Long-term mobile use and the risk of brain cancer
Using mobile phones for more than 10 years doubles the risk of brain cancer, newspapers reported. They suggest that the risk may be greater in children whose thinner skulls and developing nervous system make them more vulnerable.
The Daily Mail reported that “researchers found that long-term users had double the chance of getting a malignant tumour on the side of the brain where they held the handset”.
The stories are based on a review of studies that looked at the difference in mobile phone usage between people with and without brain tumours for more than a 10 year period.
The authors found 11 studies on people who had used mobile phones for more than a decade. Some of these showed that mobile phone use significantly increased the risk of some types of brain cancer while others did not.
Further perspective is given to this by data from Cancer Research UK, which suggests that “brain tumours” are rare and occur in less than seven in 100,000 people.
Contrary to newspaper reports, this review did not find that children are at greater risk of cancer from mobile use. The review did not look specifically at children and it would not be possible to come to this conclusion. These headlines originated from a comment by one of the authors.
However, the Stewart Report, a government sponsored independent review, recommended in 2004 that the use of mobiles by children should be minimised as a precautionary measure. This recommendation is endorsed by the findings from the 2007 MTHR Report, which found that although there was no evidence that mobile phones were associated with adverse effects in adults, further research is still needed for their use by children, and for their long-term use by adults. Proposals for the MTHR 2 study that begins in 2008 include epidemiological studies of the risk of brain tumours in children.
Where did the story come from?
Dr Lennart Hardell and colleagues from the University Hospital in Sweden conducted this review. The study was funded by grants from Cancer-och Allergifonden and Orebro University Hospital Cancer Fund. The study was published in the peer-reviewed medical journal Occupational and Environmental Medicine.
What kind of scientific study was this?
This publication was a review of previous studies that have assessed the relationship between use of mobiles or cordless phones and various types of brain tumour.
The researchers were mainly interested in whether long-term exposure to mobiles increases the risk of cancer, and therefore focussed on studies that looked at mobile use for 10 years or more.
What were the results of the study?
The researchers found 18 studies in total, but decided that only 11 of them were suitable for the study (they examined mobile use for more than a decade). The majority of the studies they found were of a case-control design and looked at the risk of two types of brain tumours; gliomas and acoustic neuromas.
Six of the studies examined the occurrence of gliomas (a type of brain tumour). Although all 6 found an increased risk, only 2 of these found a difference big enough to be statistically significant.
Four studies looked at the occurrence of acoustic neuroma. All four found there to be an increased risk of acoustic neuroma (a slow growing tumour on the acoustic nerve) on the same side of the head that the user held the mobile. Three of these studies found the increase in risk to be significant, but the fourth study did not.
The 11th study did not look at risk of glioma or neuroma specifically, but considered ‘other brain tumours’.
The researchers then carried out a meta-analysis, and pooled the data from the 6 studies of glioma and the 4 studies of acoustic neuroma. In both cases, the meta-analysis found that using a mobile for 10 years or more results in an increased risk of gliomas or neuromas on the same side of the head on which the mobile phone was used. The researchers report that the risk of gliomas is doubled.
What interpretations did the researchers draw from these results?
The researchers conclude that the results from these studies “give a consistent pattern of increased risk for acoustic neuroma and glioma” for longer-term mobile phone users.
They believe that this elevated risk is particularly pronounced for the side of the head to which the phone is most commonly held.
The researchers call for caution in mobile use and for more research to assess the risk to long-term users.
What does the NHS Knowledge Service make of this study?
As time progresses, more data on the long term effects of mobile phone use will become available. Mobile use has only been widespread for the last decade, and so longer-term studies will only become possible in the coming years.
There are several points that should be considered when interpreting the results of this study:
- The headline “Children ‘at greater risk of cancer from mobile phone use’”, in The Daily Telegraph may lead you to believe that this study revealed that children are at greater risk of cancer from mobile phones. This is not the case however and the review did not specifically look at the effects of mobile phone use in children. The reports that children are “particularly vulnerable” are based on a comment by one of the people who wrote the review.
- Of the six studies that looked at the occurrence of glioma, only two reported statistically significant differences between people with the cancer and those without. Of the studies that looked at acoustic neuroma, only three of the four were significant. The final study that looked at brain tumours in general was also not significant. Not having statistical significance means there is an increased likelyhood that the results came about through chance.
- The authors pooled the results and performed a meta-analysis. Meta-analyses are only a good, viable way of analysing data, if the characteristics of the studies included are intrinsically similar. The differences in methods and populations between these studies mean that the validity of the end result is questionable.
- Five of the case-control studies also considered how common ‘other types of brain tumour’ were (four studies were of meningiomas)’. None of them found any significant increase in risk with more than 10 years of mobile use.
- The use of questionnaires to collect information on people’s mobile use after they have developed a brain tumour may be subject to “recall bias”. This means that, following the devastating diagnosis of a brain tumour, people may report their mobile use differently to people who were chosen as normal, healthy controls.
Case control studies and reviews of this study design, may generate theories for further investigation, but cannot prove a causal link. Neither can they give us an idea of how common these tumours are. Data from Cancer Research UK suggests that “brain tumours” are rare and occur in less than seven in 100,000 people.
Therefore a small increase in this absolute background rate will be hard to detect without very large, well designed further studies.
Sir Muir Gray adds…
As a very heavy mobile phone user, I have worried about this for years. This paper reinforces my practice of using the phone as little as possible and of using a hands free set.
