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Abandoning linear no threshold

The article by Feinendegen in the January issue of British Journal of Radiology [1] suggests that there is sufficient evidence of adaptive responses and hormesis in cells and whole organisms to justify abandoning the linear no threshold (LNT) hypothesis as a basis on which to formulate radiological protection standards. In place Feinendegen favours a model incorporating a threshold below which no adverse health effects of radiation can be observed, although he does not suggest at what level that threshold should be. While we agree that there is some evidence for adaptive responses there is also much evidence suggesting a continuously increasing response between cellular endpoints and radiation dose. Significant uncertainty remains on the existence, mechanisms and basis of adaptive responses [2, 3]. Furthermore, the assertion that the induction of apoptosis by radiation is solely a high dose phenomenon is untrue [4, 5].

In brief, the arguments put forward by Feinendegen do not justify abandoning the LNT hypothesis as a basis for setting radiological protection standards. In the absence of consistent radiobiological evidence it is prudent to somewhat overestimate health effects rather than underestimate them. It should be noted that the International Commission on Radiological Protection do recommend a reduction of a factor of 2 in risk at low doses and low dose rates compared with a linear extrapolation from high doses.

Yours etc.,

A Edwards and S Bouffler

National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ, UK

Received for publication March 8, 2005. Accepted for publication March 22, 2005.

References

1. Feinendegen LE. Evidence for beneficial low level level radiation effects and radiation hormesis. Br J Radiol 2005;78:3–7.[Abstract/Free Full Text]
2. UNSCEAR. United Nations Scientific Committee on the Effects of Atomic Radiation. Report to the General Assembly, Annex G. United Nations, New York, 2000.
3. National Council on Radiation Protection and Measurements. Report 136, Evaluation of the linear-nonthreshold dose-response model for ionising radiation. Bethesda, MD: NCRP, 2001.
4. Rothkamm K, Lobrich M. Evidence for a lack of DNA double strand break repair in human cells exposed to very low x-ray doses. Proc Natl Acad Sci USA 2003;100:5057–62.[Abstract/Free Full Text]
5. Enns L, Bogen KT, Wizniak J, Murtha AD, Weinfeld M. Low-dose radiation hypersensitivity is associated with p53-dependent apoptosis. Mol Cancer Res 2004;2:557–66.[Abstract/Free Full Text]

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