NEA General Comments
Operational Quantities for External Radiation Exposure
This document was reviewed in detail by the NEA’s Expert Group on the Implication of Recommendations (EGIR) on 3 – 4 October. The clarity that the document brings is important and significant. For example, operational and protection quantities are now more closely related, and operational quantities are described as being measurable. Detriment to the lens of the eye is described as deterministic, and thus it is good that this document recommends the use of absorbed dose, in Grey, as the relevant quantity. In general, the scientific approach used here improves the modelling of responses at high and low energies for different radiations. It is good that each operational quantity is expressed in unique units, that is, the Sievert is used to express only one quantity! It is a good change if protection quantities are more accurate, particularly for high-energy radiation. However protection quantities should be defined to be measurable quantities. These and other clarifications in the document are very much appreciated.
The Value of Change
A key question identified by the EGIR addresses the need for significant change to field instrumentation calibration and use, worker training, and field documentation and procedural updating for what was seen as only a small, if any, operational gain. The significant change in operational quantities needs to be more broadly discussed. The new operational quantities of ambient dose and personal dose are the quantities indicated and displayed by dosimeters and dose rate meters (surveys instruments etc.), so these quantities will also be the ‘actual measured quantities’ in practice (Fig. 1). As the new quantities are related directly to non-physical quantity of effective dose, the conversion factors follow the changes and definitions of effective dose such as phantom models and tissue/organ weighting factors. Furthermore, development of concept of effective dose is based on consensus principle, not purely on development of physical data. Data of conversion factors for ambient dose and personal dose can be expected to change more frequently than data for current physical quantity of dose equivalent. Implementation of updated data requires updates of international/national standards and legislation, possible updates of dosimeter designs and production by manufacturers and updates of data at calibration and testing laboratories. It should be carefully evaluated whether the benefits outweigh the costs. The introduction of a replacement quantity will inevitably lead to a situation where old and new quantities (and thus measurement results) are used at the same time for a long time leading to ambiguous interpretations of measurements.
Design of dosimeters and dose rate meters is to be changed due to differences in energy/angular responses of dosimeters for the ‘new’ vs. ‘old’ operational quantities (dose equivalent). Partly the change of quantity can be solved through calibration of dosimeters, but not totally. Confusion could be expected with users as both the ‘old’ and ‘new’ quantities have the same units (Sievert) and as the dose/dose-rate meters based on ‘old’ quantities do not disappear from use rapidly. Training and guidance need to be provided to the users by the experts of radiation metrology and by the regulatory authorities for implementation of the ‘new’ operational quantities. Years of transition period could be expected when measured dose/dose-rates on field are based either on ‘old’ or ‘new’ operational quantities.
Has the benefit of making these changes now been addressed? The report discusses disadvantages of the old system, but not much on the advantages or disadvantages of the new system. This discussion has been ongoing for some time – there should be a rational for doing this now. There has not been much stakeholder engagement, e.g. no discussion at ICRP symposium. Making the suggested changes is scientifically sensible, but will be a long and resource-intensive process that will have a large operational impact. For example, if not all organisations within a country change at the same time it will be hard for the regulator to assess such different presentations of the same quantities. This is a question of document development and stakeholder involvement. If adoption is not harmonised will this cause interpretational issues, and could be a policy issue if the new approach is not uniformly adopted. While such operational questions are not explicitly part of the usual ICRP approach, the document should explain the Commission’s thinking in introduction, and note that this is an issue and could take time to implement. The document should better characterise the value of the proposed changes.
Document Target Audience
It is unclear for whom the document is written. The presentation is mostly scientific, and lacks introductory, explanatory material in virtually all sections. In a few specific cases, for example the lack of explanation of the use of mono-directional aspects that seem useless in operation where fields are in general isotropic, suggests that this document is written exclusively for experts. If this is the case, it should be stated first thing. If not, more explanation is needed – see further general and specific comments. In any case, a plain language summary should be included, focusing on the intent and key content of the document.
