What Defines a Cancer Cluster?

A cancer cluster occurs when a higher than expected number of people are diagnosed with cancer in a certain geographic area over a specific period of time. Often, they are the first indication of an important public health problem. It was a cancer cluster—an epidemic of Kaposi sarcoma among gay men—that helped lead to the discovery of AIDS.

Not all cancer clusters turn out to be real, and even if they do, they may not point to any kind of environmental carcinogen. But they can be an important tool in the identification of a carcinogen. A cluster of mesothelioma in the 1960s led investigators to the cancer-causing properties of asbestos.

Cancer clusters can occur for natural reasons or even just by chance. The challenge comes from separating a true cancer cluster from one that occurred randomly. Another challenge involves identifying the cause of a cluster.

“These are time-intensive, resourceexpensive investigations,” says Sarah Janssen, MD, PhD, of the Natural Resources Defense Council.

Disease investigators first look for signs that the cluster is real and not by chance. For example, if a rare cancer affects a lot of people in the same geographic area, it could be a sign of something abnormal going on. (A cluster of instances of male breast cancer among men with a history of being stationed at Camp Lejeune raised concerns because the disease normally strikes about 2,000 men a year nationwide. An investigation found that the drinking water at the base was contaminated with toxic substances until the late 1980s.) Or if the cancer is occurring in a group or demographic where it is not usually found. (Before AIDS, Kaposi sarcoma was a cancer that had been known to mostly afflict older men of Mediterranean, Eastern European or Middle Eastern heritage.)

One of the greatest challenges to investigating a cancer cluster is knowing where to place the geographic boundaries. A “cluster” that is not real can sometimes be created by drawing a line around known cases, rather than defining the geographic area or population beforehand. Investigation of cancer clusters can be all the more difficult because it mostly falls to cash-strapped local and state health authorities, although a bill pending in Congress, known as “Trevor’s Law,” would strengthen resources to investigate clusters, especially in children.

“I think a lot of people who may have cancer may think to themselves, ‘I did everything right.’ Everybody wants to know why,” Janssen says. “Often in situations where there is not an obvious explanation, people suspect there is an environmental cause.”