Radioactive Times. Vol.4 No 2
Health Physics paradigm crumbles
[SRP Scientific Meeting; 10th October 2000,
LLRC has been pressing for a shift of emphasis throughout its 8 year history, and a recent
meeting of the SRP in London1 heard the same message from a different source.
The meeting was entitled Dosimetry, Biology and Risks, and the description of its background
said ... as research progresses into the physical interactions of ionising radiation with tissue and
the resultant chemical and biological effects, the adequacy of accepted theories is increasingly
questioned ...
Richard Bramhall attended the meeting for LLRC.
One speaker after another confirmed that at low doses of radiation little is known about the
biology. Cells do not respond to low doses in the same way that they do for high doses; new
discoveries like low dose hypersensitivity, radiation induced genomic instability and bystander
effects have far-reaching implications for radiation protection.
Jack Simmons, Professor of radio-biophysics at the University of Westminster and co-author of
Radiation Protection Dosimetry: a Radical Reappraisal said that low levels are the significant
ones for radiation protection. The key conventional concept of absorbed dose seems valid for high
doses but breaks down for low doses. It is now clear that, as Professor Dudley Goodhead
explained, radiation damage is caused by ionisations happening along the length of discrete tracks
of radiation as they pass through or close to vulnerable targets such as cellular DNA.
Information about energy deposition is not very helpful, to put it at its mildest, Professor
Simmons said. At low levels other key concepts such as tissue weighting factors and Relative
Biological Effectiveness are useless.
Professor Eric Wright outlined some challenges to current theories in radiobiology, a field which
he said was mired in complex biology and sociology. Controversy surrounded the question of
whether there was a threshold below which radiation could not induce cancer, leading to
challenges to the ICRP standard Linear No Threshold (LNT) model.
For Wright the challenges are: to discover the biological effects of low doses, and to determine
the shape of the dose response curve and whether there is a threshold for cancer induction.
The speakers were agreed that there is almost certainly not a threshold; the other issues are more
complex. There was agreement that the dose response curve is almost certainly not the
conventional straight line, according to which cancer induction is strictly proportional to dose.
However, as the presentations showed that the number and complexity of the biological effects
undermine the very concept of dose, the debate is thrown back to a more fundamental level than
the shape of the curve.
As if it were not enough to have to drop the "average dose" idea and think about whether cells are
or are not hit, the phenomenon of radiation-induced instability has thrown a new level of
complexity into the field. Instability could appear after many cell divisions and, Wright said, could
be described as a radiation induced change at zero dose.
Speaking of Lessons from History Wright said that the radiation protection community should not
have been too surprised by radiation-induced instability - papers on it had been appearing in the
literature for decades. What is it about radiation that causes cancer?, Wright asked. That is the
key issue, and we don't know. Knocking out p53 - the tumour suppressor gene - isn't an adequate
answer.
Wright said the uncertainties in the biology fed the LNT controversy, and so did the poor
resolution of epidemiological studies at low dose (one of those fields where one can prove
anything with statistics). The controversy was further aggravated by the concept of Collective
Dose and the implications for nuclear industries having to spend large sums on
decontamination and the prevention of public exposure. That's where the sociology comes in.
Increasingly it is the courts that are deciding these issues, and it's hard to convince judges and
juries that there's a threshold.
Collective Dose
LLRC pressed Wright for an opinion on the proposal by ICRP's Chairman, Roger Clarke, to drop
Collective Dose. LLRC described the proposal as schizophrenic in view of ICRP's adherence to
the LNT model. (see Editorial RaT Vol. 4 #1).
Professor Simmons said that in many cases
Collective Dose was an administrative convenience, but since, at the very low concentrations of
radiation involved in environmental contamination, the concept of "dose" as conventionally
understood was meaningless, then multiplying a meaningless number to estimate the effect in large
populations just gave another meaningless number. This was especially true for long time-scale
projections, he said.
LLRC does not disagree with this analysis, but insisted that as Professor Wright had just
eloquently described a number of subtle effects of low dose radiation it was surely necessary to
have some measure of the effects on large populations.
