2005 ICRP Recommendation

Draft document: 2005 ICRP Recommendation
Submitted by Hans Vanmarcke, Belgian Society for Radiation Protection
Commenting on behalf of the organisation

Comments of the Belgian Society for Radiation Protection (BVS-ABR) PART 2 6 About the exclusion levels (chapter 8) [61] The suggested exclusion levels for natural radionuclides are 10 to 100 times higher than for artificial radionuclides, in spite of the comparable radiotoxicity of both groups of radionuclides. Therefore, using large amounts of material with natural radionuclide concentrations just below the exclusion levels can result in significant population exposures. According to Radiation Protection 112 of the European Commission (1999), the absorbed dose rate in air in a model room out of concrete having specific activities of 1000 Bq/kg for the uranium and thorium decay series, and 10000 Bq/kg for potassium-40 is: (0.92 x 1000 + 1.1 x 1000 + 0.08 x 10000) nGy/h = 2820 nGy/h = 2.82 µGy/h The concrete structure shields against gamma radiation of the undisturbed crust. Using the average value of 50 nGy/h for the background, the excess dose rate in the room is (2.82 - 0.05) µSv/h = 2.77 µSv/h. Assuming an occupancy factor of 80 % (7000 h/year) and a conversion coefficient from absorbed dose in air to effective dose of 0.7 Sv/Gy (adults) results in an excess effective dose from the gamma radiation originating from the concrete of: Adults: 0.7 Sv/Gy x 7000 h/year x 2.77 µGy/h = 13573 µSv/year = 14 mSv/year The smaller body size of children and infants results in higher conversion coefficients (children: 0.8 Sv/Gy and infants: 0.9 Sv/Gy) and therefore in higher excess effective doses: Children: 0.8 Sv/Gy x 7000 h/year x 2.77 µGy/h = 15512 µSv/year = 16 mSv/year Infants: 0.9 Sv/Gy x 7000 h/year x 2.77 µGy/h = 17451 µSv/year = 17 mSv/year A dose index (I ≤ 1) is suggested in Radiation Protection 112 for the regulatory control of building materials used in bulk amounts: I = CRa / 300 Bq/kg + CTh / 200 Bq/kg + CK / 3000 Bq/kg Where CRa, CTh and CK are the radium (reference is often made to radium instead of uranium), thorium and potassium specific activities in Bq/kg. The dose index calculated from the exclusion levels is 12 (1000/300 + 1000/200 + 10000/3000). This means that the suggested exclusion levels for natural radionuclides cross the dose criterion by more than an order of magnitude. The above example shows that the unconditional release of large amounts of material with natural radionuclide concentrations just below the suggested exclusion levels can result in quite high population exposures. There would not be, for instance, any restriction in using phosphogypsum, not only to plaster walls, but also as a bulk building material for the inner walls. On the other hand, an exclusion level of 1000 Bq/kg for lead-210 or polonium-210 is quite low as we find limited amounts of material with higher concentrations at many places (chimneys, filters...). The set of exclusion levels for natural radionuclides from Radiation Protection 122, part II (2001), is more balanced. The proposed values for the most relevant radionuclides are: - 500 Bq/kg: for the natural uranium and thorium decay series and radium-226; - 5000 Bq/kg: for natural uranium and thorium not in equilibrium with their long-lived decay products, for lead-210, polonium-210 and potassium-40. The working group is opposed to define a third set of (below concern) levels, in addition to the already existing exemption and clearance levels. An extra set of levels will contribute to the already confusing situation and undermine our ability to communicate to the public. Suggestion: - do not introduce a third set of levels, in addition to the already existing exemption and clearance levels or use for the natural radionuclides the set of values from Radiation Protection 122, part II. 7 About the medical exposures (chapter 9) [71] On 2 occasions (148) and (213), the Commission points out in its draft recommendations: “The constraints in Chapter 6.4, …, should apply to the workers and members of the public, but it should be recognised that some exposures have to be incurred in the care and support of patients. Members of the public may also be exposed in the course of caring for patients at home.” Although technically correct, the working group strongly feels that these sentences indicate that the protection system defined for workers and members of the public are not applicable in the medical practice. According to the Working group, all possible means of protection have to be used in order to maintain the exposure of medical workers and general members of the public below the recommended constraints. There is no objective reason to be found why another protection system should apply for medical workers, nor should members of the public be allowed to be more exposed from medical practices than from other practices. However, the working group recognises the fact that, in particular cases, consenting individuals, members of the public, can assist in the support and comfort of patients. Clearly in these cases the constraints for members of the public are not be used and specific constraints for the comforter should be set on a case-by-case basis. The Working group is however of the opinion that the presence of a comforter should not be part of daily medical practice and only after careful justification on a case-by-case basis the presence of a comforter should be considered. For that reason, the working group does not agree with the emphasis that the Commission is placing on the minimisation of the sense of isolation experienced by patients in nuclear medicine and brachytherapy in (214). Additionally, the Working group feels that (225) does not indicate sufficiently clear that occupationally exposed workers should not be