Cancer clusters

‘Cancer’ refers to a group of many different diseases. These diseases occur when cells in the body start growing uncontrollably and may spread to other parts. As different types of cancers generally have different known and/or suspected risk factors, a common cause is unlikely.

Cancer clusters are extremely rare.

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.

Age is one of the most important risk factors for cancer overall.

In WA in 2019:

• The risk of developing cancer by age 75 for men was 1 in 3 and for women was 1 in 4.

Cancers are common diseases in WA, Australia and the developed world

• In WA in 2019, there were 14,164 new diagnoses of cancer
  • The three most common cancers in males were prostate, melanoma and colorectal
  • The three most common cancers in females were breast, melanoma and colorectal

For a group 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.
• cases that share a common occupational or environmental exposure that is a known or suspected cause of cancer.

No. Just because a group 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 reflect other factors relevant to the population such as 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.

Some general points to consider before reporting a suspected cancer cluster.

• 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?

For all suspected cancer clusters that are reported to the Department of Health an initial assessment will be conducted to determine whether the cases are potentially a cluster or more likely to be a coincidental grouping.

In some instance it will be possible to alleviate a community’s fears quickly because the initial assessment will show the group of cases does not have the features of a cancer cluster.

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

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

Phase 1 - Initial assessment

The purpose of this phase is to collect informant details and information on the suspected cancer cluster to assist in making a general assessment. At the end of this phase a decision will be made on whether further investigation is necessary.

Phase 2 - Primary evaluation

The purpose of this phase is to collect cluster and case information as well as information on any significant exposures and risk factors.

Phase 3 - Secondary evaluation

The purpose of this phase is to determine whether there is a greater than expected number of cancer cases in the study population and if there are any potential exposures that could explain this.

Phase 4 - Tertiary evaluation

The purpose of this phase is to further validate the greater than expected number of cancer cases and undertake a detailed assessment of environmental agents that could have potentially led to the cancer cases.

Phase 5 (optional) - Research evaluation and surveillance

This is an optional stage of investigation to further investigate the cause of the cancer cluster and potentially implement surveillance of the cancer cluster setting to detect additional cases.

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.