Re: Use of mobile phones and risk of brain tumours: update of Danish cohort study
This study has several design flaws that should prevent the authors from any conclusions concerning the impact of mobile phone use on the development of brain cancer. These flaws are as follows:
1. Exposure data
The only information about the person’s exposure to mobile phone radiation is the length of the mobile phone subscription with the mobile phone operator. Such exposure information is irrelevant because it leads to the following: two persons, one of which spends many hours per week on the phone and the other who spends just a few minutes per week are in this study analyzed as if belonging to the same exposure group when they own the subscription of the mobile phone for the same length of time. It means that the highly exposed persons and the nearly unexposed persons are mixed up in the same exposure group. Analysis of such data can not provide valid information about correlation of the exposure to mobile phone radiation and induction of brain cancer.
2. Composition of the control group
From the starting cohort of 723 421 mobile phone subscribers were excluded all corporate subscribers (200 507 subscribers). Therefore, the people who most likely were the heaviest users and were the most exposed to mobile phone radiation were excluded from the study. Besides this, the exclusion of the corporate subscribers has likely led to “contamination” of the control group. As the authors state in the discussion section: “…Because we excluded corporate subscriptions, mobile phone users who do not have a subscription in their own name will have been misclassified as unexposed…”. This means that some of the possibly highly exposed corporate users, who did not have personal phone subscriptions, have ended up as non-exposed persons in the control group.
Because of the cut-off time of the exposure to mobile phone radiation set for year 1995 but the analysis of the cancer induction was done based on the 2007 cancer registry data, any person who got subscription after the cut-off year of the study was considered as non-exposed by the set-up of the study. It means that for example a person diagnosed in 2007 with brain cancer, who had subscription of the mobile phone from year 1996 was, by the design of the study, considered as a non-exposed person who got brain cancer. Whereas, in reality, this was a person who was exposed for 11 years and who got brain cancer.
Finally, even though the study started with over 723 421 mobile phone subscribers, and after some exclusions used 358 043 subscribers, the statistical evaluations presented in tables 1, 2 and 3 in the article are largely based on just a few, or few tens, of cases. What it means is that the statistical results are unreliable because of low numbers.
The all above mentioned flaws and shortcomings make this study unreliable and make impossible to draw any valid conclusions. The conclusions presented by the authors are not supported by the presented data.
Competing interests: No competing interests