LLRC Journal Radioactive Times. Vol.4 No 2

Radioactive Times. Vol.4 No 2

Health Physics paradigm crumbles

[SRP Scientific Meeting; 10th October 2000,
Scientific Societies Lecture Theatre, New Burlington Place, London]

"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.

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.
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.)

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.

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.

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.
What will replace Collective Dose and what will its scientific basis be? - it's a wide open question.

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.
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 RaT Vol. 4 #1 p 11) 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.

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.

There has been much criticism of LNT,
said Myers, But what is the evidence? Studies of nuclear workers and x-ray fluoroscopy patients showed no correlation between radiation exposure and cancer mortality, he said, though they did show an effect on leukaemia. LLRC noted that this is the standard line for a health physicist. Equally predictable, the studies Myers cited were all of external radiation.

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.
RaT cartoon

If you are seeing this page full screen (i.e. without a navigation bar on the left) you can't see how the rest of the site is organised.
This Home page link takes you to the index page, which has links to all the topics we discuss on the site [only use it if this page is full screen]
Use the Radioactive Times button to see links to the whole electronic edition on this site.

This page was last updated May 2001