Potential disease clusters, including cancer clusters, are a serious issue of public concern and represent on ongoing challenge for agencies responsible for disease surveillance and control.
A disease cluster happens when a large increase in a particular disease, well beyond the normal number of cases, occurs over a period of time. The cluster is defined by a group of people who share something in common, like their work or where they live. The investigation of a disease cluster is based on exposure to a defined pathogen (disease-causing substance). One example is e-coli infections. In such cases a clear cause for the disease is usually found. The investigation usually follows this route:
- A large number of people become ill; the signs and symptoms are identified
- Investigators identify the pathogen involved
- They search for the source of the pathogen
- They then control and/or remove the pathogen
Usually, the investigation is fairly quick because of the rapid onset of signs and symptoms (hours or days) after exposure to the pathogen and because of the low levels of usual occurrence of the disease. Armed with this general experience it is possible to establish surveillance strategies which lead to the rapid identification of certain kinds of outbreaks (clusters). That is why disease clusters are usually found by public health authorities and not by members of the public.
The framework described above is very useful in investigating infectious diseases. Applying this successful investigational framework to cancer clusters is more complicated. The difficulties come from the following:
- Cancer is not a single disease but a collection of over 100 different diseases. Cancers share some common characteristics, but they are very different. Even within specific cancers, there are different types – there are different kinds of leukemia, breast cancers, lung cancers, etc.
- We don’t know all the causes for most types of cancer. We do know that most cancers have multiple causes or triggers
- When we do know about something that causes a particular cancer, we also know that the cancer only appears many years after exposure and it also occurs over a prolonged period of time (e.g. smoking and lung cancer, or asbestos and mesothelioma)
That’s why sudden outbreaks of cancer are not seen in the same way they are for infectious diseases. The closest example occurs when groups are exposed to a very potent cancer-causing substance through their occupation, or have a short term exposure to a strong cancer-causing substance (a carcinogen). One example is the radiation exposure to survivors of the atomic bomb explosions in Japan. However, even that cancer cluster emerged after, and over, many years.
There is understandably an intense public interest in the causes of cancer. A lot of attention has been paid to environmental issues like electrical power transmission or pesticide use as potential cancer causes. This has fostered a public perception that there are causes of cancer which exert immediate strong influences and can lead to outbreaks of cancer in specific communities. Because of this perception, people can become very concerned when they become aware of multiple people in their community or workplace who are diagnosed with cancer.
When several people sharing a common connection (e.g. place of work) develop cancer, this might suggest a potential cancer cluster. However, we also know that all types of cancer already occur in all our communities. We can always expect some random connections between people with cancer because of the interconnected nature of society (e.g. workplace, residence, social activities).
Data on cancer occurrence is regularly monitored by the BC Cancer Agency; when concerns arise a specific investigation may be undertaken.
The first step of the investigation is to try to evaluate whether it’s possible that a real cluster may be present. The objective of the investigation is to determine whether the numbers of the specific cancer(s) of concern are within the normal limits. The investigation involves a number of elements:
- What cancer or cancers? Cancers can be grouped based on what is already known to cause them. Cancers which do not share the same causes are unlikely to form a true cluster. If there are a wide variety of cancers involved with unconnected causes in the potential cluster, the investigation may end at this point
- Who? In investigating the potential cluster it is important to identify all the cases of cancer, but it is also important to establish the non-cancerous group. These are the people (the ‘population’) who don’t have the cancer, but are considered potentially at risk because they share the same common connection (workplace, where they live, etc.) as the cancer cases in the potential cluster. It is important to establish how many people should be included in this group, in order to do the calculations on how many cancers would occur usually. Examples of a population may be all those working in a factory where some individuals have been diagnosed with cancer
- When? It is also necessary to establish a time period over which to consider cancers as being in the potential cluster. Because elements that cause cancer tend to act over long periods the “when” will usually span many years
Having made the decisions about the who, what and when, the BC Cancer Registry will be used to confirm all cases of the specific cancer(s) to be included in the analysis. Then the number of these cancers which would be expected to occur if the rate of the particular population was the same as the general population of BC will be calculated. This calculation takes into account the demographic profile of the population forming the potential cluster, using current information from local health areas, health authorities and census information.
The expected number of cancer cases for the population being studied is then compared to the actual number. Any difference between the two numbers is subjected to statistical analysis, which determines if the actual number is within normal limits or is an unusual occurrence. The BC Cancer Registry has been gathering data on cancer for many years, and these statistics show that the number of cancer cases fluctuates annually. Some years there are more cases of cancer, and some years there are less.
Frequently, investigations stop at this point because no unusual increase is found. It may be the normal number expected, or it may have been a ‘spike’ which caused the concerns of the community. If this is what happened, the community can be reassured that no evidence of excess cancer has been found.
If there appears to be evidence of an excess rate of the specific cancer, the situation is more difficult to assess. The calculations/ statistical analysis that are made can’t actually say whether there is a truly higher risk of cancer in the community. They can only determine how likely it is that the cluster could have happened by chance.
Unfortunately, if a true cancer cluster is found, the investigation can’t always identify what caused the excess numbers of cancer. This can be disappointing to those who hoped for a definitive answer.
Cancer clusters do happen. We know more about occupational than geographic or environmental causes of cancer because they are more easily identified. However, the largest known contributors to cancer occurrence in the general population are lifestyle-related (e.g. smoking, diet) so that clustering of cancer cases will not necessarily be related to the environment.
What scientists know about environmental exposure and cancer has been discovered through research. By looking at the patterns of occurrence of cancer, we can hopefully identify more potential causes and help to reduce its risk.