Cancer clusters

bulls eye

When people notice several friends or others living in their local area being diagnosed with cancer, it can sometimes feel like these cases are more than just a coincidence. People often worry that these cases of cancer may have been caused by the same thing. They suspect what researchers call a ‘cancer cluster’.

What are cancer clusters?

A cancer cluster occurs when more cases of cancer than expected are diagnosed in a group of people, geographic area or period of time. Most suspected clusters are chance coincidences. But real and apparent clusters can cause anxiety for people who spot them.

How common are cancer clusters?

Cancer clusters are rare, but it’s important to take into account people’s concerns. Clusters are more likely to be real if:

  • They involve many people with cancer within a small geographical area.
  • There are many cases of a single type of cancer, rather than cases of different cancer types.
  • The cancer type is rare.
  • The cases are affecting age groups that would not normally be associated with this type of cancer.

A cancer cluster that is actually caused by something in the environment rarely happens. But one example is from the 1960s when scientists linked cases of mesothelioma – a rare cancer – with exposure to asbestos fibres. When many cases of mesothelioma emerged in people living near asbestos mines, scientists thought there could be a link between inhaling asbestos fibres and cancer.

Further research confirmed the direct link and showed that working with asbestos is the major risk factor for developing mesothelioma. Asbestos is now banned in the UK and other countries but in the post-war years between the 1940s and 1960s, it was widely used in various manufacturing industries.

Why do people suspect a cancer cluster?

There are a number of reasons why cancer clusters seem to be more common than they actually are:

  • We notice cancer more often if we or someone we know has had it. If we’ve had a personal experience of cancer or know someone who has been diagnosed, it’s natural to want to find out what caused the disease. We might then become aware of other people around us who have also developed cancer, and think that these cancers may be linked, or worry that they resulted from exposure to something in our environment.
  • Cancer is more common with age. Sadly, this means that the older we get, the more likely it is that we will know many people with cancer.
  • Cancer is a very common disease. More than 1 in 2 people will develop some form of cancer in their lifetime, a number which is on course to keep rising because of our increasing life expectancy.
  • Even if cancer cases occur randomly in the population, there may be a few cases in one area. These 'clusters' are not necessarily due to a shared cause in that area, they just happen by chance.

How are cancer clusters investigated?

When investigating a cancer cluster, experts will look at a wide range of factors that are more likely to mean it is a genuine cluster. Most importantly, they compare the number of diagnosed cases with the number of cases you could expect in that population. As a first step, there must be more diagnosed cases of cancer than we would normally expect for there to be a genuine cancer cluster.

If this is the case, experts still need to establish whether this finding is statistically significant which means it’s unlikely to have happened by chance. Though even if a finding is statistically significant, this does not necessarily mean it‘s a genuine cancer cluster and other factors must be taken into account.

If you’re interested in more detailed information about how cancer clusters are investigated using statistics you can download this useful cancer cluster fact sheet produced by South West Public Health Observatory.

What should I do if I’m concerned about a cluster?

If you’re concerned about a large number of cancer cases in your area, you should contact your local public health authority. They will be able to decide whether an analysis of the reported cluster is needed.

If you are concerned about a possible cluster of cancer cases in your workplace then you should contact the Health and Safety Executive (HSE). The HSE is the national regulator for health and safety in the workplace.

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