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

BMJ 2005; 330 doi: https://doi.org/10.1136/bmj.330.7503.1290 (Published 02 June 2005) Cite this as: BMJ 2005;330:1290

Control selection in the Study of Childhood cancer in relation to distance from high voltage power lines in England and Wales

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

Leeka Kheifets1, Maria Feychting2,
Joachim Schuz3

1. Department of Epidemiology, School of Public Health,
UCLA, CA, USA

2. Institute of Environmental Medicine, Karolinska
Institutet, Sweden

3. Institute of Cancer Epidemiology, Danish Cancer Society,
Denmark

š

We have read with interest the paper from Draper and colleagues
[1].šš Given its large size the risk estimates in the paper should be
stable. Furthermore, because contact with the subject was not necessary
selection bias due to the differential participation among cases and controls,
which plagued some of the previous studies [2], has been avoided.š Thus we
were particularly surprised by the dependence of the results on the chosen
control group noted by the authors, (who used CNS and other cancer controls for
leukaemia cases in one of the comparisons).šš To explore this further
we combined all controls into one group and used it for comparison.š We
felt this is justified based on both 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 first decimal) from the matched results presented by authors
(data not shown).

Use of the combined control group revealed a pattern different than
the one presented in the original paper (Table 1).š As would be expected,
results for all cancers combined show no relation to the distance.šš
For both leukaemia and brain cancer results at two distances are
noteworthy:š for the 50-100 meters category an excess of leukaemia and a
deficit for brain tumours is observed.š For the 500-600 meters category we
observed a modest excess for both leukaemia and brain tumours.š Of note is
that the trend reported in the original paper is not present when the combined
control group is used, thus indicating that the trend depended on the leukaemia
controls rather than on the leukaemia cases.š We agree with the authors
that the results of this study do not support a possible magnetic field
association, as has been reported by the IARC monograph [2]. However, distance
is known to be a very poor predictor of magnetic field exposure, and therefore,
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.

1.ššššš
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.

2.ššššš
Ahlbom A, Day N, Feychting M, et al. A pooled analysis of magnetic fields
and childhood leukaemia. Br J Cancer, 83, 692-8 (2000).

3.ššššš
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.š 2002

š


š

š

š

Leukemia

CNS

Other
tumours

All cancer
combined

All
controls

Distance

No. cases

š

OR (95% CI)

No. cases

š

OR (95% CI)

No. cases

š

OR (95% CI)

No. cases

š

OR (95% CI)

No.

controls

0-49

5

0.94
(0.34-2.57)

3

0.83
(0.24-2.84)

7

1.00
(0.41-2.42)

15

0.94
(0.46-1.90)

16

50-99

19

1.73
(0.99-3.05)

4

0.53
(0.19-1.51)

15

1.04
(0.56-1.91)

38

1.15
(0.72-1.84)

33

100-199

40

1.18
(0.82-1.70)

26

1.12
(0.73-1.73)

37

0.83
(0.57-1.20)

103

1.01
(0.77-1.33)

102

200-299

44

0.93
(0.66-1.30)

38

1.17
(0.82-1.68)

66

1.05
(0.78-1.41)

148

1.04
(0.82-1.31)

143

300-399

61

1.23
(0.91-1.66)

35

1.04
(0.72-1.50)

79

1.21
(0.92-1.59)

175

1.18
(0.95-1.47)

149

400-499

78

1.15
(0.89-1.50)

40

0.86
(0.62-1.22)

80

0.89
(0.69-1.16)

198

0.97
(0.80-1.18)

204

500-599

75

1.24
(0.95-1.63)

54

1.31
(0.96-1.78)

86

1.08
(0.83-1.39)

215

1.18
(0.97-1.44)

182

™600

9378

1 (ref)

6405

1 (ref)

12406

1 (ref)

28189

1 (ref)

28252

š

š

Competing interests:
For LK work with EPRI and consulting with utilities

Competing interests: Distance of address at birth from nearest National Grid line and estimated odds ratios using all controls combined

28 June 2005
Leeka Kheifets
Professor of Epidemiology
Maria Feychting, Joachim Schuz
UCLA, Los Angeles, CA 90095