Abstract
This internal dosimetry meeting described the derivation of dose-per-unit-intake values, current ICRP thinking on the subject and state-of-the-art biokinetic and dosimetric models. In the context of screening and monitoring overall programmes and specific techniques used within the nuclear and medical fields were described.
John Stather (NRPB) opened the meeting with The Work of the ICRP in Developing Dose-per-unit Values for Radionuclides. John outlined the recent work of the Internal Dosimetry Task Groups of Committee 2 in developing a series of documents giving inhalation and ingestion dose-per-unit-intake values for both the public and workers. The work had involved the application of the new respiratory tract model; the choice of appropriate gut absorption (fL) values for radionuclides in food and for workplace chemical forms; the development of age-dependent biokinetic models and the specification of excretion pathways. John reported that within the next two years Committee 2 is expected to finalise a revision of publication 54 giving (for selected radionuclides) bioassay interpretation information based on the new biokinetic models. The long term aim is to comprehensively review the biokinetic and dosimetric models for workers and to provide information in one document that is appropriate for both dosimetry and for bioassay interpretation.
The New ICRP Lung Model and its Applications was described by Alan Birchall (NRPB) who had kindly stood in at the last moment for the published speaker. A summary of the new model was presented and compared with the ICRP 30 model. Alan described the fundamental difference in approach in the two models, i.e. the ICRP 30 model calculates only the average dose to the lungs, whereas the new model takes account of the presumed differences in radiosensitivity of respiratory tract tissues and calculates specific tissue doses. The new model is more flexible, wider in approach and compatible with later scientific evidence. It can readily incorporate information related to the subject (age, activity, health status) or the exposure (particle size, chemical form). Finally the new model is detailed enough for dosimetry purposes.
The third speaker Robert Mairs (Department of Radiation Oncology at the CRC Beatson Labs) widened the discussion to internal dosimetry in the medical field with his paper on The Implications of the Microscopic Distribution of Internal Emitters for Dosimetry and Radiotoxicity. He described how tumour seeking molecules conjugated to radionuclides are used to provide targeted radiotherapy. The current work suggests that short range emitters are well suited to the treatment of circulating tumour cells whilst beta emitters are superior in the treatment of subclinical metastases or macroscopic tumours. Also clinical strategies using combinations of radio conjugates in targeted radiotherapy were well supported.
David Taylor (University of Wales) continued on an applied subject The Metabolism and Dosimetry of Radiopharmaceuticals. Radiation doses received by patients following radiopharmaceutical administration (covering a wide range of99mTc-complexes and of natural metabolites and drugs labelled with 11C,18F, 123I and other nuclides) may range from < 1 to > 20 mSv per patient per investigation. David pointed out that ICRP in publications 53 and 62, had begun to issue dosimetric models and dose-per-unit-intake values for the most commonly used radiopharmaceuticals. However current dosimetric practice assumes uniform distribution of the radionuclide in the organs and tissues. Today, with radiopharmaceuticals increasingly being designed to deposit preferentially in small areas the question of non-uniform uptake and retention represent new challenges for the future development of radiopharmaceutical dosimetry.
Following a break for lunch Tony Gibson gave a historical account of Limiting Intakes of Radionuclides 1940 - 1985. The presentation outlined the early history of internal exposures, the earliest being exposure of miners in 1400, to the Manhattan Project 1940 - 45 and the post war years from 1946 - 1957. Quantification of Internal Dosimetry in ICRP Publications 2, 10 and 10A were described followed by a description of the ICRP 30 respiratory tract model. Tony's concluding point was that with the increasing sophistication and complexity of dosimetric models the possibility of carrying out calculations by hand has now effectively gone.
Following this historical interlude, the meeting moved onto the current monitoring programmes used in the nuclear and medical fields. Alan Brither (BNFL) presented Design and Operation of a Screening Programme for Internal Contamination. The BNFL Sellafield statutory monitoring programme employs air sampling supported by urine sampling and in vivo monitoring (plutonium being of prime concern). In addition to the statutory programme, routine monitoring is also carried out for reassurance and to permit the assessment of received dose should the need arise. Indicators of accidental exposure are built into normal operations and routine monitoring designed to assess dose following suspected accidental intakes. The importance of worker cooperation in the success of any monitoring programme was stressed. To conclude, Alan looked to the future technical challenges to ADSs with the introduction of new models and the need to assess doses to progressively lower levels.
