2005 ICRP Recommendation

Draft document: 2005 ICRP Recommendation
Submitted by Staff of the NRPB, National Radiological Protection Board
Commenting on behalf of the organisation

PART 2 5. ICRP Draft Section 2 - The aim and scope of the Commission’s recommendations As stated previously there should be a clear statement of the fundamental ICRP principles of justification, optimisation and limitation here. Although justification is discussed it would be helpful if more guidance could be given. Detailed technical points on this section of the ICRP report are given in Table 3. TABLE 3 Technical comments on the aim and scope of the recommendations 15 This paragraph could benefit from a bit more explanation. Also is the term “controllable source” used elsewhere in the recommendations? 17 It is very difficult to give a simple definition of “practice”. As written a practice would include a terrorist deliberately exposing people to radiation. Some reference to the alternative, ie, “intervention” is needed. Otherwise, there is nothing to be gained from defining a “practice”. Also, current definitions of practices mostly exclude natural sources, which is of practical importance for industries using minerals, etc. It should be made clear whether this distinction is still recommended by ICRP. 18-19 The reasons why the methods for ensuring the justification of medical exposures are in the scope of these recommendations, but not of occupational or public exposures, are not very clear. Perhaps the individual (rather than societal) nature of the benefit and harm from medical exposures needs more emphasis as a reason for ICRP retaining responsibility for recommending methods for their justification? In these new recommendations ICRP appears to have considerably softened the justification principle. It no longer explicitly states that the benefit of a practice should offset the radiation detriment it causes (even for medical exposures). Is this omission in recognition of the impossibility of making such a quantitative judgement in most circumstances or is it assumed that the basic principle as defined in ICRP 60 still applies and there is no need to repeat it here? 20 The last sentence is not at all clear, ie, it suggests that a non-justified practice may still be undertaken. Examples are needed, or at least a reference to paragraph 22. 22 This paragraph seems very muddled. Many (non-emergency) existing practices can still be periodically reviewed to see if they are justified (eg, lightning conductors in the UK). Also, the terminology is confusing – it includes “exposure situations” and “practices”. Also “exclusion” is used in relation to “sources”. Does “existing controllable exposure situations” mean intervention? If so it should be made clear. 23 The last sentence is problematic – natural sources may be covered by the Commission’s recommendations, but not always in the same way as artificial sources. 24 There may be problems with the idea that anything giving rise to doses below a particular level is outside the scope of the advice. Basically, the system should provide a level of dose below which no further consideration is needed. This screening off procedure can and should be included in the wide-ranging system of protection that is being put forward here. 28 Exemption levels can be activity concentrations OR total activity levels. 29 There is a need for further clarification of practices and intervention and there may be difficulties in interpreting this paragraph in practice. It is not always possible to avoid treating waste disposal as a practice in its own right. For example, introducing restrictions on food supplies following an accidental release is an intervention but disposal of the contaminated food as a waste is a practice. It is stated that the advice in ICRP 81 and ICRP 82 continue to represent the Commission’s views. However both of these documents refer to two intervention reference levels – 10 mSv as a level below which intervention would not normally be justified and 100 mSv as a level above which intervention would almost certainly be justified. Whilst the 100 mSv level is clearly mentioned in the new recommendations the lower level would appear to now be 20 mSv rather than 10 mSv. ICRP need to make the position absolutely clear. If the lower level is now 20 mSv they need to say that ICRP 81, 82, etc, apply but that when the lower intervention level is discussed this should now be considered to be 20 mSv. 6. ICRP Draft Section 3 - Quantities used in radiological protection This section is relatively hard to read and most of it seems too detailed for a top-level document. It would be better if much of this material was in a supporting document or an annex. This is done for tissue weighting factors (Section 3.4.2) and should also be done for other sections. The introduction in ICRP Publication 60 of the quantity effective dose with the new approach of using radiation weighting factors defined in terms of the radiation incident on the body was not universally well received. The introduction of wR was considered by many to have been done without the foundations having been established or the practical consequences investigated. Much has been written and said since on this topic, and many calculations made. Partly as a result of this, there is a much firmer basis for the updated recommendations on quantities. However, there remain some areas where recommendations could be made less ambiguously or the arguments in support made more transparently. Firstly, the link of the values of wR for neutrons to Q(L) via qE, or to RBE more directly could be explained more clearly. Since there are no human epidemiological data for neutron exposure, which can be used to determine RBE values as a function of neutron energy, animal and/or in vitro cell data must be used. Using these directly or via the Q(L) relationship and qE, one obtains a similar weighting factor dependence on energy. But this relationship needs to be modified to obtain agreement with the presently recommended values of wR for neutron energies around 1 MeV. It would be helpful if the reasons for preserving these values were given. If the relationship were not modified, there would not be the increased