Alan Edwards of NRPB defended Clarke's proposal, saying that while Collective Dose had some
useful applications, other approximations might be more appropriate in some cases. ICRP, he
added, Don't know the relationship between dose and risk any better than anyone else.
Second Event Questions
LLRC pointed out that the Monte Carlo modelling from which Professor Goodhead had derived
his analysis of radiation tracks was concerned with the spatial distribution of energy but took no
account of temporal distribution. What about Busby's Second Event theory, which compares the
hazard of double hits from sequentially emitting nuclides such as Strontium-90? Bramhall asked.
Scientific Societies Lecture Theatre, New Burlington Place, London]
) said that since animal studies did not confirm the statistical predictions of the theory he
concluded that it was not valid. But in discussion with LLRC after the meeting he realised that the
studies he had in mind were not relevant. This has opened a new round of correspondence on the
theory in which LLRC is urging NRPB to reconsider its position, especially since the SRP
meeting had left so many open questions. We hope to report on this in a future issue of
Radioactive Times.
"Nearly a decade ago, when LLRC began saying that low levels of
radioactivity were causing health problems far beyond the predictions of conventional
models, we were dismissed as madmen. But at the SRP meeting one speaker after another
confirmed that at these low doses little is known about the biology; conventional concepts
break down, and new discoveries have far-reaching implications for radiation protection."
The Society for Radiological Protection is the professional association for people who work in
radiation protection in industry, hospitals, government, advisory bodies and universities.
These people are apt to call themselves "Health Physicists" - a term which raises a smile at
LLRC, since it reminds us that the conventional model of radiation hazard puts undue emphasis
on the physical properties of radioactive substances, rather than their bio-chemical behaviour or
medical effects.
Professor Simmons told the meeting that at low levels there will be no radiation hits to a large
proportion of the cells, while just a few cells will receive all the energy. He suggested that at low
doses it would be better to drop dose and talk in terms of the fluence of charged particles
through the tissue being considered; It is only relevant to talk in conventional "dose" terms when
all the cells in that volume have been hit at least once.
(LLRC has long been illustrating this point with the analogy of a gunman emptying his pistol at a
football crowd: the concept of absorbed dose insists that the bullets must be averaged out across
all the people in the stadium.)
Effects are observed after cytoplasmic
irradiation or, more dramatically, in cells that are not themselves irradiated but are in the
neighbourhood of irradiated cells.
These are the so-called "bystander" effects, which include p53
protein expression, sister chromatid exchanges, micronucleus formation, cytotoxicity, gene
mutation and chromosomal instability.
What will replace Collective Dose and what will its scientific basis be? - it's a wide open question.
Goodhead reiterated his opinion that the Second Event theory is well specified and testable, but
that the problem was finding the political will to pay for the experiments. Alan Edwards, one of
the authors of a critique of the Second Event theory published in January (see
Summing up:- An old Titan gets a new job
Reviewing the day's business, Melvyn Myers introduced the figure of Atlas with the world on his shoulders.
The radiation protection world rests on the utility of LNT and Collective Dose, he said. For the sake of prudence we assume low level radiation is unsafe, but there's a trade-off - public fear and distrust leading to thousands of [elective] abortions after Chernobyl, refusal of nuclear medicine and obstacles placed in the way of hospital practice, measures [of dubious necessity] to protect sewage workers, air crew and workers in the industries which process naturally occurring radioactivity. Billions are spent in radon reduction and relocating contaminated soil.
But in the end he proposed a useful acronym to add to the jargon; The meaning of ATLAS, he
said is At These Levels Act Scientifically.
LLRC agrees with that - coming from within the
profession, it's pretty radical.
RaT's Response
The laboratory Rat, who had a classical education, thinks it's a neat idea to get Atlas to sort out the problems, given that it was his brother Prometheus who caused them in the first place by teaching mortals to use fire, which he had stolen from the gods in a hollow tube (or fuel rod), and that Atlas's sister-in-law Pandora and her little box of fallout were the means by which Zeus spread disease across the world to punish mankind's presumption.
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