considered as comforters. Furthermore, the Working group disagrees with the fact that exposures of families and friends of patients discharged from a hospital after diagnostic or therapeutic nuclear medicine procedures are to be considered as part of the medical exposure. According to the Working group, a medical exposure can only be incurred by the patient or by a comforter in the special case where one would be present. Relatives and friends of patients have to be considered as members of the public and their exposures as a result of contact with the discharged radioactive patient have to be seen as exposures of the general public to the medical practice, for which the recommended dose constraints are applicable. This is furthermore consistent with (227) where it is stated that constraints for public exposure are to be applied for exposures resulting from wastes discharged by nuclear medicine departments. [72] The first sentence of (222) consists of a generic justification of all medical exposures. The working group discourages the Commission to make such statements without sound arguments. [73] The working group also feels that the formulations of the usefulness of diagnostic reference levels in (222) and (223) is not consistent with the fact that these reference levels typically correspond with the 3rd quartile of the doses administered to patients nationwide or even internationally. If 75% of all cases of a particular procedure are performed at a lower dose level, the working group feels that diagnostic reference levels should be more than “useful reminders to check that doses are not excessive” or “unusually high”. It is unclear to the working group why the Commission in (227) of its draft recommendations does not take into account the contamination risk that is related with nuclear medicine patients. If this risk would also be taken into account, there would be no reason to limit the requirement to have special restrictions on nuclear medicine patients to those being treated with radioiodine for thyroid cancer. Several palliative treatments would have to be considered as well. [74] Under paragraph (213) it is mentioned ‘First and most important, the limitation of the dose to the individual patient is not recommended because it may, ....’. What about ALARA? Of course, adequate quality is necessary for effective diagnosis but the prescriber of an examination should not only justify the practice but also take adequate measures to keep the exposure as low as possible. This is more or less mentioned in (217) but not stressed enough in (213). [75] In both paragraphs (225) and (226) the ALARA principle should also be stressed. And in the same way the justification of the practice (which is at least required e.g. for insurance companies). The group receiving the dose should indeed be “informed”, including a dose estimate (which is not mentioned in the text), in order to obtain its consent. Suggestion: - keep the same protection system for workers and members of the public in medical practice. - indicate that the presence of a comforter should only be considered after a case-by-case justification - not extent the concept of medical exposure to other persons than the patient and the comforter if present - not make statements on the generic justification of medical exposures without sound arguments - reconsider the meaning of diagnostic reference levels - not limit the requirement to have special restrictions on nuclear medicine patients to those being treated with radioiodine for thyroid cancer. Several other therapeutic procedures where such special restrictions are also applicable are being performed today and future procedure could require special restrictions as well. - underline the importance of ALARA in the considerations made under paragraphs (213), (225) and (226). 8 About the potential exposures (chapter 10) [81] In chapter 10 it is suggested to consider the ‘probability of attributable death’ to deal with potential exposures of individuals. The working group believes that the introduction of this ‘concept’ is psychologically not very opportune. No worker will be happy to work in an installation where its probability of attributable death is 2.10-4, even if he receives the explanation this value is very low. No person will be happy to live close to a nuclear installation knowing that its probability of attributable death is 10-5, even if he receives the explanation this is very low. Moreover, using this concept will reintroduce the chronic debate on the conservatism (or not) of the linear non threshold approach, as the probabilities are directly the result of the LNT consideration. Suggestion: - consider the probability of a dose higher than the background when dealing with potential exposures of the public. Consider the probability of a dose higher than a constraint when dealing with occupational potential exposures. 9 About the protection of the environment (chapter 11) The approach to the protection of the environment is limited to animals and plants, although it is realized that environmental protection is much more than safeguard non-human species. It also includes sustainability, pollution control, general hygiene, land use, safeguard abiotic compartments (atmosphere, deep sea, geological layers...). The purpose of the new framework is a harmonized approach with other (non-radiological) pollutants. However, the proposed protection system (reference organisms) is similar to the existing ICRP-system for the protection of humans (reference man). ICRP selected a small set of reference animals and plants that are typical for the major environments (rat, duck, frog, freshwater fish, marine flatfish, bee, crab, marine snail, worm, pine tree, grass and seaweed). It is questionable whether a pine tree is a good point of reference for a mangrove