Identifying a possible cancer cluster: Cancer is a very common disease. The National Cancer Institute estimates, based on 2009–11 data, that about 40.4 percent of Americans will develop cancer at some point during their lifetimes. With a disease as common as cancer, it is not unusual for two friends, coworkers, or neighbors to be diagnosed with the same kind of cancer or to be diagnosed at similar times. However, sometimes it may seem like more than just a normal coincidence. Sometimes it may seem as if many people in the same workplace, neighborhood, or family are being diagnosed with cancer. In these cases, a cancer cluster may exist.
Many suspected cancer clusters are reported each year. According to the American Cancer Society, as of 2014 more than one thousand reports of suspected cancer clusters were being made to state health agencies every year. The vast majority of these suspected cancer clusters turn out not to be true cancer clusters after careful investigation and statistical analysis. For many of the clusters that are believed to be true cancer clusters, an underlying cancer agent is never satisfactorily identified. However, in some cases, a cancer cluster is identified and an environmental or other agent is found to be responsible. This is one way that substances can be identified as causing an increased risk of cancer.
Just having a group of people who all seem to have been diagnosed with cancer at a similar time or who have a similar type of cancer is not enough to indicate that a true cancer cluster exists. A group of cancer cases is more likely to be a cancer cluster if one or more of certain factors exist. It is more likely to be a true cancer cluster if there is only one primary diagnosis of cancer involved and the cancer is an uncommon one. According to the National Cancer Institute, it is estimated that more than 232,600 new cases of breast cancer were diagnosed in the United States in 2014. Therefore, if ten women in the same moderately sized town were diagnosed with breast cancer, it would be unlikely to indicate a cancer cluster, but if the same number of women were diagnosed with an extremely rare form of cancer, a cancer cluster might be more likely.
If a very large number of cases of a common form of cancer occur, this may be a possible indication of a cancer cluster. Additionally, if cancer occurs in a particular group of people in whom it is not usually seen, such as breast cancer in men, or a type of cancer that usually develops in adults is diagnosed in children, it may be a sign of a possible cancer cluster. If a set of people who share a well-defined characteristic, such as all working in the same factory over the same period of time, are involved in the suspected cluster, it is generally more likely to be a true cancer cluster than if the suspected cancer cluster involves people who do not share a very specific common trait.
Investigating a suspected cancer cluster: When a cancer cluster is suspected, it is usually reported to a state board of health or to a state cancer registry. Cancer registries collect information about cancer occurrences and cancer-related mortality statistics. Anyone can report a suspected cancer cluster; it does not have to be reported by a government employee or by a doctor or other health professional.
Once a suspected cancer cluster has been reported, the state health department or other agency will collect information about the people in the suspected cluster. This will include information about the age and ethnic background of the people involved, the type or types of cancers, the places in which the individuals have lived and worked, the time period during which the cancers occurred, specific medical information about the cancers, and any other available information. The health agency will also try to gather information about what possible causes for the cancers exist, such as if there are any known or suspected carcinogens in the places in which the affected people work or live. They may also collect information about any other possible causes of the cancer, such as a history of smoking.
After analyzing all the gathered information, experts at the health agency will decide if there is reason to suspect that a true cancer cluster exists. According to the American Cancer Society, four out of five of all reported possible cancer clusters are determined not to be true clusters based on preliminary information. This is because often what may seem like a large number of cases of cancer is still within reasonable expectations given the population and other variables. One of the ways that experts can determine whether it is possible that the group of cancers constitute a true cancer cluster is by doing statistical analysis to determine whether the number of cancers is statistically significant. A group of cancers is statistically significant if it is more than the number of cancers that are expected to occur in the group by chance. In most cases, 5 percent is considered the benchmark to determine if something is statistically significant. This means that out of one hundred towns of the same size and population, for example, the number of cancers seen in the town in question would occur in five or fewer of the towns by chance alone.
If it is determined that there may be a true cancer cluster, the state health agency, scientists, medical professionals, and sometimes federal agencies such as the US Centers for Disease Control and Prevention (CDC) will begin to investigate in more depth. This may involve getting detailed medical records and histories, interviewing people in the area in question, and testing possible sources of carcinogens. It is usually a very time-consuming, laborious, and money-intensive investigation. Once all the evidence has been gathered, experts try to determine whether there is a true cancer cluster and what caused the cluster. Unfortunately, in many cases, it is never possible to determine a definite cause. This is because the factors that cause cancer are complex and interactive, and cancer may not be diagnosed for years or even decades after exposure to the believed carcinogen.
Historic cancer clusters: Several well-known cancer clusters have been documented in the United States since the 1960s. In some cases these clusters were linked to environmental toxins, such as toxic waste spills, and in other cases they have been linked to workplace-related exposures. Some cancer clusters have even helped identify substances that are carcinogenic that were not formerly known to be harmful.
Possibly the most famous cancer cluster is the cluster in Hinkley, California, which was the basis for the film Erin Brockovich (2000). The cause believed to be at the root of the town’s problems was chromium-6, or hexavalent chromium, that had been dumped into unlined ponds by the Pacific Gas and Electric plant near the town. The chromuim-6 was believed to have leached out of the ponds and into the ground and drinking water of the residents of Hinkley.
Another well-known cancer cluster was in Toms River, New Jersey. In Toms River, 118 children were found to have cancer between 1979 and 1995, far more than could be reasonably attributed to chance for that population. Investigations found that 4,500 barrels of toxic waste had been dumped at a landfill and much of the toxic waste had leached into the Toms River water supply.
In the 1960s, a very high number of cases of mesothelioma were found and were determined to have been caused by prolonged exposure to asbestos. At the time, asbestos was used frequently in consumer products and in shipbuilding. The discovery of the cancer cluster and the subsequent discovery of its cause led to a drastic change in the way asbestos is handled, its removal from schools and other public places, and the development of safe handling practices and alternative technologies. Without someone noticing and reporting the high incidence of mesothelioma, many more people might have been harmed by asbestos exposure.
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