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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: (Published 02 June 2005) Cite this as: BMJ 2005;330:1290

Authors’ reply to Roman et al

Roman and colleagues say that we used distance as a proxy for
magnetic field exposure; this is correct only in a rather weak sense of
the word “proxy”. They go on to say that we “acknowledge [that] this is a
crude estimate [of power-frequency magnetic field exposure]”; we said
nothing like this. The distance analyses are similar to those used by the
writers of the letter in their capacity as authors of the United Kingdom
Childhood Cancer Study (UKCCS) paper [1]. We shall be presenting our
analysis of calculated magnetic fields in a subsequent paper, and we
regard the distance analysis in this paper as a separate analysis in its
own right.

We drew attention in our paper to the possibility that the leukaemia
controls are, by chance, unrepresentative.

In discussing this point, however, Roman et al make invalid
comparisons in the graph accompanying their letter. Their two sets of
comparison data refer to addresses in the 1990s. Our study extends over a
much longer period (1962-1995), during which there were increases in the
numbers of lines and of houses situated close to lines. The numbers quoted
in our paper relate to the whole of this period. Their finding that there
is a smaller proportion of addresses close to the line when comparing the
average over the whole of this period with data for the 1990s is
unsurprising given the time trend in the number of houses near lines.
Our unpublished data show that when data relating to more closely
comparable periods are used we actually have, for most of the distances
considered, higher proportions of leukaemia controls living near lines
than are found for the two comparison groups, not lower as they suggest:
see attached graph. (We have taken the values for UKCCS controls from
table 1 of [1].) Even when the periods are comparable, the distributions
of birth addresses (our data) and diagnosis addresses and all homes (their
comparison data) are not necessarily expected to be the same .

To summarise: we suggested ourselves that the distribution of our
leukaemia controls means that chance has to be more seriously considered
as an explanation for our results, but the evidence for this comes from
internal comparisons within our data and not from suggested comparisons to
other data.

1 UK Childhood Cancer Study Investigators. Childhood cancer and
residential proximity to power lines. Br J Cancer 2000; 83:1573-80.

Competing interests:
GJD, TJV and MEK: no conflict of interest. JS is employed by National Grid Transco and worked on this project with their permission

Competing interests: No competing interests

25 July 2005
Gerald Draper
Honorary Senior Research Fellow
Tim Vincent, Mary Kroll, John Swanson
Childhood Cancer Research Group, University of Oxford, Oxford OX2 6HJ