values given in paragraph 73 for higher neutron energies (of importance for cosmic radiation exposure). Although there is some discussion in paragraphs S13, 54, 55, and 86, it could be stated more clearly in what situations it is appropriate to use the quantity effective dose. In paragraph 86 it is stated that effective dose is formally defined for a hypothetical individual using age- and sex- averaged tissue weighting factors, but that the quantity can also be used when interpreting data for a specific individual using some individual-specific values of some parameters. A definition which is not specific to a particular individual fits in better with the general application of the quantity in most applications, for example where it is included in national legislation, or international regulations and guidance, with tables of recommended conversion coefficients. For demonstration of compliance with such legislation and regulations the quantity becomes single-valued for a given radiation field and orientation of the reference phantom chosen. The section raises concerns in a number of places about dose averaging within tissues and organs. It is important that the current ICRP methodology is clearly presented, including the point that doses take account of regional distribution of radionuclides and target cells in tissues where considered appropriate. Concerning uncertainties in dose estimates for internal emitters, a clear message is needed that uncertainties are small for some radionuclides (eg, tritium as tritiated water, caesium-137) but may be substantially greater for others (eg, plutonium-239). Detailed technical comments on Section 3 of the ICRP report are given in Table 4. TABLE 4 Technical comments on quantities used in radiological protection 37 Section 3.3 (41, 42) and 3.3.2 It would be clearer if this paragraph and these sections on dose averaging were combined and made more succinct, but with some indication of the extent to which microdosimetry might help understand and therefore inform judgements on the effects of different radiation types and energies. 36 1st line: suggest replace the term “dosimetric quantities” with “radiation protection quantities” (“dosimetric quantities” is specifically applied by ICRU to quantities which are essentially the product of radiometric quantities and interaction coefficients, eg, kerma, exposure, cema and absorbed dose). Perhaps the last sentence could be expanded a little to say that the intended use of ICRP quantities is for planning protection measures and controlling exposures to individuals and populations. The phrase “assessment of risk in general terms” is very vague. 40 LET should be defined/explained. 43 3rd sentence: this sentence would be more precisely in line with the definition if rearranged slightly to read “Absorbed dose, D, is defined as the quotient of the mean energy imparted by ionizing radiation in a volume element, d , and the mass dm of the matter in that volume.” 44 2nd sentence: should be “….. and, in principle, if averaged over a finite mass, is a measurable quantity.” (This is in line with paragraph 48.) 47 There is the explicit admission that the treatment of radiation quality is “pragmatic and empirical” and that the selection of radiation weighting factors is basically a “judgement based on the results of radiobiological experiments”. This is supported. 50 This paragraph requires further attention. It refers to examples of heterogeneous dose distribution from internal emitters – plutonium on bone surfaces and radon progeny in bronchial airways. It says that organ-averaged absorbed dose may not be a good dose quantity in these cases. It is only in the last sentence that it says that ICRP do not do this - they calculate doses to the putative target tissues in these cases. 51 NRPB supports the change to ‘radiation weighted dose’ but on balance do not think that the unit needs to be changed due to the confusion that it will cause at a working level. 54 This restriction on the use of effective dose to prospective guidance seems too strong. It implies that effective dose can not be used for any study based on measurements in the environment as they are inevitably retrospective. Also studies to demonstrate that the dose limit or constraint has been complied with or to assess the collective dose from particular practices or types of exposure for comparison with other sources of population exposure, are also retrospective and for these it is perfectly valid to use effective dose. The issue here is the level of dose and it would be better to keep the previous advice that equivalent (radiation weighted) and effective dose only apply well below thresholds for deterministic effects. NRPB agrees that in most cases it is not valid to use effective dose in studies to investigate the risks of particular health effects in identified populations but this is not the only type of retrospective dose assessment. 54 and 55 Perhaps it should be made clearer whether ICRP intend that in many applications the quantity becomes single-valued for a given radiation field and orientation of the reference phantom chosen. For example, where the quantity effective dose is included in national legislation or international regulations or guidance, with tables of recommended conversion coefficients, for demonstration of compliance with such legislation and regulations. ICRP might state that the use of phantom calculations is acceptable for these purposes. 55 Line 3: delete “of a person”. The phantom may be sex and age dependent but calculations are still to a representative phantom not a person. 56 “It is in order to improve the correlation between dose quantities applied in radiation protection and the effects considered two types of weighting factors have been introduced …” This should be rephrased - it implies a more arbitrary process than is the case. 61 The Q(L) function is introduced with no explanation, the role of Q(L) in helping the selection of wR could be better explained. 62 Last sentence: for completeness, change to “…. eg, 60Co gamma rays, medium and low energy x rays.” “radiation quality” is generally taken to relate to the spectral distribution of the photon energies in a field of photon radiation. You do not need to conduct in vitro experiments on cells to measure “radiation quality”. 