On a similar theme Richard Birch (AEA Technology) presented The Interpretation of Excretion Measurements for Internal Dosimetry. He described the general principles involved with the interpretation of bioassay data focusing on the occupational exposure to long-lived radionuclides, such as plutonium. The types of monitoring available, i.e. air sampling, bioassay sampling and in vivomonitoring were identified followed by the main steps in the dose assessment procedure. To illustrate two practical examples were provided, the first outlining the steps taken following a single high urine result (incident) and the second following a high routine annual result. The presentation concluded with a summary of the key issues in a biological sampling regime.
The final speaker of the day was Cathy Griffiths (Royal Hallamshire Hospital, Sheffield) on the Monitoring of Radionuclides in Hospital Employees. Cathy outlined the results of a survey of 25 nuclear medicine departments on their use of specific radionuclides and the monitoring of the hospital employees that was subsequently performed. The radionuclides generally considered for internal monitoring are99mTc, 125I, 131I, 8lmKr, 133Xe and 3H. The availability and use of thyroid monitoring following the use of 131I for management of an overactive thyroid gland and air monitoring following lung ventilation scanning using133Xe, 99mTc DTPA aerosol or 8lmKr were described. Generally doses from both types of studies were found to be in the µSv range.
Overall the meeting proved to be informative and a useful forum for the exchange of ideas. A transcript of the meeting is held by the Society for Radiological Protection.
Eileen Cowling
Science, Policy and Risk: the Royal Society's conference on 18 March 1997 at Carlton House Terrace, London
John Adams gave the opening presentation in this follow-up to the 1992 conference, but I missed it because of transport problems. Fortunately he has written a book ( Risk, UCL, 1995 ISBN 1-85728-068-7) and, along with the other presentations, his script appeared in the Times Higher Education Supplement of 14 March. The proceedings were available on the internet at http://thesis.newsint.co.uk - but whether they will still be there in June is uncertain.
The most eye-catching element in Adams' presentation was clearly the concept of `virtual risk' - by which he meant risks that have great uncertainties about them. To quote the THES article `Virtual risks are products of the imagination that work upon the imagination. We do not respond blankly to reality, we impose meaning upon it'. Predictably, Adams gave BSE and global warming as examples of virtual risks and suggested that there is more to dealing with them than getting the science right. If the day had a theme, it was this. Themes apart, I was alarmed to find that the Davy safety lamp had actually caused more accidents than it prevented because it made people exploit more hazardous mines. I had always seen it as a kind of Albert Schweitzer among inventions.
Tim O'Riordan from the University of East Anglia produced some evidence that the public do not greatly trust scientists to give them information on risks and have a marked distrust of Government scientists. In a survey of people in Norwich it was found that the most trusted sources for this kind of information were family, then friends. Scientists from environmental pressure groups were almost as reliable as friends and doctors were close behind. Government came bottom - as you feel it must - but religious organisations did particularly badly. So if you are worried about genetic manipulation give your Aunt Gladys a ring; don't bother the bishop. You will not be alone. Well, not if you live in Norwich.
The main conclusion was that, where we are subject to uncertain risks, the fairness and integrity of the consultative process we go through before any decisions are made is as important to acceptance as any scientific information. This O'Riordan called `civic' science. I would call it `politics' myself because if politicians aren't making decisions about uncertain alternative futures I don't know who is unless it's managers or parents or everyone.
John Krebs, the Chief Executive of the Natural Environment Research Council, made the apparently contentious point that you can always be scientific even when there are great uncertainties in a situation. He thought the problems stemmed from: lack of openness in decision making, lack of consensus and perceived lack of independence of scientists. Everyone agreed here that unnecessary secrecy was a bad thing.
In the discussion period a hypnotherapist stood up and said that 90% of his patients were obsessed about infinitesimal risks. How could we help him was, to me, the implied question. I wondered whether he was hoping that the conference would induce sufficient extra anxiety to bring him more business or whether he just needed to talk to someone.
John Durant of Imperial College argued that risk assessments need to involve lay people because there is a distrust of the experts after the sagas of nuclear power, pesticides, beef and food irradiation. He suggested `a period of cautious experimentation in the integration of expert and lay perspectives in this risks assessment process'.
