|Response of the Health Protection Agency Radiation Protection Division to the ICRP Draft Committee 2 Foundation Document (FD-C-2)
2 General comments
3 Foundation document draft section 1 - Introduction
4 Foundation document draft section 2 ¡V Health effects
5 Foundation document draft section 3 ¡V Dose quantities in radiological protection
6 Foundation document section 4 ¡V Weighting factors
7 Foundation document section 5 ¡V Practical application in radiological protection
8 Foundation document section 6 ¡V Uncertainties and judgements in radiological protection
9 Foundation document section 8 - Glossary
In April 2005 the International Commission on Radiological Protection (ICRP) published on their website a draft of the Committee 2 Foundation Document, Basis for dosimetric quantities used in radiological protection. This document is one of several that underpin the 2005 ICRP recommendations, a draft of which has already been the subject of consultation through the ICRP website. NRPB provided comments on the draft recommendations and NRPB¡¦s successor division within the Health Protection Agency (HPA), Radiation Protection Division (RPD) welcomes the opportunity to provide comment on this foundation document.
This report has been produced primarily by the authors named some of whom had an opportunity to comment on previous drafts. Several HPA-RPD/NRPB staff contributed to the development of the foundation document. Comments include both substantive and editorial points.
2 GENERAL COMMENTS
a) The Foundation Document draws heavily on previous reports, particularly ICRP 92. Full understanding of the foundation document requires a good knowledge of the material contained in this review. Not all the recommendations in that report have been implemented.
b) The principal comment is that effective dose is not defined unambiguously and is actually used in the document with different meanings. Neither is it defined unambiguously in ICRP 92. Two contradictory statements appear here.
1. Page 15 end of section on protection quantities
¡§¡K.indicates that the quantity effective dose is not aimed at providing an individual dose value for a specific individual human body¡K.¡¨
2. Page 39 section on reference person, first sentence
¡§In principle, effective dose is defined and estimated in a person (worker or person of the public) and is intended for use prospectively in the protection of these persons.¡¨
The first of these statements assumes there is one and only one definition of effective dose and that is within a specified phantom. This is the view expounded by Ralph Thomas and shared by some members of HPA-RPD. It is also shared by pure physicists who demand that the physical aspects of radiological protection are precisely defined and that any judgement applies only to the biology.
The second statement assumes effective dose is defined in any human being either individual or representative or in one or more specified phantoms. This is the view held by Alan Edwards and the difference formed the nub of the topics under debate on ICRP quantities, (Thomas and Edwards, 2003)
The first of these views means that there is a uniquely defined relationship between fluence and effective dose (external) and between intake and effective dose (internal). Thus legal dose limits are defined rigorously in terms of fluence or intake. Therefore effective dose is directly measurable, by measuring fluence as a function of particle type and energy or by inferring the intake. Natural variations in human stature, build, organ size, metabolic activity or location do not affect effective dose in a given radiation field or for a particular intake. Therefore there is no room whatsoever to legally argue individual circumstances should it be necessary. ¡§I only had my hand in the beam¡¨ is not an allowed defence against exceeding the dose limit.
The second view allows legal dose limits to mean to the individual concerned. You cannot measure effective dose and you would not bother to calculate it in an individual unless it was really necessary. Nevertheless there is the practical problem of planning activities to comply with effective dose limits and to demonstrate that compliance. For these purposes, calculations in defined phantoms to produce standard conversion coefficients are used to provide estimates of effective dose to a group or to an individual. The estimates will be adequate for the purposes of planning and for demonstrating compliance with dose limits and constraints in most situations. In this restricted sense they are accepted values but it does not follow that these values form the definition of effective dose. With this second view there is no embarrassment that different phantoms are chosen for external and internal caculations or that different coefficients are given for intakes dependent on age and even gender.
Without clarification, confusion abounds. This foundation document must make it clear which of these two views Committee 2 is taking. They are mutually exclusive and different members of staff at HPA-RPD have come to opposite views about the intentions of committee 2 and therefore of the ICRP Main Commission.
