The Report warns also that newly discovered effects of radiation, genomic instability (ongoing, long-term increase in mutations within cells and their offspring), bystander effects (cells next to those that were irradiated can also be damaged), and minisatellite mutations (inherited germline DNA changes) are real biological events that need further research. However the Report finds no clear evidence to date that current radiation risks are substantially wrong.

Scientific Briefing on the CERRIE Report

Uncertainties

The Committee were agreed that insufficient attention had been paid in the past to uncertainties in dose and risk estimates for internal emitters. Reliable quantitative estimates of uncertainties in dose coefficients for a range of radionuclides were not yet available. Uncertainties in estimating equivalent dose were significant and varied in magnitude from factors of 2 in the most favourable case than 10 or more in the least favourable, above and below the central estimate. This would depend on factors such as the type of radionuclide, its chemical form, the mode of exposure and the body organ under consideration. Thus, under some circumstances the equivalent dose may be substantially greater, or substantially smaller, than current best estimates. For effective doses, there were additional uncertainties in the use of tissue weighting factors. Further work was required to quantify uncertainties in dose estimates for important radionuclides, with transparent identification of all the underlying contributions to overall uncertainties and how to compound them. The Committee concluded that dose and risk estimates should contain an explicit indication of the uncertainties involved. This approach would help identify those situations in which a precautionary approach was appropriate, which was preferred to the use of conservative/pessimistic assumptions in models.

Current models used for dose/risk estimation are particularly limited in their treatment of radionuclides that emit very short-ranged particles, causing inhomogeneities of dose within tissue on a microscopic scale. These include alpha-particle emitters, low-energy beta-particle emitters and Auger-electron emitters. Microdosimetric analyses should be applied to such emitters.

Biological Evidence of Novel Effects

The Committee considered a number of novel aspects of radiation biology that had emerged since the ICRP made its 1990 recommendations, which currently form the basis for radiation protection. These aspects include: induced genomic instability (whereby radiation can induce an ongoing long-term increase in mutation rate in cells and their progeny, which may contribute towards cancer), bystander effects (whereby unhit cells in the vicinity of cells that have been hit by radiation may also be affected by the radiation), and minisatellite mutation induction in the germline (which leads to inherited DNA changes which may have health effects). The Committee agreed that these were real biological phenomena, which could in principle have implications for assessments of radiation risk. Members differed on the likely implications but agreed that these differences were primarily due to lack of firm information at present. They could in principle lead to underestimation, or overestimation, of risk at low doses under some circumstances.

Almost half the Committee was of the view that the biological evidence on these mechanisms was not adequately reflected in current ICRP models. Current risks could therefore be underestimated at least to some degree, and perhaps significantly for some nuclides. The remaining members of the Committee were unsure of the implications. Of these, some were inclined to the view that risks were adequately taken into account in current models and epidemiological observations. These differences of view existed because of current lack of knowledge, particularly for these effects at low doses of radiation in in vivo situations. The Committee were agreed that new findings on radiation-induced bystander effects and radiation-induced genomic instability should be included in consideration of health risks at low doses and their quantitative uncertainties. In this respect, the Committee recognised that the current ICRP recommendations, formulated in 1990, predated much of the biological information discussed in the Report.

Almost half the Committee considered that the biological evidence on these new effects had immediate implications for radiological protection standards and that Government should give consideration to the Precautionary Principle. Other members, whilst generally supportive of a precautionary approach, did not agree, principally because of their perception of a current lack of coherence in the experimental data and absence of clear links with health effects.

Epidemiological evidence

The Committee considered a number of epidemiological studies that that had been put forward by some as definitive evidence of inadequacy of predictions of risk from ICRP models. The Committee scrutinized these claims and, to provide further tests, it also obtained original data to carry out two analyses of its own: one on the incidence of infant leukaemia in Great Britain after the Chernobyl accident, and the other on trends in childhood leukaemia incidence in Great Britain to include the period after radioactive fallout from atmospheric nuclear weapons testing in the northern hemisphere. The Committee’s analysis found no indication at all of any increase in childhood leukaemia in GB associated with weapons fallout. Whilst its other study produced results that are generally in the direction expected from Chernobyl contamination increasing the risk of infant leukaemia, the increase is not statistically significant; numbers of case are so small that the findings cannot exclude the possibility of there being no increase in risk at all.

The Committee agreed with the standard view that epidemiological evidence is compelling for there being a raised risk of adverse health effects in those exposed to moderate and high levels of internally incorporated radionuclides. For low level intake of radionuclides, all but one member of the Committee accepted that there was probably some increased risk of adverse health effects as a result of the internal irradiation of organs and tissues, although this increase may be undetectably small.