The Nature of Quantities
The document precisely describes the mathematical nature of the quantities in this report, but does not describe their use or bounds. For example chapter 3, Definitions of Quantities, could begin with a paragraph describing the chapter’s intent, e.g. to present the definitions, bounds and uses of the quantities used in radiological protection. Then, each sub-section (e.g. Radiometric Quantities; Dosimetric Quantities; Protection Quantities; Operational Quantities for External Exposure) should do the same, beginning with a short paragraph specifically describing the nature and function of the types of quantities listed. Finally, for each quantity in each sub-section, an introductory paragraph should describe the quantity’s bounds (e.g. dose, particle / photon energy range, etc.) and uses.
A section of the report could also be dedicated to the relation that quantities have to risk. The first paragraph of chapter 1, Introduction, states: “The protection quantities are used for the implementation of dose limits and for guiding quantitatively the optimization principles.” This implies that protection quantities are related to risk, but could be more explicit.
It would also be useful if this document described the appropriate use and range-of-validity of each quantity. In each case, the relevant uses, bounds and assumptions should be specified. For example:
Through its analysis of the document the EGIR developed several other questions and general comments, as well as a series of specific suggestions for adding, deleting or altering specific parts of the text. These general and specific comments are presented here:
While ICRP is not operational, the impact will be significant so the document should have been discussed by C4.
Different dosimetric quantities in the ICRU system (e.g. absorbed dose and kerma) are measured in the same unit (e.g. Gray). Should new units been considered to better distinguish between dosimetric quantities?
Ambient exposure fields of operational interest are generally isotropic. The use and value of “directional” quantities are not clear and should be explained.
Implementation of ambient dose and personal dose produce significant changes to values of measured/measurable ‘doses’ and ‘dose rates’ when compared to numerical values of current quantities of dose equivalents. This could be expected both for area monitoring and for personal dose measurements at least for the realistic low kV photon spectra. Due to changes in operational quantities the regulatory dose constraints given by operational quantities, requirements for and measured results of radiation shielding (e.g. in medical imaging), and evaluation of effective doses based on operational quantities need to be considered according to new approach. In addition, the new approach will lead to significantly lower measured personal doses especially in use of x-rays in medicine (the highest exposures come from interventional radiology and cardiology). This may lead to attitude that radiation is considered to be less harmful than before and potentially leading to neglect of personal shields and degrading radiation hygiene.
The new quantities of Directional Absorbed Dose in the Lens of the Eye, Directional Absorbed Dose in Local Skin, Personal Absorbed Dose in the Lens of the Eye and Personal Absorbed Dose in Local Skin are defined directly by physical quantity of absorbed dose without consideration of tissue/organ sensitivity for radiation (quantities measurable in Gy units, not in Sv). This is a clear approach from point of view for measurements but differs from current practice of using directional dose equivalents (Q-factor). Differences of new and old conversion factors (Eq. Fig 4.15, Fig.4.20) are large and accuracy of new quantities for estimating the equivalent doses of eye lens and skin should be described and discussed more comprehensively in the Report.
In practice, large uncertainties in measured personal dose equivalent (personal dose) result from unknown direction of incident radiation. A dosimeter worn on the chest is not sensitive to radiation impinging from the back of the person. The definition of personal dose includes the integration over all angles of incidence, so the actual exposure conditions are taken into account. However, from the point of view of personal monitoring, the new approach does no solve this fundamental problem of incident radiation direction.
Subscript “p” is used to denote particle type in Ep, but also “I” is used for indexing particles. Subscript “p” is used also in Hp. A consistent use should be developed
The point of investigation in the definition of Hp is somewhat superficial – it only defines the point where the incident fluence is determined. The point where fluence is determined does not have an effect on personal dose, which is defined in broad parallel incident beams. As incident fluence is homogeneous, the personal dose is fixed if energy spectrum and exposure geometry are fixed. This should be clarified
It would be convenient to include an image or drawing to clarify and thus better understand the definition of "Charged-particle equilibrium".
The paragraph beginning on line 333 requires clarification.
“Although the conversion coefficients from fluence or air kerma are to values of protection quantities that are not point quantities but are averaged over an organ or tissue, or the sum of these averages, the operational quantities that are recommended are point quantities.”
