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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. Around 1 in 3 people will develop some form of cancer in their lifetime. This means that even when there seem to be lots of cases of cancer in the same place, it doesn’t mean that anything unusual is going on. Sometimes, many people in an area develop cancer just by chance.

Cancer might seem more common than it used to be. This is because:

  • People are living longer and cancer is largely a disease of old age
  • Better medical care means that people don’t die from other diseases (such as infections) as much as they used to
  • We can diagnose cancer more accurately
  • Cancer is less of a taboo and cases are more common knowledge than 50 years ago 
  • Lifestyle changes are taking their toll on cancer incidence. It is not just smoking and drinking alcohol that affect cancer risk, but also factors such as body weight, eating unhealthy foods and lack of physical activity

The Texas sharp shooter effect. This is a common statistical problem where coincidences can seem like they have hidden meaning. 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 cancer clusters are investigated, experts will establish the number of actual cancer cases. They will also look for factors that are more likely to indicate a genuine cluster are satisfied. And they will seek to find out if a specific source for the cancers is suspected in the environment.

First, scientists calculate a Standardised Incidence Ratio (SIR). This is done by comparing the number of observed cases in the suspected cluster with the number of cases of cancer that would be normally expected in a population. If a cluster is real, there must be more actual cases of cancer than would be normally expected.

Even 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.
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.

Chance and cancer clusters. Even if a finding is statistically significant, this does not rule out a chance association. Unfortunately, cancer is a very common disease. In an area with a population of 100,000 people in the UK, there will be on average around 370 new cases of cancer diagnosed per year. And in this same population, about 175 people will die from the disease in a year. Even if these cases occur randomly, there may be, just by coincidence, a few cases that will be clustered in one area. These clusters are not real but a result of a chance association.

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 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: 6 August 2010