1 Institute of Cancer Epidemiology, Danish Cancer Society, DK-2100 Copenhagen, Denmark
2 Institute for Medical Biostatistics, Epidemiology, and Informatics, University of Mainz, D-55101 Mainz, Germany
3 Neurosurgical Department, Neuroscience Centre, University Hospital of Copenhagen, DK-2100 Copenhagen, Denmark
4 Department of Radiation Biology, Finsen Centre, University Hospital of Copenhagen, DK-2100 Copenhagen, Denmark
5 Department of Otolaryngology-Head and Neck Surgery, Gentofte Hospital, University of Copenhagen, DK-2900 Hellerup, Denmark
6 Unit of Radiation and Cancer, International Agency for Research on Cancer, F-69 372 Lyon, France
We thank Dr. Hardell and Professor Hansson Mild (1) and also Dr. Kundi (2) for their observations on our recently published study on cellular telephone use and acoustic neuroma (3). Hardell and Hansson Mild raise a number of issues regarding our study. In the Interphone Study, no information was collected concerning the use of cordless telephones (other than digital enhanced cordless telecommunications (DECT)), because they emit in a different frequency band than mobile phones and because the time-averaged power emitted by such telephones is much smaller than that emitted by cellular telephones.
It is true that 82 percent of eligible cases were interviewed and, for each case, a corresponding pair of controls were ascertained. If a control denied participation, another control was sampled from the pool of randomly selected potential controls. This process was repeated until we had established a case-control study base consisting of 106 cases and 212 controls, all included in the conditional logistic regression analyses representing 106 complete matched triplets.
We do not agree with Hardell and Hansson Mild (1) that persons reporting less than two calls per week and less than 6 months of use should be reported in this context. To separate the data into two groupsanalog users and digital userswill not give meaningful results because all regular users of cellular telephones in our study either started with a digital telephone or switched over from an analog telephone to a digital telephone within a few years after the introduction of the digital system.
Our paper (3) reports the results of a population-based study of use of cellular telephones and risk of acoustic neuroma, with cases diagnosed between 2000 and 2002. Contrary to this, our previously published cohort study (4) reported on a cohort of subscribers from 1982 to 1995 with an explanation of the change in proportions of analog and digital users and the high prevalence of mobile phone use. The digital system was introduced in Denmark in 1992. We have no further comments on this historical reality.
We did not exclude any case or any control from the analysis because of their use of a hands-free device. What Hardell and Hansson Mild (1) express in relation to laterality does not contradict what we explain in our paper (3). Hearing problems, as we describe, are the most likely explanation that our risk estimates were biased toward a reduction in risk for heavier users. Consideration of hearing problems in the analytical model reveals an odds ratio close to 1. It is true that blinding is not possible for face-to-face interviews, but the major advantage of this method is the fact that the interview is more standardized for cases and controls because you are able to have spontaneous answers from both groups. Use of postal questionnaires has the disadvantage that often cases spend more time thinking about the questions, being eager not to miss an exposure, while controls tend to forget minor exposures, which leads to an overestimation of exposure by cases (5).
Concerning the issues raised by Dr. Kundi (2), we have no particular comment on the overall reflections concerning initiation and/or promotion of radio-frequency exposure in relation to the risk of acoustic neuroma. In general, the Interphone Study will be analyzed in ways that will allow the investigation of the association between radio-frequency exposure and the risk of tumors under study and, if relevant and possible, of the mechanism for such an association.
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