BMJ  2005;331:635 (17 September), doi:10.1136/bmj.331.7517.635

Letter

Childhood cancer and power lines

Results depend on chosen control group

Editor—Given the large size of the study by Draper et al,¹ the risk estimates should be stable. Furthermore, because contact with the subject was not necessary, selection bias due to the differential participation among cases and controls as in previous studies has been avoided.²

We were therefore surprised by the dependence of the results on the chosen control group noted by the authors (who used the central nervous system and other cancer controls for leukaemia cases in one of the comparisons). To explore this further, we combined all controls into one group and used it for comparison. We thought this was justified on the basis of theoretical and empirical grounds: exposure at birth among controls chosen for leukaemia, brain tumours, and other cancers should not depend on the cancer subtype; crude odds ratios calculated by us did not differ (beyond the first decimal) from the matched results presented by the authors (data not shown).

Use of the combined control group showed a pattern that was different to the one presented in the original paper (table). As would be expected, results for all cancers combined show no relation to the distance. For leukaemia and brain cancer, results at two distances are noteworthy: for the 50-100 m category we observed an excess of leukaemia and a deficit for brain tumours. For the 500-600 m category we observed a modest excess for both leukaemia and brain tumours. The trend reported in the original paper is not present when the combined control group is used, which indicates that the trend depended on the leukaemia controls rather than on the leukaemia cases.

View this table: [in this window] [in a new window]   Distance of address at birth from nearest National Grid line and estimated odds ratios, using all controls combined

 

We agree with Draper et al that the results of this study do not support a possible association with magnetic fields, as has been reported by the International Agency for Research on Cancer.³ However, distance is known to be a poor predictor of magnetic field exposure, and therefore the results of this material based on calculated magnetic fields, when completed, should be much more informative.

Further insight might be gained by details on the methods used for the control selection and sensitivity analyses by age, sex, and time period.

UCLA School of Public Health, Department of Epidemiology, 73-284 CHS, 650 Charles E Young Drive South, Los Angeles, CA 90095-1772, USA kheifets{at}ucla.edu

Institute of Environmental Medicine, Karolinska Institutet, Box 210, S-17177 Sweden

Department of Biostatistics and Epidemiology, Institute of Cancer Epidemiology, DK-2100 Copenhagen, Denmark

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Competing interests: LK works with the Electric Power Research Institute in Palo Alto, California, and consults with utilities.

References

1. Draper G, Vincent T, Kroll ME, Swanson J. Childhood cancer in relation to distance from high voltage power lines in England and Wales: a case-control study. BMJ 2005; 330: 1290-2. (4 June.)[Abstract/Free Full Text]
2. Ahlbom A, Day N, Feychting M, Roman E, Skinner J, Dockerty J, et al. A pooled analysis of magnetic fields and childhood leukaemia. Br J Cancer 2000;83: 692-8.[CrossRef][ISI][Medline]
3. International Agency for Research on Cancer. IARC monographs on the evaluation of carcinogenic risks to humans. Vol 80: non ionizing radiation. Part 1: static and extremely low-frequency electric and magnetic fields. Lyons: IARC, 2002. www-cie.iarc.fr/htdocs/monographs/vol80/80.html (accessed 28 Aug 2005).

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Childhood cancer in relation to distance from high voltage power lines in England and Wales: a case-control study

Gerald Draper, Tim Vincent, Mary E Kroll, and John Swanson
BMJ 2005 330: 1290. [Abstract] [Full Text] [PDF]