Explainer: Concern over cancer cases on the Bellarine Peninsula
Concerns raised by residents on the Bellarine Peninsula about an alarming number of cancer deaths among young adults in the area, particularly in the coastal town of Barwon Heads, have prompted many questions since they were revealed in The Age.
Some families believe the incidence of cancers is connected with a former agricultural chemical, a pesticide called dieldrin, that was widely used by farmers in the area until it was banned in the late 1980s.
The family of Georgie Stephenson, a Barwon Heads nurse who died in late 2017 aged 26 after her second bout with acute myeloid leukaemia, say they are convinced there was an environmental link to her illness.
Five of her peers – some of whom were from Barwon Heads, and others who attended Bellarine Secondary College in Drysdale – have died of cancer in recent years. Her family say they know of another 20 people in her age group in the area who have had cancer.
A leading law firm is also preparing a test case related to the death of a man in his early 30s who grew up in Ocean Grove and attended the high school, and dieldrin.
The state government says the school has been operating safely, and that soil tests over many years have found no threat to human health.
Does this constitute a cancer cluster?
Cancer clusters are notoriously difficult to identify.
"There are extremely few cancer clusters around the world that have actually been confirmed after thorough investigations," according to the Australian Cancer Atlas, which maps the risk of cancers by location.
The deaths of 16 people and higher incidence of cancer among those who worked at the Country Fire Authority training college at Fiskville, west of Melbourne, is one of the few confirmed cancer clusters in Victoria.
A common misconception, according to the Cancer Atlas, is referring to a "cluster" to identify an area or community that has a higher rate of cancer than other areas.
A true cancer cluster is rare and is said to occur when the number of people in a particular area, workplace or group diagnosed with a specific type of cancer is significantly higher than would otherwise be expected, after other contributing factors are taken into account.
Professor Lin Fritschi from Western Australia's John Curtin University has led a number of studies into occupational and non-occupational causes of cancer.
Professor Fritschi said the first question to ask was whether there was a particular type of cancer shared among a group.
The next steps were to try to work out what the common denominator was between those people, and whether any others might be at risk.
It was natural for people with cancer and their families to speculate on a potential cause, especially when people around them were also diagnosed, Professor Fritschi said.
"But almost always when you do these investigations you don’t find any association."
Does dieldrin cause cancer?
Professor Fritschi was part of a 2016 global review of published research into the carcinogenity of pentachlorophenol – an organochlorine – and related compounds including dieldrin.
"They are extremely persistent in the environment and in humans and animals," the review found. "Dieldrin ... was classified as probably carcinogenic to humans."
While dieldrin had been investigated in relation to breast cancer, there was "limited evidence" of an association, the scientists said.
However, they found "sufficient evidence" of a link between dieldrin and cancer in some animals used in laboratory testing.
Professor Fritschi said research into organochlorines and cancer was limited.
"The problem was there wasn’t very many studies, and the only thing we could look at was breast cancer because those studies were into breast cancer," she said of the study. "So the things that cause breast cancer don’t necessarily cause blood cancer.
"There has been a number of pesticides which have been associated with blood cancer, but no studies. From the point of view of literature, it’s quite difficult."
How are people exposed to dieldrin?
Food – particularly animal fat and breast milk – is one of the main sources of exposure to dieldrin for children and adults.
People can also be exposed to dieldrin by coming into contact with contaminated water, or through air or soil near hazardous waste sites, the US Agency for Toxic Substances and Disease Registry says.
"We do not know whether children differ from adults in their susceptibility to health effects from aldrin or dieldrin exposure."
Most dieldrin in the environment attaches to soil and to sediments at the bottoms of lakes, ponds, and streams, the agency says.
"Dieldrin can travel large distances by attaching to dust particles, which can then be transported great distances by the wind. Dieldrin can evaporate slowly from surface water or soil. Plants can take up dieldrin from the soil and store it in their leaves and roots.
"Fish or animals that eat dieldrin-contaminated materials store a large amount of the dieldrin in their fat. Animals or fish that eat other animals have levels of dieldrin in their fat many times higher than animals or fish that eat plants."
Where else has dieldrin been investigated?
One of the most high-profile suspected cancer clusters in Australia in recent years involved the ABC's former Brisbane newsroom in the suburb of Toowong.
An exhaustive investigation was conducted into the site after 18 women who had worked there were diagnosed with breast cancer between 1994 and 2006.
Monash University Centre for Occupational and Environmental Health epidemiologist Professor Malcolm Sim was a member of the three-person panel who led the investigation into ABC Toowong.
Dieldrin was investigated as part of that inquiry.
However, Professor Sim said there was no evidence the pesticide was used to excess in that workplace.
“Existing research strategies provide conflicting and mostly negative evidence with respect to DDT, DDE and dieldrin,” the final report of the inquiry said. “Lack of evidence for an association between organochlorine pesticides and breast cancer may be due to a true lack of association or to shared methodological weakness across a large number of studies.”
Dieldrin was also part of a 1987 Queensland parliamentary inquiry into the incidence of childhood leukaemia in the cotton-growing area of Emerald, 900 kilometres north-west of Brisbane.
Eight children who lived in the Emerald area died of leukaemia between 1980 and 1985. The study found that the levels of dieldrin contamination observed there were “unlikely to pose health risk”.
Are cancer rates higher on the Bellarine?
There is no statistically significant difference between the overall numbers of cancers diagnosed in the region from 2010-2014 and the national average, according to the Cancer Atlas.
It shows the number of non-Hodgkin lymphoma cases diagnosed in Barwon Heads-Ocean Grove in that period was 8 per cent above the national average. In the Drysdale area, NHL diagnoses were 6 per cent above average.
The number of leukaemia diagnoses was 8 per cent below average in Barwon Heads-Ocean Grove and 11 per cent below average in Drysdale.