or a rat for an elephant. The working group takes the view that the countless number of living species on earth cannot be reduced into a manageable number of reference organisms. Suggestion - replace the reference animal and plant approach by an environmental protection approach. As mentioned in the draft recommendations, a set of ambient specific activity levels would be the simplest tool. This implies the development of a framework to deal with the unavoidable very high contamination levels of small abiotic compartments (geological layers (waste disposal), remote areas (testing of nuclear weapons) and with large scale dilutions (atmosphere (85Kr), deep sea (sea dumping)). The real challenge will be the interaction and acceptance of the environmental protection approach by the general public. A practical way to develop the ambient specific activity approach is through an extension of the critical group concept to uninhabited places. We could hypothetically (for the sake of the environment) assume people living in remote areas, building tunnels in geological layers, farming activities in the deep sea or flying at stratospheric altitudes. 10 About the quantities and definitions [101] Absorbed dose as defined (under paragraph (44)) is a non-stochastic quantity, and thus it is defined in a point. As such it can be determined by computation, but is not directly measurable. See ICRU33,I.B.: stochastic quantities can be experimentally determined, non-stochastic quantities: '… can, in principle, be calculated', but '… can be estimated as the average of observed values of the associated stochastic quantity'. Therefore, in radiation protection the averaged absorbed dose is used (see paragraph (49)). Suggestion: - change the text into: 'As a non-stochastic quantity absorbed dose is defined at any point in matter and can be determined by computation. It can be experimentally estimated by the average of observed values over the volume of a specified organ or tissue, for which in radiation protection we use the averaged absorbed dose (see (49))'. [102] Paragraph (45) mentions the ‘frequency of energy deposition events’. Energy deposition events are not periodic, so there is no frequency. Suggestion: - change the 'frequency of energy deposition events' into the 'rate of energy deposition events'. [103] The working group welcomes the new name radiation weighted dose as this name is explicitly clear by itself. It is reasonable to choose for a different name for the unities of radiation weighted dose (equivalent dose) and effective dose, in order to avoid confusion between these two quantities. In the past both quantities got the dimension of sievert. For reasons of continuity, it is better to keep the sievert. To make the difference between the radiation weighted dose and the effective dose more clear, one can make a distinction between the radiation-sievert (Svrad) for the radiation weighted dose, and the effective sievert (Sveff) for the effective dose: HR,T: [Svrad] E: [Sveff] The radiation weighted dose is a dose in a generic sense, but not in its physical sense. The dimension of a radiation weighted dose can be the sievert (or radiation-sievert), but this is definitely not a J/kg, neither in a generic sense, nor in a physical or mathematical sense. A radiation weighted dose has nothing to do with a joule (J). The reason for this confusion is the choice for a dimensionless radiation weighting coefficient. Logically and mathematically correct is to choose for a dimensional radiation weighting factor while transforming the absorbed dose into a radiation weighted dose with the dimension of Svrad/Gy: HR,T [Svrad] = wRDT,R [ .Gy = Svrad] Suggestions: - the name radiation weighted dose is welcomed; - keep the name ‘sievert’ by introducing the unit radiation-sievert (Svrad) for the radiation weighted dose, and the unit effective-sievert (Sveff) for the effective dose ; - change the dimensionless radiation weighting coefficient into a dimensional radiation weighting factor with the dimension of Svrad/Gy. [104] Under paragraph (55), describing the use of the quantity ‘effective dose’, it could be useful to add a further clarification related to the use of ‘radiation weighted dose’ in case of partial irradiation. Suggestion: - add: ‘In cases where only part of the body (special organs or tissues) is irradiated (e.g. in medical diagnosis: mammography, CT-scan, …), averaging over the whole body can be unhelpful in e.g. the judgement of the patient's irradiation exposure and in the quality control mechanisms of the procedures. In these cases the radiation weighted dose (equivalent dose) is a more appropriate and meaningful quantity.’ [105] Other punctual suggestions for clarifications related to quantities and definitions are prescribed here below. Suggestions: - under paragraph (59), about the radiation weighting factors, change 'Values of wR are taken to be …' into: 'Values of wR are averaged so as to be …' - in paragraph (186), table 9, the limits should be expressed in Gyeq not in Sv, as these are tissue reactions (deterministic effects), to be coherent with paragraph (94). - the basic SI-unit for specific activity is Bq/kg, not Bq/g (and justly kBq/kg, MBq/kg, …) - the internationally recommended name for 'activity concentration' is 'specific activity (Bq/kg)' or 'activity density (Bq/m3)'. Members of the BVS-ABR working Group Michel Bovy, Belgian Nuclear Research Centre (SCK•CEN) Henri Drymael, Association Vinçotte Nuclear (AVN) Gilbert Eggermont, Belgian Nuclear Research Centre (SCK•CEN) Herwig Janssens, Hogeschool Limburg (HL) Pierre Kockerols, Institute for Reference Materials and Measurements (EC-JRC-IRMM) Michel Sonck, Association Vinçotte Nuclear (AVN) Marc Van Eijkeren, University of Ghent (UGent) Hans Vanmarcke, Belgian Nuclear Research Centre (SCK•CEN)