© The Author 2007. Published by Oxford University Press.
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CORRESPONDENCE |
Re: Cellular Telephone Use and Cancer Risk: Update of a Nationwide Danish Cohort Study
Affiliations of authors: Karolinska Institute, Stockholm, Sweden (AA, MF); International Agency for Research on Cancer, Lyon, France (EC); Imperial College London, UK (PE)
Correspondence to: Anders Ahlbom, PhD, Institute of Environmental Medicine, Karolinska Institute, Box 210, 171 77 Stockholm, Sweden (e-mail: anders.ahlbom{at}ki.se).
Schüz et al. (1) have extended the follow-up of their nationwide Danish cohort study (2). The longer follow-up is a considerable improvement because the small number of long-term users limited the interpretation of the previous results, as the same issue has limited the interpretation of most other studies presented to date. However, time since first subscription was not considered in many of the analyses performed in the extended follow-up study.
Because the study does not find evidence for an association between mobile phone use and cancer, the possibility of attenuation of risks is critical. The standardized incidence ratio (SIR) value for brain and central nervous system (CNS) tumors in cellular telephone users assessed at more than 10 years since first subscription is reduced (SIR = 0.66, 95% confidence interval = 0.44 to 0.95). The authors fail to provide a plausible explanation for this, although it is notable that among the male participants, the overall standardized incidence ratios for all cancers and many specific types of cancer are also reduced. This implies that the population of mobile telephone subscribers is different from (and healthier than) the general population, as acknowledged by the authors. Because of this implication, we can safely assume that the reported standardized incidence ratio value for brain and CNS tumors is too low, although it is not possible to evaluate the level of the underestimation. The use of information on subscriptions obtained from the operators rather than from the users is a strength and a valuable alternative to the approach in most previous studies. However, this approach has its own problems, specifically the lack of information about the actual user of the phone and the amount of use. It is known that some subscribers do not use the phones themselves but provide them for family members to use. That this happens is supported by the observation that only 61% of a small sample of the subscribers reported use of mobile phones when responding to a questionnaire. Another concern, also discussed in the article, is the fact that a large proportion of the population started to use mobile phones after the cohort was defined and thus are included in the reference population. The same problem applies also to corporate users, who are not included as subscribers in the study. All these circumstances would dilute any excess risk, were it to exist, and push the estimate toward the null. A similar dilution may arise from the fact that it has not been possible to evaluate the risk of specific subtypes of brain and CNS tumors in relation to time since first subscription. In particular, previous research (3) has shown excess risks for acoustic neuroma in relation to mobile phone use of more than 10 years, even without considering laterality of phone use in relation to tumor, but no corresponding analyses are presented in this article.
The authors conclude that any large association of risk of cancer and cellular telephone use can be excluded. We believe that such a global conclusion is premature and not supported by the data. Although results from this and other studies concerning short-term use are reassuring, we do not see how these data provide convincing evidence against an effect of long-term use, which for some time has been the critical issue in evaluations of potential health risks related to mobile phone use.
REFERENCES
(1) Schüz J, Jacobsen R, Olsen JH, Boice JD Jr, McLaughlin JK, Johansen C. Cellular telephone use and cancer risk: update of a nationwide Danish cohort. J Natl Cancer Inst (2006) 98:1707–13.
(2) Johansen C, Boice J Jr, McLaughlin J, Olsen J. Cellular telephones and cancer—a nationwide cohort study in Denmark. J Natl Cancer Inst (2001) 93:203–7.
(3) Lonn S, Ahlbom A, Hall P, Feychting M. Mobile phone use and the risk of acoustic neuroma. Epidemiology (2004) 15:653–9.[CrossRef][ISI][Medline]
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J Natl Cancer Inst 2006 98: 1707-1713.
J Natl Cancer Inst 2007 99: 655-656.
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