© 2001 by Oxford University Press
Journal of the National Cancer Institute, Vol. 93, No. 11, 878-879,
June 6, 2001
© 2001 Oxford University Press
CORRESPONDENCE |
Response: Cellular Telephones and Cancer—a Nationwide Cohort Study in Denmark
Affiliations of authors: C. Johansen, J. H. Olsen, Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark; J. D. Boice, Jr., J. K. McLaughlin, International Epidemiology Institute, Rockville, MD, and Department of Medicine, Vanderbilt University Medical Center and Vanderbilt–Ingram Cancer Center, Nashville, TN.
Correspondence to: Christoffer Johansen, M.D., Institute of Cancer Epidemiology, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenhagan, Denmark (e-mail: christof{at}cancer.dk).
Godward et al. raise an interesting issue regarding our study on the risk of cancer among cellular telephone subscribers in Denmark. When cancer rates of the general population are used to calculate the expected number of cancers in a given cohort, these incidence rates may be influenced by the cancer rates of the cohort under study to a certain degree. However, this influence is of minor importance if the number of cancers among members of the cohort is small compared with the general population. In the period of study, from 1982 through 1996, cohort members developed 3391 cancers that were less than 1% of the cancer cases occurring in the general population in the same period. We also addressed the possibility that the relative risk estimates tended to be biased toward the null by calculating internal risk comparisons among cellular users themselves. These internal analyses confirmed our standardized incidence ratio findings. Thus, it is unlikely that the cancers among cohort members influenced comparisons with the general population to any meaningful degree.
We were not able to obtain comprehensive information on socioeconomic status in this large nationwide cohort of telephone subscribers. Since World War II, a major political achievement of the Danish government has been the establishment of a welfare state, including public-supported hospitals, schools and housing—and a very high tax load compared with the United States and even other European countries. This distribution of wealth is largely independent of place of residence. Therefore, postcode of residence is not a good indicator of socioeconomic status in Denmark.
The power to detect a trend in the risk of cancer over years of digital subscription depends, in part, on the level of risk to detect. Our study had sufficient power to rule out moderate to high risks. Furthermore, although the average follow-up was only 1.5 years, we did follow more than 10 000 persons for more than 10 years. We agree with Godward et al. on the need for individual exposure information and the need for longer follow-up before a final conclusion can be drawn; we hope to obtain such data in the future. We note, however, that two carefully conducted case–control studies reached the same conclusion as did our cohort study (1,2).
Hocking raises the possibility of bias associated with excluding corporate customers. Unfortunately, we have no way to confirm whether corporate customers were heavy cellular telephone users, but, if so, we would have missed an important high-exposure group. As above, it is unlikely that their contribution to general population rates would have been meaningful, so any bias would almost surely be small. Given the high number of children using cellular telephones, any future studies should include this important population. It is unlikely that the testes receive more than negligible radiofrequency exposure for any cellular telephone worn on a belt or in a pocket; furthermore, the majority of phone models used in the study period were of a size that did not fit comfortably in the pocket.
REFERENCES
1
Muscat JE, Malkin MG, Thompson S, Shore RE, Stellman SD, McRee D, et al. Handheld cellular telephone use and risk of brain cancer. JAMA 2000;284:3001–7.
2
Inskip PD, Tarone RE, Hatch EE, Wilcosky TC, Shapiro WR, Selker RG, et al. Cellular-telephone use and brain tumors. N Engl J Med 2001;344:79–86.
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