The abandonment by ICRP and ICRU of the depth of 10 mm for the definition of the personal dose needs explanation, and is not at all clear in the document. The depth between 50 and 100 μm for dose absorbed locally on the skin in older definitions for the personal dose equivalent were, in skin specified as a depth of 0.07 mm, and as 3 mm for the lens of the eye. The reasons that have led ICRU and ICRP to abandon a specific depth in the definition of personal doses for total body, for the lens of the eye and for the skin are not discussed or questioned. Although this has probably been unnecessary in the past, there remain some concerns about the consequences of this. In particular, in the near future manufacturers of personal dosimeters will abandon the current harmonized practice of designing personal dosimeters with an equivalent depth of 10 mm for the determination of the personal dose and 0.07 for the skin, since this is no longer a condition of definitions of the operational magnitudes. ICRP and ICRU are therefore encouraged to include in the text a sentence of the following style: “ICRU and ICRP continue to recommend (or see as a good practice) (or do not oppose) that equivalent skin depths continue to be used of 10 mm and of 0,07 for the manufacture of the personal dosimeters for the determination of the personal and local dose in skin.” This will ensure that the current non-written standard or practice of designing personal dosimeters with these common characteristics will not be lost.
Irrespective of whether the new definitions of operational quantities require recalibration of all calibration rooms and all personal dosimeters, which will undoubtedly result in a significant effort of time and human resources, it is mainly concerned that personal doses are not now defined on the basis of the equivalent doses, but on the effective dose, that is the protection quantity to be monitored. For that reason, the joint effort of ICRU and ICRP with this publication is recognized: Paragraph 215 “The previous operational quantities were based on the dose equivalent that would be produced at a depth in the hypothetical ICRU 4-element sphere, and on dose equivalent in soft tissue at a point in the body”, and paragraph 1261 “The recommendations of operational quantities in terms of conversion coefficients to ambient dose, directional absorbed dose in the lens of the eye, directional absorbed dose in local skin, personal dose, personal absorbed dose to the lens of the eye, and personal absorbed dose to local skin, are a clean break from the existing recommendations for operational quantities that are based on dose equivalent in the ICRU 4-element sphere and at a depth in the body, and will give a better estimate of the protection quantities”.
However, the calibration of personal dosimeters in the old and now obsolete operational quantity of the personal dose equivalent at the depth of 10 mm had an important advantage that now has been lost: as was well known, personal dose equivalent at the depth of 10 mm would probably overestimate the effective dose, but it would never underestimate. An example will help to understand this better.
For the Cs-137 gamma energy, the air kerma, physical quantity, conversion factor to personal dose equivalent Hp (10) is 1.21 Sv/Gy (ICRP-74 table A.23 and ISO-4037, Part 3, Table 33), while the air kerma conversion factor to effective dose is 1.02 Sv/Gy (ICRP-116, Table A-2). That is, calibrating personal dosimeters in Hp (10) had a certain operating margin of 1.21/1.02 = 1.186 for not to underestimate the effective dose. Now, since the conversion factor of the personal dose Hp is defined as a function of the effective dose, this margin of maneuver is lost and given the high uncertainties that arise in personal dosimetry, as also recognized in the ICRP document "For doses below dose constraints or investigation levels, in common practice, the protection quantities are assessed in terms of the operational quantities” there is no guarantee that the reading of personal dosimeters will underestimate the most important protection quantity, the effective dose.
Even if one can think that the problem is solved by maintaining the calibration of personal dosimeters with the old definition the personal dose equivalent to a depth of 10 mm, Hp (10), which will soon be obsolete, this will not be possible because future national or international intercomparisons, where the new definition of personal dose would be used, would leave users calibrating their personal dosimeters in Hp (10) in a poor position or situation, at least 18.6% above the actual effective doses used in the intercomparison.
It would be helpful to discuss this more clearly in the document.
The document uses both “absorbed dose TO the lens of the eye”, and, “absorbed dose IN the lens of the eye”. Need consistent language.
Skin absorbed dose still uses ICRU tissue model, not a monte-carlo phantom – WHY not move to monte-carlo phantom, like for lens of the eye.
In the introductory text, the effect of new ICRP phantoms on the calculation of conversion coefficients in this document should be discussed.