Understanding Cancer Clusters

Every year in the United States, state and local health departments have to respond to more than 1,000 calls about possible cancer clusters from concerned citizens and citizens' rights groups. Authorities can actually dispense with three-quarters of these inquiries over the phone.

For the rest of them, the authorities need to follow up. In doing so they're looking for:

• The number of verified persons affected
• Their age
• Diagnosis/subtype of cancer
• Date of diagnosis
• Other related factors

They will then compare cancer incidence among the affected to the cancer incidence expected for that region or area. In as many as 15 percent of all instances, authorities find that the number of observed cases is indeed higher than what would be expected. This doesn't make it a cancer cluster, however, since chance is always a possibility.

On top of that, even after careful epidemiological investigations, it is extremely difficult to determine with any confidence just what the causes of these diseases was. "The few exceptions have involved clusters of extremely rare cancers occurring in well-defined occupational or medical settings, generally involving intense and sustained exposure to an unusual chemical, occupation, infection, or drug." (1)

Defining a Cancer Cluster

Both 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 fact the CDC has a set of parameters that must be met in order to go from being a suspected cancer cluster to a cancer cluster (2):

• A greater than expected number:
A greater than expected number is when the observed number of cases is higher than one would typically observe in a similar setting (in a group with similar population, age, race, or gender). This may involve comparison with rates for comparable groups of people over a much larger geographic area – e.g., an entire state.
• Of cancer cases:
All of the cases must involve the same type of cancer, or types of cancer scientifically proven to have the same cause.
• That occurs within a group of people:
The population in which the cancers are occurring is carefully defined by factors such as race/ethnicity, age, and gender, for purposes of calculating cancer rates.
• In a geographic area:
Both the number of cancer cases included in the cluster and calculation of the expected number of cases can depend on how we define the geographic area where the cluster occurred. The boundaries must be defined carefully. It is possible to “create” or “obscure” a cluster by selection of a specific area.
• Over a period of time:
The number of cases included in the cluster – and calculation of the expected number of cases – will depend on how we define the time period over which the cases occurred.

As the American Cancer Society points out, "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." (3)

Truth be told, cancer clusters are extremely rare. People who worked around asbestos and developed mesothelioma are considered part of an infamous cancer cluster, and while there have been some other smaller ones—notably the Hinkley groundwater case made famous by Erin Brockovich—they are in fact rare in occurrence.

1. Thun MF, Sinks T. Understanding Cancer Clusters. CA Cancer J Clin. 2004 Sep-Oct;54(5):273-80.
2. CDC: Cancer Clusters
3. Cancer Clusters: American Cancer Society