Cancer clusters

Cancer clusters are extremely uncommon.

Cancer is one of the most common causes of death and disability in Western Australia (and Australia) so it is not unusual to find multiple people diagnosed with cancer in a workplace, neighbourhood or other geographical area.

About half of all Australians will have had a cancer diagnosis by the time they turn 85 and that by the time of their 75th birthday:

• 1 in 8 men can expect to have had a prostate cancer diagnosis and one in 153 will have died from it
• 1 in 10 women will likely have developed breast cancer, with one in 85 likely to have died from the disease
• 1 in 24 men and one in 29 women will have been affected by colorectal cancer.

These statistics help explain why a number of cancer cases are likely to occur in the same workplace, neighbourhood or other setting. 

For a spate of cancer cases to be considered a cancer cluster it will likely have:

• a large number of cases of one type of cancer – beyond what would normally be expected among the same population group
• a rare type of cancer
• cancer cases involving an age group not normally affected by that cancer (for example a cancer that normally affects only elderly people occurring in a large number of young people).

No. Just because a spate of cancer cases has the characteristics of a cluster does not mean those cases will be the result of a common cancer-causing agent.

The cluster is more likely to be a reflection of other factors relevant to the population such as ready access to healthcare, which would make members more likely to undergo regular cancer screening and therefore more likely to have cancer detected early.

Likewise, higher rates of cancer could be expected in a community that has high rates of smoking and alcohol consumption because these are leading risk factors for cancer.

If you suspect a cluster in:

• a workplace, you should report it to the relevant company or employer
• your neighbourhood, you can report it to the Department of Health.

Reports to the Department of Health can be emailed to epi@health.wa.gov.au or by calling the Director Epidemiology on 6373 3914.

Some general points to consider before reporting a suspected cancer cluster are:

• Most cancer clusters will turn out to be chance groupings.
• Even those with some characteristics of clusters will turn out to be coincidental groupings of individuals with cancer.
• A thorough investigation of a potential cluster may take many months – sometimes even years – to complete.
• There is no guarantee that a cause for the cancers will be found even if the investigation concludes that a common environmental agent was likely to have caused the cluster of cases. This might be because the individuals concerned were exposed to the agent many years earlier in another environment.
• A person from the population group should be nominated as a point of contact for the investigators. This person may be the cluster informant (the person reporting the potential cluster) or a person from within the cluster community.
• The informant and members of the cluster population will need to be willing to provide detailed information to the investigating team and may also be asked to consent to the investigators accessing their medical records.

If you plan to report a suspected cancer cluster to the Department of Health, we encourage you to consider the following questions:

• What is the suspected affected population?
• How many cancer cases are there?
• Over what period have these cancer cases been diagnosed?
• What makes you believe it is a cancer cluster?
• Do you belong to the cluster population?
• When did you become aware of the cluster?
• How have you come to be aware of this cluster?
• How many cases of cancer have been identified within this population?
• What is the age range of the cancer cases?
• What types of cancer have been identified?

The Department will conduct a preliminary investigation to determine whether the cases are potentially a cluster or just a coincidental grouping.

In some instances it will be possible to allay a community’s fears quickly because these early inquiries will show the spate of cases does not have the hallmarks of a cancer cluster.

For others, further investigations will be necessary. These investigations can take many months to complete.

The cancer cluster investigation will consist of up to three phases and may be terminated at any phase.

Phase 1 – Primary evaluation

The purpose of this phase of the investigation is to collect information from the informant (the person reporting the suspected cluster) to determine whether further follow-up is required.

During this initial phase, the investigators will determine whether the cases reported within the population are beyond what might be considered normal for that population using comparative population data.

Phase 2 - Secondary evaluation

This phase is required if the data gathered in Phase 1 suggest the need for further evaluation. In this phase, more detailed information will be sought from the cluster population and meetings with the cluster community will be organised.

Phase 3 - tertiary evaluation

If a higher than expected number of cases is still evident after verifying known cases in Phase 2, the investigation will move to Phase 3.

The duration of the investigation will depend on various factors including the groups potentially affected and the level to which the investigation goes. It could take many months – even years – to complete.

An investigation may be terminated without going through all the phases if epidemiological analyses find no basis for a cluster or if an environmental assessment rules out cancer-causing agents.

The informant (the person who reported the cluster) and other relevant parties will be given detailed information about the investigators’ findings and every endeavour will be made to keep members of the relevant population informed of the progress of inquiries.