health department may ask for the help of federal agencies, which have more resources they can use. The Centers for Disease Control and Prevention (CDC) is the agency most commonly involved. Other agencies, such as the Environmental Protection Agency (EPA) and the National Institute of Environmental and Health Sciences (NIEHS), may also help in the investigation. Federal agencies may do a more in-depth investigation, including getting more thorough medical histories. These agencies may also obtain and test samples from the environment (air, soil, drinking water, etc.), and 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 is very important that government agencies keep concerned 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?

• A true cluster may not exist

In many cases, investigators can determine that a "cluster" of cancers isn't a true cluster. For example, the number of cases may not turn out to be higher than expected when other factors that could explain the increase (such as age, gender, and race/ethnicity) are taken into account.

• A cancer cluster may be found and the cause can be identified

In some cases, especially when there may be an obvious potential cause, a true cancer cluster may be confirmed. 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 a true cluster may be present, it's important to realize that it's very rare that a cause is found. There are several reasons why this is the case.

Random patterns can create the illusion of a cluster: Even if the number of cases in an area is higher than expected, it still may not be a true cancer cluster. More cancer cases in the United Stated are expected in large population centers or in places where the population tends to be older. But allowing for this, cancer cases in the United States for the most part are spread randomly across the country. Even with a random pattern, there will be more cases than expected in some spots, and fewer cases than expected in others. Certainly 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 and in different locations. Eventually you would find that some of the circles would contain more cancers than expected. Some might be true clusters, but most would be due to chance.

It is very hard to determine 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 a wide array of 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 assuming 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 single exposure that may be the cause can be like looking for the so-called needle in the haystack.

There is often a long delay between exposure and cancer: In clusters where the cause was known (again, mostly from 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. Again, it's not easy to study people and their environments. Suppose a group of people live in a community where there's 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, or 10, or 20? And what did the people with cancer have in common during that time? Added to this, some people would have moved into the community, while others may have moved away since then. Should the cases of people who moved into town in recent years be included? And can the people who moved away be tracked?

The boundaries of the cluster area can be hard to define: Defining the geographic area is another issue that's not always as clear-cut as one might think it would be. Just how big should the "bull's eye" be drawn? Should it include only a local neighborhood where most of the cases occurred? 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 compound the matter even further, people may be more or less prone to getting cancer based on their genes. It's unlikely that everyone exposed will develop cancer, and there may be others who were not exposed who develop the same cancer by chance.

Scientists do their best to piece the puzzle together, but more times than not, they don't find a likely link. This doesn't necessarily mean that there isn't one; it may just be that one can't be teased out with the methods scientists have at the time. This may be an unsatisfactory answer for people in a community being affected, but it may be the case.

What should I do if I suspect a cancer cluster?

Concern about cancer clusters most often comes up in schools, in workplaces, and in localized areas of communities. If you are concerned about a possible cancer cluster, you may want to contact your local or state health department. (If you're having trouble figuring out who to call in your area, the CDC has a list of Web links to local and state contacts at http://www.cdc.gov/nceh/clusters/statelocal.htm. See the "Additional resources" section below.)

Some concerns may be relieved by making a phone call. If not, the health department will likely look into doing a brief investigation and, if needed, will conduct a more thorough review of the situation. For more complicated or urgent situations, the department may also call in experts from the CDC or other agencies to perform a more in-depth investigation, as described above.

The investigators may be able to determine with a fair amount of certainty that there is no true cluster, or that a true cluster is present and is likely caused by a certain exposure. But in many cases, a single cause may not be found.

Suspected cancer clusters often cause great distress to those involved. It is very important that these situations be handled openly, promptly, and professionally. Even though thorough investigations of potential cancer clusters rarely yield clear-cut answers, it is important that communities continue to report suspected cancer clusters, and that health organizations respond to those concerns.