Whilst the document does not come down clearly in the text in defining effective dose as a quantity related to a standard phantom or a specific individual, the formulation in equation 3.6 and the final section of paragraph 3.3 seem to exclude its application to individuals. If this is the case all reference to individual effective dose should be excluded. If the Commission wish to retain the possibilty of calculating an individual effective dose this may lead to the assigning of separate tissue weighting factors for males and females, both nomalised to a total of 1.0
c) There is still concern that radiation weighting factors are expressed in terms of the radiation type incident on the body. The reasoning is that biological effects are scientifically related to dose and radiation quality in the organ concerned. Physics calculations show radiation type and therefore quality varies for different organs for the same incident radiation on the body. Therefore any calculations based on wR are unscientific. This comment is only valid if view 1 above is accepted. If view 2 is accepted then then the incident based wR values are perfectly adequate for doing the required calculations without ambiguity.
d) There is nothing in this document about the dosimetric quantities to be used in radiological protection for medical exposures. ¡¥Operational¡¦ dose quantities for measuring exposures to patients and methods for converting them to appropriate radiological protection quantities have been developed by the international medical physics community without any guidance from ICRP. Not surprisingly, a plethora of dosimetric quantities and methods have evolved and clear guidance from ICRP on the most appropriate ones for helping medical practitioners in the justification and optimisation of this major source of population exposure would be very welcome. It is to be hoped that such guidance will be included in the forthcoming Foundation Document from Committee 3 on ¡¥Protection of the Patient in Medical Procedures¡¦ and that it will be available in time to influence the final version of ICRP¡¦s new recommendations.
3 FOUNDATION DOCUMENT DRAFT SECTION 1 - INTRODUCTION
4 FOUNDATION DOCUMENT DRAFT SECTION 2 ¡V HEALTH EFFECTS
N is used in equations 2.1 (page 7) and 3.1 (page 9) to mean different things.
Page 8 paragraph starting ¡§High LET radiation¡K¡¨. We suggest:
¡§High LET radiation causes more severe tissue reaction than low LET.¡¨ The word ¡§damage¡¨ might be misunderstood as initial damage, e. g. double strand breaks.
5 FOUNDATION DOCUMENT DRAFT SECTION 3 ¡V DOSE QUANTITIES IN RADIOLOGICAL PROTECTION
Page 9 Section 3 Para 2 line 3.
Replace ¡§vectorial¡¨ by ¡§vector¡¨
Paragraph just above equation 3.2
¡§Fluence is independent of the directional distribution of particles passing the sphere¡¨. ¡§Passing¡¨ suggests ¡§missing¡¨. Use ¡§entering¡¨, ¡§crossing¡¨, or ¡§passing through¡¨ instead. The same comment applies to the previous paragraph. ICRU and ISO use the term ¡§direction distribution¡¨ rather than ¡§directional distribution¡¨.
Paragraph just below equation 3.2
Delete ¡¨(stochastic process)¡¨ as it adds nothing to the meaning.
Page 9 last paragraph
Fluence is not appropriate for use in radiological protection because it does not correlate with risk. It is a simple quantity, if anything too simple. Remove the words ¡§and simple enough¡¨.
Page 10 paragraph 3
¡§A concept of body-related¡¨ helps readability (hyphen inserted).
Page 12 Section 3.2 Averaging of dose
¡§The averaging of absorbed doses¡K¡Kis the basis for the definition of protection quantities. This approach implies ¡K¡K..LNT and¡K. additivity of doses¡¨. Isn¡¦t it the other way round? The LNT hypothesis implies the validity of additivity and organ averaging.
Page 14 Section 3.3 Radiation weighted dose and effective dose Para above equation 3.5
Although it is said in Section 4, the statement that wR is set for incident radiation and initial energies for internal exposure should be stated here. We suggest:
¡§A set of wR values for various radiations incident on the body was given in ICRP 60 (1991)¡¨.
Page 15-16 Section 3.4.1 Internal and external exposure
The bullet points at the end are written as if view 2 above is taken.
