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Cancer clusters

An archery targetWhen people notice several friends or others living on their street being diagnosed with cancer, it can sometimes feel like these cases are not just a coincidence. 

People often worry if these cases have been caused by the same thing. They suspect 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 coincidences that occur by chance. But real and apparent clusters can cause anxiety for people who spot them.

Real cancer clusters are rare

It is important not to dismiss people’s concerns about cancer clusters. But we have to bear in mind that many apparent clusters are just down to coincidence. Clusters are more likely to be real if:

  • They involve many people with cancer within a small geographical area
  • There are many cases of one type of cancer, rather than cases of different cancers
  • The cancers are of a rare and not a common type
  • The cases are affecting age-groups that would not normally be associated with this type of cancer

Real cancer clusters caused by something in the environment are rare. A good example was found in the 1960s when scientists linked cases of mesothelioma with exposure to asbestos fibres. 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.

Mesothelioma is a very rare cancer. So when many cases of mesothelioma emerged in people living near asbestos mines, the direct link between inhaling asbestos fibres and cancer was proposed. Further research confirmed the direct link and showed that working with asbestos is the major risk factor for developing mesothelioma.

Why do people often suspect a cancer cluster?

There are many reasons why real 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 have had a personal experience of cancer or know someone who has been diagnosed, it is 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, such as our neighbours or work colleagues. In such circumstances, it is common for people to 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 3 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, just by coincidence, a few cases in one area. These 'clusters' are not necessarily due to a shared cause in that area, they just happen by chance.

This can be explained by something called the Texas Sharp Shooter effect. The name comes from an imaginary Texan who fires his weapon at a barn door. After he has fired a hundred shots, he finds the closest cluster of bullet holes, draws a bulls-eye around them, and claims to be a sharpshooter.

To a passer-by, his claim might seem true but they have seen the bullet-hole cluster out of context. This often happens with cancer clusters. We might suddenly become aware of people developing cancers at the same time, which might have nothing to do with a common cause.

Investigating cancer clusters

When investigating a cancer cluster experts will look at a wide range of factors including the number of actual cases and other factors that are more likely to mean it is a genuine cluster. 

Most importantly they compare the number of actual cases with the number of cases you could expect in that population, there must be more actual 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, meaning that it is unlikely to have occurred by chance. And even if a finding is statistically significant, this does not necssarily mean it is a genuine cancer cluster and other factors must be taken into account.

If you are interested in a more detailed description of the statistics involved, the South West Public Health Observatory have produced a good cancer cluster fact sheet.

What to do if you are concerned about a cluster

If you are concerned about a large number of cancer cases in your area then you should contact your local public health authority. They will be best placed 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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Question about cancer? Contact our information nurse team
Updated: 13 January 2015