The Sellafield nuclear site with Seascale in the distance. Credit: Simon Ledingham/CC BY-SA 2.0

When an ITV documentary in 1983 revealed a high number of childhood leukaemia cases between 1955 and 1983 in the village of Seascale, in north-west England, it caused a public outcry.

That’s because the village is just a few miles south of the Sellafield nuclear site.

The documentary, which found 7 cases of leukaemia (when less than 1 would have been expected), prompted a series of investigations into what was happening. And ever since, the question of what caused the Seascale cancer cluster has been a matter of much debate.

Now, a report by the Committee on Medical Aspects of Radiation in the Environment (COMARE) – a group of independent experts that provides advice to the government about issues relating to radiation and health – confirms that the cancer cluster is no longer present, and supports earlier suggestions that radiation wasn’t to blame.

So what was?

What’s a cancer cluster?

A cancer cluster is the name given to a situation where more cases occur than would normally be expected for a type or group of cancers. And it could be in a group of people, in a certain area or over a period of time.

While it can be worrying if someone notices several cases of one type of cancer in their community, most suspected cancer clusters actually turn out to be chance coincidences.

Imagine you took a pen, closed your eyes, and randomly made 100 dots on a piece of paper.

Just by chance, there may be some areas on the paper that have more dots than others – the dots are unlikely to be evenly distributed. It’s the same with cases of cancer.

But that doesn’t mean that clusters can always be safely explained as random. In fact, when cancer clusters do appear, it’s important that scientists investigate to find out what caused them.

And that’s exactly what happened with Seascale.

The Seascale cancer cluster

The combination of more cases of childhood leukaemia and a nearby nuclear site formed a perfect storm for speculation.

The main finding of the new report was that there was no significantly increased leukaemia and non-Hodgkin lymphoma around Sellafield or Dounreay for the period 1991‐2006

– Dr Chris Gibson, chair of COMARE

Unsurprisingly, some suggested that radioactive discharges from Sellafield may be to blame. And residents, politicians and the public were shocked and understandably worried.

The 1983 documentary led to COMARE being set up, which has since carried out a whole series of investigations confirming that between the 1950s and the year 1990, rates of two types of childhood cancer, leukaemia and Non-Hodgkin Lymphoma (NHL), were significantly higher in Seascale than expected.

And investigations of childhood cancers near the nuclear site in Dounreay, on the north coast of Scotland, revealed a similar cluster there.

The good news, according to the recent COMARE report, is that since the early 1990s these clusters have disappeared.

“The main finding of the new report was that there was no significantly increased leukaemia and non-Hodgkin lymphoma (NHL) around Sellafield or Dounreay for the period 1991‐2006,” says Dr Chris Gibson, chair of COMARE. “And no new cases of leukaemia or NHL in children were registered close to either site for the same period,” he adds.

So it seems that whatever caused the earlier increased rate of childhood cancers is no longer happening.

While that’s reassuring, the question of what was to blame for the higher rate of these cancers up to 1990 remains.

What’s the evidence so far?

When researchers started to investigate the cancer rates in Cumbria they found that, weirdly, the rise in childhood cancer rates wasn’t mirrored in other nearby areas, including around the Sellafield site.

It was suggested that if radiation from Sellafield was to blame, then there should be higher childhood cancer rates in other areas too.

Researchers also worked out the extra radiation doses children and young people born in Seascale and Thurso (a village near Dounreay) could have been exposed to as a result of living near the sites.

And they found that the amount of extra radiation from the sites was dwarfed by the amount of radiation from natural sources – such as radon gas from the ground and naturally occurring radioactivity in foods, such as brazil nuts and bananas.

This suggests, again, the nuclear sites weren’t to blame.

“The best estimates of the radiation doses to both the Seascale and Thurso populations are much too small to account for the observed numbers of cases of leukaemia and NHL that occurred in the young people in these areas,” explains Gibson. “And current radioactive discharges are substantially lower than historic levels.”

But what if the damage was done far earlier, before the children were even born? The experts also looked at this possibility in detail and found no evidence that the amount of radiation parents were exposed to before getting pregnant was responsible for the cases observed.

So from all this evidence, COMARE’s experts think it seems less and less likely that radiation from nuclear sites was to blame.

So if it wasn’t radiation, what else could it have been?

Nearly 100 years ago, in 1917, Gordon Ward suggested a possible alternative explanation for the cause of childhood leukaemia: infection.

Childhood leukaemia and many other types of childhood cancers do not occur evenly within the population of Great Britain

– Dr Chris Gibson

There is evidence to support the suggestion that, while leukaemia is not an infectious disease, infections may play a part in its development. Firstly, cases of infectious diseases often cluster in different locations and over periods of time. Evidence from the UK found several clusters of childhood leukaemia throughout history that follow this sort of pattern. And not just near nuclear sites.

“Childhood leukaemia and many other types of childhood cancers do not occur evenly within the population of Great Britain,” says Gibson. “There are a variety of incidence rates in different geographical and social circumstances and these differ more than would be expected from simple random or chance variations.”

Secondly, stimulation of the immune system early in life, for example by attending day-care, seems to help protect against childhood leukaemia – possibly hinting at the fact that this type of cancer may be linked to an infection.

Thirdly, evidence has been accumulating that a process called population-mixing could be playing a role.

This process occurs when relatively large numbers of people from urban areas move into previously isolated rural communities, exposing the people already living there to new infections. “The population-mixing hypothesis proposes that childhood leukaemia can be a rare response to a common but, as yet, unidentified infection,” says Gibson.

But does this tally with what happened at Seascale?

Population-mixing – a possible explanation?

In the cases of Seascale and Thurso, an influx of workers who moved to the villages to work at the nuclear sites could have led to population-mixing. Potentially exposing long-term residents and their families to new infections, which could have played a role in the rise in childhood cancer rates.

This kind of effect had been observed before. During the Second World War, three ordnance factories were built in west Cumbria, leading to an influx of workers which coincided with a rise in childhood leukaemia deaths in the area.

And on the remote Scottish islands of Orkney and Shetland, there was a sudden rise in deaths from childhood leukaemia when hundreds of soldiers were stationed there.

Gibson also points to British and German studies that looked at the risk of leukaemia around proposed nuclear sites where construction never actually took place. These studies found similar risk levels in some of those sites to risks seen around active nuclear sites. “This is suggestive of a risk associated with factors other than the operation of the plant, such as the nature of the location itself,” says Gibson.

According to COMARE, population-mixing could have been a key factor leading to the rise in childhood cancer rates in Seascale and Thurso. “Given the highly unusual conditions experienced in Seascale and around Dounreay, it is likely that infectious agents are responsible, at least in part, for the excesses of leukaemia and NHL among young people there,” explains Gibson.

What’s next?

While the COMARE report strongly suggests that radiation from the nuclear sites doesn’t explain the cancer clusters in Seascale and around Dounreay, we don’t have all the answers yet.

And Gibson cautions that further research is still needed.

While there may be some evidence of a possible infectious element, there still isn’t a clear explanation of the role that infection and population-mixing might have played. Or whether it alone can explain why these cancer clusters occurred.

That’s why research into these possible explanations must continue. Because the people directly, or indirectly, affected by these cancers, and those still living in the area, deserve answers.

Jana Witt is a health information officer at Cancer Research UK