A cancer cluster is the occurrence of a greater-than-expected number of cancer cases among a group of people in a defined geographic area over a specific period of time. In very rare cases, cancer clusters can indicate the presence of carcinogens—cancer-causing agents—in the environment, but for the most part, they are the result of random patterns.

A cancer cluster can be defined as such when there is a larger than expected number of cases of a more common type of cancer, several cases of a rare type of cancer, or cases of a type of cancer that does not usually manifest in a certain group—such as children getting a cancer usually seen in adults.

Many exposures can cause cancer, making it tricky for researchers to define a clear, causal link between environmental contaminants and cancer clusters. There is also usually a delay between exposure to a substance and cancer, meaning that any cancer causing exposure could have occurred in the past or in a different geographic area from the one being investigated. Either way, random patterns can form clusters, and this is the most common explanation behind clusters that don’t have an identified cause.

Cancer cluster reports are usually fielded by a public health agency over the phone. The agency, often a local or state health department, gathers 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, and specific information about each person thought to be affected and the cancers themselves. If compelled by these responses, the agency will conduct a formal investigation with assistance from federal agencies, such as the Centers for Disease Control and Prevention (CDC) and the Environmental Protections Agency (EPA), if needed.

Next, investigators look to state cancer registries and census data to compare the suspected group with the number of cases that is typically seen in a similar group, such as age, gender, and ethnicity. This step determines if there actually is an excess of cancer cases that warrants an epidemiologic study.

Finally, investigators try to discover if a cluster is associated with risk factors in the local environment. Even if a possible environmental contaminant is found, further research needs to confirm that the contaminant caused the cluster. This is how scientists were able to link the relationship between malignant mesothelioma, a rare type of cancer, and asbestos exposure.

Even as new methodologies of investigating cancer clusters develop, those efforts from the past forty years have rarely been successful in terms of understanding cancer etiology—the causes of the disease. In a 2012 study that assembled and reviewed reports pertaining to over 400 cancer cluster investigations, the cause was identified with certainty in only one of them.

Clusters aren’t always limited to small geographic regions, according to Dana-Farber’s Shuji Ogino, MD, PhD, MS. Ogino says there have been higher-than-normal breast and colorectal cancer cases in the United States and other areas of the Western world. “We found that the causes of some of these cancers include obesity, red and processed meat, lack of physical activity, and diabetes,” Ogino says.

About one-third of people in the United States will develop cancer in their lifetime. Genetic predisposition, lifestyle characteristics, and many other factors play a role in this potential risk. Environmental contaminants, such as radiation and other environmental chemicals, add another category of risk factors, but there are challenges to find methods that can specify any relationships between exposure and diagnosis.

This article was originally published on March 6, 2018, by Dana-Farber Cancer Institute. It is republished with permission.