Explainer: what are cancer clusters?

A cancer cluster generally features an unusually high number of the same type of cancer occurring in a group of people with a common exposure. Shutterstock

Most of us are living longer and we are all expected to be working longer. Because the likelihood of cancer increases as we age, we’re more likely to be diagnosed with cancer while still a member of the paid workforce.

That is equally true of our ageing workmates. An increasingly common phenomenon is that of a group of people – who might have been working in the same organisation for decades – will see their peers diagnosed with cancer around the same time.

Concerns that there may be a “cancer cluster” are sometimes raised when there is something unusual about the workplace: its location, the use of a certain types of equipment, old buildings that may contain known carcinogens such as asbestos, and so on.

Some of these factors seem to be at play in the most recent suspected cancer cluster at what is known as “the chook house” at the Victorian state Parliament.

Without knowing all the details it’s important to avoid making any specific comment on that case. But it raises an important issue: how do you differentiate between a “cancer cluster” and a normal pattern of cancer diagnoses?

A cancer cluster generally features an unusually high number of the same type of cancer occurring in a group of people with a common exposure – be it where they live, where they work or some other usually long-term exposure to an agent that has brought about the cancer.

The cancer cluster that attracted the most recent attention was the 2006 Brisbane ABC case. Of the 550 female staff, ten women were diagnosed with invasive breast cancer between 1994 and 2006 – a six-fold increase over the number of cases that might have been expected.

Governments are still working out how to deal with the enormous (but as yet unquantified) amount of asbestos in our workplaces, homes and public buildings. Shutterstock

The precise cause of those cancers remain uncertain, and the result being due to “chance” is still considered likely. The investigation did, however, lead to the building of a new ABC complex in Brisbane.

An earlier and now famous case was the Wittenoom asbestos miners and their families who have been diagnosed with a range of asbestos-related diseases, including mesothelioma, since the mine closed in the 1960s. This case helped confirm the now well-accepted link between exposure to asbestos and a number of respiratory diseases.

Uncertainty remains the most common outcome of cancer cluster investigations. A group in the United States examined the outcome of 428 cancer clusters investigations over two decades in that country and found no real increase in cancer cases above what might have been expected in 87% of “clusters”.

The study found one cluster investigation where the genuine increase in cancer cases had an identifiable cause: ship builders living in South Carolina who were exposed to asbestos through their work at a naval shipyard.

Of the other 69 suspected clusters where a substantial increase in cancer rates were observed, the cause remained unclear. In the two remaining cases, there was some indication of an (inconclusive) association between an identified exposure – contaminated water and/or air due to industrial pollution – to leukaemia.

In Australia, potential cancer cluster investigations are conducted by epidemiologists, with involvement from occupational hygienists, statisticians and occupational physicians who are independent of the employer, and are often government employees or consulting academics.

Cancer risk increases dramatically with age. Shutterstock

Half of all men and a third of all women in Australia are likely to have a cancer diagnosis by the age of 85.

While common, cancer it is also the most feared illness. It’s therefore important that an appropriate expert takes the time to listen carefully to the employees’ or residents’ concerns and any theories that might explain the phenomenon.

Cancer is not one disease but a category of disease that is made up of over 200 different illnesses with different causes, pathways and treatments. So if the suspected cluster involves lots of different types of cancer – some people effected by breast cancer, some by lung cancer, some bowel cancer and some leukaemias, for instance – then the workplace (or suburb or other exposure) is an unlikely culprit.

Likewise, if some of the people with cancer have been at that workplace or lived in that street (or had that exposure) for a short period, or had previous disease or risk factors that might explain the cancer, its unlikely to be a cluster.

After looking at the situation systematically, the employees’ or residents’ concerns may be allayed and the initial assessment concluded.

Of course, there may be legitimate grounds for concern. For example, exposure to pesticides, solvents, wood dust, diesel exhaust and radiation are established carcinogens that occur in some workplaces.

In Australia, around 5,000 cases of cancer a year are a result of occupational exposures to things such as environmental tobacco smoke, UV radiation in addition to those mentioned above.

If health authorities have cause to proceed with a formal investigation and a known carcinogen is identified, employers or landlords should act immediately to remove or reduce exposure – waiting for the final results before taking action makes no sense.

Wood dust is a cancer-causing agent. Shutterstock

Finally, it’s important to keep things in perspective. While we don’t know for sure what causes about half of all cancers, lifestyle factors are probably far more likely to be contributing to cancer risk than where we work or where we live.

We can all reduce our cancer risk by quitting or not taking up smoking, establishing and maintaining a healthy body weight, avoiding excessive UV exposure and excessive alcohol consumption, being more physically active and eating healthy food.

In broad terms Australia has a safe, well-managed environment with low levels of pollution by world standards. Constant effort is required to maintain and improve on that status.

And there is room for improvement. Like everything we do - even if we are doing okay, it makes sense to try and do even better. It’s a balancing act. But one that we will be increasingly required to face.