© 2001 by Oxford University Press
Journal of the National Cancer Institute, Vol. 93, No. 12, 952,
June 20, 2001
© 2001 Oxford University Press
CORRESPONDENCE |
Re: Cellular Telephones and Cancer—a Nationwide Cohort Study in Denmark
Affiliations of authors: L. Hardell, Department of Oncology, Örebro Medical Centre, Sweden; K. Hanson Mild, National Institute for Working Life, Umeå, Sweden, and Department of Natural Sciences, Örebro University, Sweden.
Correspondence to: Lennart Hardell, M.D., Ph.D., Department of Oncology, Örebro Medical Centre, S-701 85, Örebro, Sweden (e-mail: lennart. hardell{at}orebroll.se).
Johansen et al. (1) reported cancer incidence in a Danish cohort of 420 095 persons who reported ever having used a cellular phone; an accompanying editorial concluded that the study gave a "convincing answer" of no association (2). Also, media have echoed the results as giving "proof" of no increased risk. However, there are several shortcomings in the study that make such conclusions premature.
The analyses were based on "ever" use of a cellular phone from 1982 through 1995. The subjects were followed in the Danish Cancer Registry from 1982 through 1996, users of the analogue (Nordic Mobile Telephone; NMT) system on average 3.5 years and users of the digital (Group Special Mobile; GSM) system on average only 1.9 years. Duration of subscription was available only for digital use; 15 584 of the digital phone users were subscribers for greater than or equal to 3 years, representing 3.7% of the whole study population. Only 32 997 (7.9%) subjects had their first subscription from 1982 through 1990.
Of the 154 persons with brain or nervous system tumors, 24 (16%) had a latency period of greater than or equal to 5 years. Only 11 of 144 intracranial tumors were located in the temporal lobe and five were located in the occipital lobe, areas with highest exposure. No information was provided on which ear was used during a phone call or if a car-mounted cellular phone with an external antenna was used, which significantly reduces exposure.
Standardized incidence ratio (SIR) should be calculated for tumors in the temporal and occipital lobes, with a latency period of greater than or equal to 5 years. Analogue use should be displayed separately, since an analogue phone gives two to three times higher exposure than a digital phone. However, such an estimate would probably only include very few cases.
Of the digital phone subscribers, 44.1% had duration of less than 1 year, too short a latency period for the development of cancer. Brain tumors often have a preclinical time period that might entail a sick-leave period from employment before diagnosis. Since some use of a cellular phone would be related to occupation, sick leave might result in even lower exposure among case patients than among control subjects using a latency period of less than 1 year. The SIR for a digital subscription of less than 1 year was calculated to 0.7 (95% confidence interval [CI] = 0.6 to 2.2), increasing to a SIR of 1.2 (95% CI = 0.6 to 2.3) for a subscription of greater than or equal to 3 years (n = 9 persons with a brain tumor).
The study showed a SIR of 0.86 (95% CI = 0.83 to 0.90) for all cancers, and the authors concluded that the results were confounded by social class. It is not clear why the results were not adjusted for social class, since such data exist in the Danish population registries.
We performed a case–control study on brain tumors and the use of cellular phones (3,4). In the report by Johansen et al. (1), reference was made to questions by Ahlbom and Feychting (5) on the inclusion of cases in our investigation. However, the authors omitted to discuss that, in the very same issue of that particular journal, we described in detail the inclusion of cases (6). Thus, the cases fulfilling the inclusion criteria were included.
REFERENCES
1
Johansen C, Boice JD Jr, McLaughlin JK, Olsen JH. Cellular telephones and cancer— a nationwide cohort study in Denmark. J Natl Cancer Inst 2001;93:203–7.
2
Park RL. Cellular telephones and cancer: how should science respond? [editorial]. J Natl Cancer Inst 2001;93:166–7.
3 Hardell L, Nasman A, Pahlson A, Hallquist A, Hansson Mild K. Use of cellular phones and the risk for brain tumours. A case–control study. Int J Oncol 1999;15:113–6.[ISI][Medline]cancerlit;99307661
4 Hardell L, Nasman A, Pahlson A, Hallquist A. Case–control study on radiology work, medical x-ray investigations, and use of cellular telephones as risk factors for brain tumors. MedGenMed 2000;E2 (available at: http://www.medscape.com/Medscape/GeneralMedicine/journal/2000/v02.n03/mgm0504.hard/mgm0504.hard.html).
5 Ahlbom A, Feychting M. Re: Use of cellular phones and the risk of brain tumours: a case– control study [letter]. Int J Oncol 1999;15:1045.[ISI][Medline]cancerlit;20077490
6 Hardell L, Nasman A, Pahlson A, Hallquist A, Hansson Mild K. Re: Use of cellular phones and the risk for brain tumours: a case–control study [letter]. Int J Oncol 1999;15:1045–7.cancerlit;20077490
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