CORRESPONDENCE

Re: Cellular Telephones and Cancer—a Nationwide Cohort Study in Denmark

Bruce Hocking

Correspondence to: Bruce Hocking, MBBS. FAFOM, Specialist in Occupational Medicine, 9 Tyrone St., Camberwell, Victoria, Australia 3124.

Johansen et al. (1) have assessed the risk of cancer in Danish cellular telephone users. They conducted a retrospective cohort study of cancer incidence in 420 095 residential subscribers compared with national cancer data and found no excess of brain cancer or other cancers of primary interest.

In constructing their cohort, however, they initially identified 723 421 subscribers and then excluded 200 507 (27.7%) who were "corporate customers" because data on the individuals who used the phones were not available. While the exclusion is understandable on grounds of practicality, it introduces a potential bias. The corporate customers will include many company executives who are likely to be among the earliest and most extensive users of telephones because the individual does not have to pay the bill for long and frequent business calls. Removal of this most exposed group from the cohort will lessen the likelihood of finding an effect; however, they will contribute to the national data used for reference. This excluded group contrasts with the residential customers who formed the cohort and for whom cost is a disincentive to use.

The intensity of use of the phone by residential subscribers was not available; it may have ranged from a few calls a week to nearly continuous use. Also, the extent of usage of hands-free kits by residential subscribers concerned by the publicity about brain tumors over several years was not known. These facts point to the appreciable imprecision of the exposure data. It is also noteworthy that the cohort excluded persons younger than 18 years of age. For these various reasons, the study should be interpreted carefully regarding the hypothesis of an association between use of cellular telephones and tumors of the brain or salivary gland, leukemia, or other cancers.

The authors did note a slight increase in the risk of testicular cancer (standardized incidence ratio = 1.12; 95% confidence interval = 0.97 to 1.30). This increase could be related to exposure to the testis incurred by men carrying the phone on the front of their belt or in the pocket while switched on. (A phone that is switched on sends repeated brief signals—polling—to the nearest cellular telephone base station to identify its location for incoming calls.) A case–control study could resolve this.

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.[Abstract/Free Full Text]



             
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