63 Tritium should be included in the low LET discussion. A mixture of units of x-ray energy is used, ie, keV and kV; presumably the latter is kVp. The distinction may be clear in a report on x-rays or in a more detailed Annex, but it is confusing here. 64 Last sentence: “Often the parameters used in these models contain large uncertainties and many parameters can only be considered as rough estimates.” Suggest replace with something like: “Uncertainties in model predictions depend on the radionuclide concerned and in some cases can be substantial.” 65 Last line: “… obtained from investigations of small cell probes.” What are they? 66 4th sentence: need to add Hp thus, “……the operational dose quantities H* and Hp are used …..” 66 and 67 Discussion of wR of 1 for all low LET radiations. This could be extended to include, for example, tritium betas and Auger electrons. 68 It is interesting that the reasons that are given for selecting wR=1 in paragraph 63 are ignored when neutrons are considered here. 70 Effective dose for neutrons has only ever been calculated for a well defined human phantom, so lines 2-3 and 7 should not refer to a human body. Last sentence: “not symmetrically” is superfluous; but suggest overall change to last part of sentence to make clearer thus : “Owing to the different wT values of the organs and tissues, and the different depths in the body at which they are located, the value of qE depends on the direction distribution of the radiation incident on the body.” 73, 74 Figure 1 and equation 8 All the physics of high energy interactions indicates that above a few GeV (important energies for fields near high energy research accelerators, and, to lesser extent, the cosmic radiation exposure of aircraft crew) the radiation weighting factors for all hadrons (specifically in this context protons, neutrons and pions) should become the same (a value of 2 is appropriate). 74 There is no change in the wR for neutrons above 1 MeV which is good as it means ICRP is not putting undue faith in Q(L). 76 In this discussion of the epidemiological data for alpha emitters, the uncertainties are perhaps overstated. It is worth saying that RBE data for animals support the use of a wR of 20 for alphas and this is consistent with human data for alpha induced liver and lung cancer. The human and animal data suggest that RBE values are tissue specific, eg, close to 1 for leukaemia induction. 78 Pions are mentioned but no value of wR is given in the table. 79 and Table 2 Muons are missing (see Table S3); should not positrons be included also (unless it is assumed that the term electrons applies to e- and e+), and pions? 81 The new tissue weighting factors will lead to significantly different numerical estimates of effective dose for many types of diagnostic medical exposure. Much confusion will be caused by comparing effective doses calculated with the new tissue weighting factors with effective dose calculated using the old tissue weighting factors. ICRP’s decision to retain the same name for “effective dose” despite changing the way it is calculated, will only add to the confusion. Table 3 Perhaps the footnote about remainder tissues could explain that the average dose is the simple arithmetic average of the individual tissue doses, not the mass-weighted average that was recommended in Publication 60. Current dosimetric models do not permit the explicit calculation of doses to adipose tissue, connective tissue or lymphatic tissue. Perhaps the voxel phantoms discussed in paragraph 89 will make this possible for adipose tissue and the main lymphatic nodes, but it would help to pick up this point in paragraph 89. 82-93 This section on practical application seems to be aimed entirely at occupational exposure and does not really address the application to public or medical exposure. There is no discussion of the practical radiation quantities to be used in radiological protection for medical exposures and how organ and tissue doses are to be estimated. Although protection quantities are not required for the limitation of medical exposures, a reliable method for estimating organ doses and for deriving individual risks is necessary to comply with ICRP’s recommendations on justification and optimisation. Why does ICRP not give as much advice on the dose quantities to use for medical exposures as it does for occupational exposures? After all, medical exposures are a far greater source of population exposure than occupational exposures. 83 Last line: ICRU report 33 (ICRU, 1980) is not the most appropriate reference. It has been superseded by reports 51 and 60. The more appropriate of these in this context is Report 51 (ICRU, 1993). 84 1st sentence: replace “take account” with “include”. It would also make the sense clearer if the first 2 sentences of this paragraph were put at the end of paragraph 83. 86 The last part of this paragraph is not at all clear. Does effective dose really have a ‘formal definition’ or is it more as a way of doing a calculation? Tissue and radiation weighting factors are specified formally but metabolic and anatomical factors are not. The latter factors are just as important. Does it mean that effective dose is formally defined for a hypothetical individual using age- and sex- averaged tissue weighting factors, but that the term can also be used when interpreting data for a specific individual using individual-specific values of some parameters? This refers back to the single/multi-valued nature of E. 87 There is another route of intake that has not so far been mentioned and that is through the skin. Will it be considered? 88 This paragraph could be clarified. “If doses approach or exceed the dose constraints, then investigations may need to be undertaken to address workplace and individual specific characteristics in the dose assessment.” This appears to need strengthening. Individual assessments would surely be needed well in advance of an approach to constraints because individual characteristics may take doses over the constraint. It would probably also be appropriate to consider uncertainties in such circumstances. 89 See comment on Table 3 above.