Over the 25 years since cellphones have gone from being uncommon to common, the incidence of brain tumours has scarcely changed. Photo / Getty Images

Over the 25 years since cellphones have gone from being uncommon to common, the incidence of brain tumours has scarcely changed. Photo / Getty Images

With a pending World Health Organisation report on the effects of mobile phone usage on the brain, the evidence so far is pointing to good news. However, much is still unknown.

Is your mobile phone giving you a brain tumour? Yeah, nah, maybe: depending on who you listen to, all of the above may be true.

At the end of October a flurry of news reports kicked off another round in a debate that has been raging for 20 years. "Long-term use of mobile phones 'may be linked to cancer'," proclaimed London's Daily Telegraph. A few days later, by the time the story got to Fox News it had become: "WHO to announce cellphone, brain tumour link."

In fact there was no announcement, just speculation on what might be in a long-awaited report yet to be released. Most research to date finds no link between brain tumours and cellphone use. There is, however, a small question mark about "heavy" mobile phone users - although the data about such cellphone devotees isn't at all clear.

Based on what we know today, cellphone tumour remains rumour.

The report that many are hoping will clarify the picture is the final results paper of the Interphone study - a 20 million (NZ$45.5 million), decade-long international collaboration overseen by the World Health Organisation (WHO).

It's due out soon, but no one is saying when.

"It will be the largest study ever taken on the cause of brain cancer and the largest study taken of the link, if there is one, with cellphone use," says Professor Alistair Woodward, who oversees the New Zealand arm of the Interphone investigation.

About half the 13 countries participating have already published their results. The New Zealand study involved 156 cases of cellphone users in the 30-59 age group who had brain tumours, and 172 matched controls. Like most of the results from other countries published to date, its finding did not support an association between the use of cellphones and brain tumours.

"The finding is, overall, there is no association," says Woodward, who is head of the School of Population Health at Auckland University. "But then when you look closely there are these wrinkles."

He's talking about the rather odd finding of what seems to be a reduced risk for light and moderate users, which is difficult to fathom. Could using a phone a little bit actually reduce cancer risk? At the other end of the scale, there is the suggestion of a slightly increased risk for the top 20 per cent of cases - the heaviest users, on their phone several hours a day.