Page 16-18 Section 3.4.2 Operational dose equivalent quantities Equation 3.9
Would D(L) be better than DL?
¡§While measurements with an area monitor are performed mostly free in air, personal monitors are usually worn at the body¡¨. Replace ¡§mostly¡¨ by ¡§preferably¡¨ and ¡§usually¡¨ by ¡§always¡¨. Neutron area monitors are hand-held close to the body and personal monitors are placed on the body
Strictly HP(0.07) monitors skin dose for strongly penetrating radiations as well
We think a few users have measured HP(3) but the usual argument is that the other two are sufficiently protective. We support the suggestion that the use of HP(3) is discontinued.
Page 18-20 Section 3.4.3 Operational quantities for radiation monitoring
Nowhere in these definitions does it say that the ICRU sphere is in a vacuum. The concepts of expansion and alignment depend on calculations
Page 18 Section 188.8.131.52 Aligned and expanded radiation field Last paragraph.
Would ¡§hypothetical¡¨ be better than ¡§fictitious¡¨?
Page 19 Section 184.108.40.206 Ambient dose equivalent
A reference should be given for the definition of ambient dose equivalent. This applies also to the other dose quantities.
Page 19 Section 220.127.116.11 Directional dose equivalent Para 4
Preferable to use ¡§weakly penetrating radiation¡¨?
Page 20 Section 3.4.4 Operational quantities for individual monitoring
Yes, Hp(10) is defined in the human body. BUT, nobody calculates it in the human body. All of the calculated data that I have seen have been for geometric phantoms: the ICRP and ICRU have published conversion coefficients for the quantity but never any calculated in ¡§the body¡¨. This is an area of some confusion, which this is perhaps an optimum moment to address. It needs to be stated that the quantity can also be defined in a geometric phantom, and specify the dimensions and composition of that phantom.
6 FOUNDATION DOCUMENT SECTION 4 ¡V WEIGHTING FACTORS
Page 23 Section 4.1.2 Radiation weighting factor for photons, electrons and muons Para 6.
Kellerer and Roos (2005) have calculated the effect on electron energy spectra of large absorbers for photon energies in the range 150 keV to 6 MeV. They say the magnitude of the effect of Compton scattering is much lower than that reported by Harder. The effect of Compton scattering on electron spectra in large absorbers is small compared with the inherent differences between photon energies.
Page 23 Section 4.1.3 Radiation weighting factor for neutrons Para 1
¡§Radionuclides emitting neutrons are infrequent¡K¡¨ Odd word ¡§infrequent¡¨. ¡§Scarce¡¨? Of course, a high fraction of neutrons encountered in the workplace are actually from fission events.
¡§This is due to the fact that most neutron fields contain neutrons with broad energy spectrum and very often calculations using energy dependent conversion coefficients are performed for estimating doses.¡¨ No. The people doing the calculations simply did not like the step function. As plain and simple as that.
¡§The production of secondary photons in the human body if exposed to neutron energies below 1 MeV, is mainly responsible for the decrease of the neutron weighting with decreasing energy.¡¨ Strictly it is the increasing fraction of dose caused by the secondary photons that is responsible. Is the following any better? ¡§When the body is exposed to neutrons with energies below 1 MeV, a significant fraction of the dose is deposited by secondary photons from the H(n, £^) reaction, so the neutron weighting is reduced.¡¨
¡§e.q. 4.1 can only be applied to strongly penetrating external radiation, e.g. neutrons, protons and heavy ions¡¨. It should be explicitly stated that the protons and heavy ions are only strongly penetrating if they have very high energies!
Page 30 Section on £\-particles.
There is no discussion on how RBEm varies with the way dose is calculated. This affects the judgement of wR. For example, if a judged wR is suitable when dose is calculated to the whole organ, it is probably not suitable for doses calculated to specific cells. Is this worth mentioning?
Replace £\-rays by £\-particles throughout
7 FOUNDATION DOCUMENT SECTION 5 ¡V PRACTICAL APPLICATION IN RADIOLOGICAL PROTECTION
Section 5.2.1 Application of effective dose Para 3
AMAD ¡V this is the first use of this acronym. It needs to be defined. It should also be in the glossary.
