“Heavy mobile phone use may be linked to an increased risk of cancer of the salivary gland”, according to a BBC News report.
The Daily Telegraph also reports on research that people using their mobile phones for “hours a day are 50% more likely to develop mouth cancer than those who do not talk on them at all”. The research behind this story is a case-control study that explored the link between mobile use and cancer. In this case, researchers investigated the link between mobile phone use and parotid gland (salivary gland) tumours (both benign and cancerous).
The researchers recognise that there are weaknesses associated with the study design and the way that the findings were analysed that should be taken into account when interpreting the results. The main comparison of the study, of non-users with regular users, found they were equally likely to have tumours. The researchers only obtained significant associations when they looked at subgroups of people with particular ways of using their mobile phones and, as these subgroups are smaller, their results are less reliable. The “50% increased likelihood of developing mouth cancer” reported by the newspapers is mostly due to an increased risk of benign salivary gland tumours.
It is worth noting that the risk of developing a salivary gland mouth tumour is low and the risk that that tumour is cancerous is even lower. In the face of conflicting and unclear evidence, more research is needed before the risks associated with mobile phone use, if any, are identified.
Where did the story come from?
Dr Siegal Sadetzki and colleagues from the Chaim Sheba Medical Centre; the Sackler School of Medicine at Tel Aviv University in Israel and the International Agency for Research on Cancer in Lyon, France carried out the research. The study was funded by the European Commission Fifth Framework Program, the Union Internationale Contre le Cancer, and a grant from the Israel Cancer Association. The study was published in the peer-reviewed medical journal: American Journal of Epidemiology.
What kind of scientific study was this?
This case-control study was of Jewish adults who had benign or cancerous parotid gland tumours (PGTs, which are tumours in the largest of the salivary glands). They were compared to randomly selected Israeli adults from the National Population Registry. Up to seven randomly selected “controls” were matched to each of the adults with PGTs - the “cases” - based on their gender, interview date, age and continent of birth.
The researchers interviewed all participants on their mobile phone usage, other possible risk factors and demographic variables. The amount of mobile use was divided into ‘regular users’, who had made or received more than one call per week for at least six months and ‘non-regular users’.
The regular users gave up to a 10-year history of their mobile phone use. This included all the mobile phones they had owned, when they started and stopped using them, the number of calls they made and received, call duration, use of hands-free devices, whether they were left or right handed, which side of the head the phone was usually held and the main area of use (rural/urban or both). In total, the researchers identified 531 eligible cases of people with salivary gland tumours and 1920 controls. Of these, 460 eligible cases and 1266 eligible controls agreed to participate.
What were the results of the study?
The researchers found that regular users and non-regular users were just as likely to develop a tumour at one year, five years and 10 years. The researchers found that those who developed cancer were significantly more likely to be smokers.
When the tumours were separated into either malignant or benign, there was still no significant effect of mobile use on either type. As the researchers state, “for the entire group, no increased risk of PGTs was observed for ever having been a regular cellular phone user… or for any other measure of exposure investigated?”.
The researchers noted that there was a greater number of regular mobile phone users with tumours on the same side of the head they held the phone to than those with a tumour on the opposite side, or who had tumours on both sides or used the phone equally on both sides. When the researchers analysed the data in terms of the side of the head that the phone was used, there were a few statistically significant results, though most differences were no bigger than would be expected by chance.
The breakdown into urban or rural/urban phone use found few significant results, but did show that more than 18,996 calls or more than 1034 hours of use was associated with increased risks of tumours.
What interpretations did the researchers draw from these results?
The researchers concluded that their results suggest a relationship between long-term and heavy cellular phone use and PGTs. They say this association was seen when they limited their analyses to regular users; analysed the side of the head on which the phone was used, and when they analysed the area of main use.
What does the NHS Knowledge Service make of this study?
This case-control has some design weaknesses that should be kept in mind when interpreting the results.
- Most of the analyses in this study did not find a link between mobile phone use and tumours. In the main analyses, where the researchers compared regular users with non-users, non-users were just as likely to have tumours as users of mobile phones. For people who did use mobile phones, the researchers broke the results down further to explore whether particular characteristics of their phone use had an effect on the presence of tumour. Results from these ‘subgroup analyses’ are more likely to be affected by chance as they are conducted in a smaller number of individuals. The increased odds of having a tumour on the same side of the head that the phone was held to were only significant for benign (non-cancerous) tumours. The result was not significant for cancerous tumours.
- It is worth noting that cigarette smoking was significantly more common among the cases. Cigarette smoking is a well known risk factor for cancers of the mouth and throat. The researchers adjusted for this in their analysis saying that it had no effects on differences in risk.
- The study relies on the participants’ recollection of their past use of mobile phones (up to 10 years before their diagnosis or study entry). It is unlikely that people can remember how much they have used their mobile phone over such a long time period.
- There may have also been systematic differences in the way that cases and controls remember their usage history. If people with tumours felt that their mobile phone was responsible, they are possibly more likely to remember heavy use. The researchers say that there was no difference between cases and controls in terms of their “apparent recall of cellular phone use” as rated by interviewers, but this is unlikely to have been a robust measure of this bias.
Though the researchers are confident their findings suggest a positive association between mobile phone usage and the risk of PGTs, they state that, “results from a single epidemiologic study do not…form a strong enough basis to assume causality, and additional investigations of this association, with longer latency periods and large numbers of heavy users are needed to confirm our findings”. They recommend a “cautionary approach” in the face of conflicting and unclear evidence.
Sir Muir Gray adds...
A handsfree seems sensible for those who talk on the phone for hours.