Some members considered that available epidemiological evidence did not suggest that the predictions from current risk models were materially in error. Other members considered that these models could underestimate risks from intakes of certain radionuclides by relatively modest factors. And two members thought that current models underestimate risks from intakes of radionuclides by very large factors. The disagreements stemmed partly from differences of view about the appropriateness of the data and methodologies used in epidemiological studies and from different interpretations of the findings. A core methodological limitation was the inherently reduced statistical power of epidemiological studies at low levels of exposure and risk.

The Committee agreed that all epidemiological studies should employ rigorous scientific methods and establish protocols and checks to prevent errors, such as they noted in some of the studies. In addition, it recommends that epidemiological studies should be published in recognized peer-review journals. Where results are self-published, authors have the scientific and public responsibility to ensure that their analyses are carefully checked and closely examined, prior to publication, by other scientists willing to check their work.

Hypotheses proposed by two Members

The Committee was set up to examine the risks of internal radiation. Implicit was the expectation that it would examine a number of hypotheses proposed over the past decade by two Members of the Committee, Dr Busby and Mr Bramhall. Much the Committee’s time and efforts were spent examining these theories in considerable detail: this is fully described in the Report. These theories included the so-called second event theory (SET) (whereby cells are assumed to be enormously more sensitive to pairs of coupled radiation hits in a specific time window), hot particle theory, biphasic dose responses, and the theory that differences exist between man-made and natural radionuclides as classes. The Committee, apart from Dr Busby and Mr Bramhall, considered that these theories were not supported by the available scientific evidence. For example, the evidence substantially contradicted the SET. The Report found a lack of biological plausibility for the basic preconditions of the SET; a lack of supporting evidence in the proponents’ reviews of the SET; weakness in the few studies cited in support of the SET; and no supporting evidence from experimental studies in an independent review of commissioned by the Committee.

Conclusions (also refer to chapter 5)

The Committee emphasised that the ICRP recommends reserving the use of effective dose for only radiological protection purposes at doses below dose limits. For specific assessments, the ICRP recommended the use of absorbed dose and specific data on relative biological effectiveness (RBE) for the particular radiations and organs concerned. The Committee considered that the use of such specific information should be applied in cases when doses were close to dose limits, in retrospective dose assessments and in interpreting epidemiological data. The Committee further concluded that it was important that the scientific basis of the ICRP methodology should continue to be challenged, and that ongoing developments in microdosimetry and radiobiology should inform judgements on their reliability.

Dose limits, constraints, and indeed tissue weighting factors were based largely on risk estimates for radiation-induced cancer resulting from external gamma ray exposure of the Japanese populations of Hiroshima and Nagasaki. The applicability of these risk estimates to internal exposure from short-range charged particle emissions was questioned, although some human data on risks from alpha particle emitters, provided some support for the use of these risk estimates.

Most Committee members agreed that there were no fundamental differences between internal and external radiation that could not be accommodated through the use of appropriate parameters (eg RBE or kinetic factors). Some members did not accept this view, and considered that there were biophysical and biochemical mechanisms that resulted in an enhanced effectiveness of internal emitters over external radiation in specific instances that was not taken into account in current methodology. There was agreement that enhanced effectiveness may occur as a result of radionuclide binding to DNA, but most members considered that this was an issue specific to some low energy beta emitters and Auger emitters.

Recommendations (also refer to chapter 6)

The Committee supported the COMARE recommendation that organisations and research groups should establish scientific protocols and internal controls to prevent errors before distributing data or conducting epidemiological analyses and making public their results. In addition, the Committee recommends that epidemiological results should be published in recognised peer-reviewed scientific publications. However, the Committee recognises that the peer-review process may tend to reject evidence that does not conform to existing paradigms. Where epidemiological results are self-published, authors have a scientific and public responsibility to ensure that their analyses are carefully checked and closely examined prior to publication by other scientists willing to review their work.

Members were agreed that long-term research was needed on the implications of some of the novel biological mechanisms for radiation risks, from both internal and external radiation.

Oct 20/04
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CERRIE Membership

Chairman
Professor Dudley Goodhead OBE
MRC Radiation and Genome
Stability Unit, Harwell, Oxford

Members
Mr Richard Bramhall
UK Low Level Radiation Campaign

Dr Chris Busby
Green Audit

Dr Roger Cox
NRPB

Professor Sarah Darby
University of Oxford

Dr Philip Day
University of Manchester

Dr John D Harrison
NRPB

Dr Colin Muirhead
NRPB

Mr Peter Roche
Formerly Greenpeace UK

Professor Jack Simmons
Formerly University of Westminster

Dr Richard Wakeford
BNFL

Professor Eric Wright
University of Dundee

Attachments

• CERRIE Report

  1931 Kb - Format pdf
  CERRIE
  Final report of the UK Government's Committee Examining the radiation Risks of Internal Emitters.