Page 34 line below Equation 5.3
Would ¡§comparison¡¨ be a better word than ¡§compliance¡¨?
The double summation over j is a little clumsy and unconventional. It would be more normal to use a different index for the second summation, even if it is intended to represent the same set of radionuclides. Better still the equation would read: [GRAPHICS CANNOT BE REPRODUCED HERE BUT ARE AVAILABLE ON REQUEST]
This comment applies equally to Equation 5.8 and some others.
Missing ¡§([GREEK LETTER TAO])¡¨. The parameter is described in the text as ¡§e(ƒä)¡¨. Surely, it is effective dose as defined in Equation 5.4 that has an annual limit of 20 mSv? The paragraph that precedes Equation 5.5 needs to make it clear that this for a worker who is exposed only by this route.
Section 5.4 Application of effective dose
The section is written as if effective dose is defined in one phantom, the first view in the main comments. If the second view is taken then:
First sentence. Delete ¡§effective dose or corresponding¡¨.
Paragraph 3. Replace ¡§determination¡¨ by ¡§estimation¡¨.
Paragraph 4. Replace ¡§effective dose¡¨ by the ¡§conversion coefficients¡¨.
The section would more appropriately be entitled ¡§Application of the conversion coefficients¡¨.
Section 5.5 Reference person
Para 1 sentence 1
¡§member of the public¡¨ rather than ¡§person of the public¡¨.
Para 2 last sentence but one. We suggest:
¡§However, a specific reference model has hitherto not been defined by ICRP or other international organisations, but this will change with the adoption of the computational phantoms (based on voxels) described in section 5.5¡¨ Maybe a set of reference models is more appropriate.
Page 40 para 1 last sentence but one
¡§radioactive sources¡¨ should be ¡§sources of radiation¡¨.
Page 40 para 2
Here it says voxel phantoms have been adopted. Previously it was ¡§will be¡¨.
Page 40 Para 3
Does the Commission understand the practical implications of what it is proposing? Earlier it states that when an assessment of a worker¡¦s effective dose exceeds 20 mSv an individual risk assessment needs to be made. It is here acknowledging that the assessed risk for some workers, females in particular, may be a factor of 2 higher than the assessment of effective dose had implied.
For consistency with Equation 3.6, the male and female labels should be written in full as subscripts not superscripts. Subscripts are almost always better, because superscripts are used to denote powers.
Page 41 Last sentence of section 5.5. We suggest
¡§However, since the wT assigned to the thyroid is given as 0.05 to allow for the high susceptibility of young children, the difference in detriment between genders is not important¡¨.
8 FOUNDATION DOCUMENT SECTION 6 ¡V UNCERTAINTIES AND JUDGEMENTS IN RADIOLOGICAL PROTECTION
Page 45 6-8 lines from the bottom
Dosimetric models are not used to recommend dose limits or constraints. They are needed to demonstrate compliance with dose limits. The phantom conversion coefficients are not uncertain only if the first view is taken. If the second view is taken, (see main comments) the coefficients lead to estimates of effective dose, which do have uncertainty.
9 FOUNDATION DOCUMENT SECTION 8 - GLOSSARY
Here effective dose is defined in the body not in a specified phantom. This suggests the second view is taken (see main comments).
If the Commission want to avoid the interpretation that effective dose can apply to a specified individual, they should avoid using the formulation of effective dose that does not include the male and female averaging. Otherwise, they do appear to be leaving open View 2, which might not be their intent.
A couple of the terms defined here do not appear to have been used in the document: ¡§biological half-life¡¨ and ¡§specific effective energy¡¨.
Thomas R. H. and Edwards A. A. Topics under debate. The present ICRP quantities are seriously flawed. Moderator J. C. McDonald. Radiation Protection Dosimetry 104, 79-87 (2003).
Kellerer A. M. and Roos H. Are all photon radiations similar in large absorbers? A comparison of electron spectra Rad. Prot . Dosim. 113, 245-250 (2005).