Robin Grove-White, University of Lancaster, used the phrase `plural rationalities'. This I took to mean that everyone is right and that sociologists (or at least the more relativistic ones) will defend unto the death our right to all be right. There is a chronic mismatch between perceptions of Government, business, scientists and the public. For example, in the Brent Spar farrago, Shell and the Government had a different perspective from the men in the rubber boats. The Government and Shell were short-termists and parochial while environmentalists were thinking well ahead and globally. Of course, on the day, it was a hands-down victory for the inflatables (even though they got their sums wrong) and Shell's particular rationality was up the creek. Our refusal to acknowledge that there are views other than scientific ones about the acceptability of particular risks, and that these are at least as important and more likely paramount, undermines our system of government. He thought that the demand for more information on BSE was a surrogate for mistrust of Government. I am not sure we are quite as close to the edge as all that but I take the point.
Sir Robin Nicholson talked about the pressures of Government, the need to make decisions in uncertain circumstances but recounted memorably his first encounter with Mrs Thatcher. It was the time when the Channel crossing was being debated under the title of the Channel Fixed Link and choices had to be made between tunnels and bridges. Nicholson was summoned and asked his advice. Before he could give it, he was told that all he had to do was tell Mrs T what Brunel would have done in the circumstances. She went on to demand of him why he was needed anyway. Such are the risks faced by Government scientists.
Lewis Wolpert made a good point in the discussion; it's scientists who uncover most of these risks so why does everyone distrust them. Someone else stood up and said that `expertise is a deeply political concept'. While I wondered whether he was an expert in this field himself and whether he should be mistrusted the Chairman of the session said that he thought it a deeply silly point and passed on.
John Rimmington introduced the next session with a short advert for the HSE. Many of the points made about consultation and the involvement of non-experts had been taken into account by them; they did consult widely before taking decisions. He also gave some impression of the broad flow of a wide range of decisions that are everyday matters to regulators.
Lucy Neville-Rolfe, director of the Deregulation Unit in the Cabinet Office, talked about deregulation and the process of regulatory appraisal that has to be followed before regulation is even considered for the statute book. Essentially - and I hesitate to summarise such an elegant presentation - the appraisal requires a comparison of the benefits of regulation with the total costs and the typical ones for a (usually small) business. Business has to be consulted - especially small business - and there has to be explicit consideration of political factors such as equity. This applied across the board but where risks are being regulated there are special problems: risks are not always easy to assess, it is not always straightforward to distinguish cause and effect and the benefits of risk reduction are sometimes difficult to qualify.
Derek Burke, Chairman of the Advisory Committee on Novel Foods and Processes, said essentially that the layperson may see risks quite differently from the scientists. A full evaluation of the risks of food irradiation might show it to be completely harmless but the very mention of radiation conjured up dangerous ideas. A genetically modified yeast had been developed and proclaimed safe in 1990. The Times had responded soberly (`Genetic yeast passed for use?') but The Star had `Are the boffins taking the rise out of our bread?'. The negative reaction meant that the product has never been used.
More recently the committee had been asked to pronounce on the safety of meat from sheep that carried the human gene for factor IX - a protein required for the treatment of haemophilia. For every 100 modified sheep only one can produce the protein in useful quantities so someone wondered if we should not eat the other 99 with the inactive gene. Consultation had shown a wide divergence of view ranging from an abhorrence of the whole idea (tantamount to cannibalism) to a pragmatic rabbinical reaction that if it looked like a sheep then it was a sheep.
This emphasised the importance, Burke thought, of involving lay people in the decision making process. As I recall, a churchman is now on the committee giving moral guidance - although of course O'Riordan's results throw doubt on his real credibility. Scientists are regarded as arrogant, distant and uncaring.
Angela Wilkinson is the corporate issues and reputation manager of Shell UK Exploration and Production. She described how Shell are reviewing their decision making process to avoid generating unnecessary public outrage (I am quoting the THES here). They are having a dialogue with the public about whether the Brent Spar platform should be dumped at sea and plan to incorporate the results of this into the solution they will recommend to Government. While there is more to this than science and regulatory compliance it helps business to have clear standards set and monitored by trusted, independent and authoritative bodies.
The day was rounded off with a media panel.
I was rather disappointed in the day. It was all well presented, everyone was very fluent and spoke with great conviction but there seemed little new to say compared with 1992. What there was seemed to be: the public do not trust Government and scientists and they need to feel involved in decision making on risk. Either very profound or very obvious - ask Aunt Gladys.
Geoff Meggitt