The most comprehensive study to date on whether cellphones raise the risk of brain tumours has proven inconclusive.

The 10-year study, called Interphone, is due to be published Tuesday in the International Journal of Epidemiology but results were released early, on Sunday.

The huge study followed more than 10,000 people, including cellphone users, non-users, users who survived brain cancer and survivors who had never used cellphones.

It found most cellphone use didn't increase the incidence of glioma, an aggressive, fatal form of brain cancer, or of meningioma, a slower growing, often benign tumour.

But the results did suggest that heavy cellphone users may be at an elevated risk of developing tumours. The frequent users had a 40 per cent higher risk of glioma, compared to people who never used the phones. They also had about twice the risk of developing tumours on the same side of their heads where they normally held their phones while talking.

It's important to note that the study was conducted between 2000 and 2005, and usage described as "heavy" may not be considered so by today's standards. Heavy users reported chatting on the phones for the equivalent of a half an hour a day over 10 years.

Despite the findings of higher tumour risk among frequent cellphone use, the researchers concluded their evidence was not strong enough to link the devices to the disease.

"All we can say at this point is that the study does not demonstrate an increased risk, but it doesn't either say there is no risk," Elisabeth Cardis, the study's lead author and a professor at the Centre for Research in Environmental Epidemiology in Barcelona told CTV News.

Low use might decrease tumour risk?

One aspect of the study puzzled researchers. Data showed that overall, most cellphone users actually had a lower risk of brain cancer than people who had never used one. The researchers described that finding as "implausible."

The 21 scientists who conducted suggested faulty research methods might be to blame for that finding. For example, the study was interview-based, not interventional, and required the subjects to try to recall how much time they spent on their cell phones, a method that can lead to inaccuracies.

Dr. Jack Siemiatycki, an epidemiologist at the Centre hospitalier de l'Université de Montréal Research Centre and one of the scientists who contributed to the research, cited other problems. In a statement he issued before the study's publication, he said restricted access to participants compromised the validity of the research results.

"The findings... are ambiguous, surprising and puzzling," he said.

He said only about 50 to 60 per cent of eligible subjects participated in the study, which may indicate the sample wasn't representative. As a result, they may have provided an inaccurate portrait of cellphone usage among cancer patients and the healthy control group.

Siemiatycki said rigid constraints imposed by ethics committees forced researchers to recruit subjects through physicians, which may have also skewed results.

He stressed that any risks of developing cancer through cellphone use would be small, but urged concerned consumers to use hands-free headsets as a preventative measure.

More study needed 

The researchers called for more study, particularly among young cellphone users. They also noted that more study is needed because the latest mobile phones aren't like those used between 2000 and 2005. Today's phones have smaller batteries, lower emissions, and the popularity of hands-free devices and texting has reduced exposure to the head.

Another factor not examined was the risk of having a cellphone on while not making calls, such as in a pocket, or next to the bed while sleeping.

"I don't think this is the definitive study or the last word," Dr. Mary McBride of the BC Cancer Agency, told CTV News.

The study was co-coordinated by the International Agency for Research on Cancer, an intergovernmental agency that's part of the World Health Organization. It was performed by scientists in 13 countries, including Canada, the United Kingdom, France and Japan.

The Canadian portion took place in Quebec, backed by the government-funded Canadian Institutes of Health Research.

Almost a quarter of the 19.2 million euros (US$24 million) required to fund the study was provided by the cellphone industry, though WHO said measures were taken to ensure the scientists' independence was protected.

European scientists last month launched what will become the largest ever study into the effects of cellphone use. It aims to track at least a quarter of a million of people in five European countries for up to 30 years.

This kind of study, called a prospective study, is considered more accurate because it tracks cellphone use in real time, rather than requiring subjects to recall their use.