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Comments on the ‘Draft for Consultation - 2005 Recommendations of the International Commission for Radiological Protection (no date)’
The Commission is to be thanked for providing the opportunity and privilege of commenting on its Draft for Consultation –‘2005 Recommendations of the International Commission for Radiological Protection’, (Hereinafter referred to as ‘The Draft for Consultation’). This act of ’Glasnost’ is most welcome and to be highly commended.
The recommendations of the International Commission for Radiological Protection (ICRP) are pre-eminently influential in the setting of international standards for the protection of human health and safety against any risks presented by the use and application of ionising radiations. Such standards must be set wisely, so that not only is the health of individuals and communities safe-guarded but the secondary concerns of the economic and sociological consequences of such standards on the application of these radiations for the general good are taken into account. The proper setting of such standards therefore is a public trust and duty of the radiological protection profession.
The comments offered here are limited to parts of Section 3, entitled ‘Quantities and Units used in Radiological Protection’, of The Draft for Consultation. Particular emphasis is addressed to the recommendations for neutrons, other high-LET particles and high-energies.
Nota Bene: In the context of these comments the term “high energy” is intended to indicate above about 10 MeV
Radiation weighted dose and effective dose (Draft Section 3.3.3)
Paragraph 55 of The Draft for Consultation states that ‘Effective Dose is -----in principle as well as in practice a non-measurable quantity’. Nevertheless, E is determinable by current techniques of measurement and calculation. Furthermore, only minor modifications in the definition of Effective Dose would make the quantity measurable.
This subtle distinction between ‘measurable’ and ‘determinable’ has resulted in the evolution of a dual system of radiation protection quantities (limiting [or protection] quantities and operational quantities). It is respectfully requested and suggested that the Commission review this policy to determine whether its benefits outweigh the concomitant misunderstanding introduced. It is urged that a single system of quantities that would result by eliminating the distinction between the protection and the operational quantities would have great benefits yet carry no corresponding detriment (see also Comments on sub-section 3.5.1-Control of stochastic effects).
Weighting Factors (Draft Section 3.4)
3.4.1 Radiation Weighting Factors
Paragraphs 59-61. Both ICRP publication 92 and The Draft for Consultation place a major emphasis on the development of an average radiation-weighting factor, wR, applicable to the whole body. This emphasis necessarily raises the fundamental question of the relevance of radiation weighting factors to a system of radiological protection for the twenty-first century.
Some twenty-five years ago average radiation weighting factors (then called average quality factors) were of great value to the application of the ‘critical organ-MADE’ system to radiological protection against external irradiation by neutrons. The weighting factors provided a necessary simplification, imposed by the limitations of metrological techniques, radiation transport codes and mathematical models of the human body, in dealing with the complexity of the radiation fields generated within the human body.
An average radiation weighting factor may still be of utility for internal and external exposure by low-energy photons [where the important parameters RBE, Qav, H*(10) and wR are constrained to take the value 1, thus making the any necessary calculations trivial]. However, in the cases of external exposure by high–energy particles (nota bene: including photons) and for neutrons of all energies, physical considerations make the practical utility of an average radiation-weighting factor uncertain.
An important question is ‘what quantity does wR modify’? The answer may be deduced from the relationship:
= wR x (sum over all tissues of the product wT x average DT) (1)
where the symbols are well understood. Evidently, from equation (1), the answer is the tissue-weighted average absorbed dose, the sum over all tissues of the product wT x average DT. Both the absorbed dose and (dE/dX) distributions may vary greatly with location in the body, when it is externally irradiated by high-energy particles. In such cases values of average organ quality factors, average QT, may show a correspondingly wide variation between tissues (for example see ICRP Publication 74 for 14 MeV neutrons). Under such conditions the tissue-weighted average absorbed dose is a complicated quantity, not readily accessible to simple measurement. Thus the physical nature of high-energy radiation fields denies us the apparent simplicity suggested by equation (1).
With the great improvements, in fairly recent times, of metrological techniques, radiation transport codes and mathematical models of the human body, the arithmetical constraints of earlier times no longer exist. Complex calculations may now be made with great facility and speed. There is much less necessity, therefore, for the ‘simplification’ afforded in earlier times by the average radiation weighting factors. For neutron measurements and at high-energies, and particularly at accelerator laboratories, there is more interest in using conversion coefficients that relate field quantities (e.g. fluence) to determine the radiological protection quantities.
Because of these trends the author suggests that greater emphasis needs to placed in The Draft for Consultation on an acceptable definition of the model, or convention for the Q(L)-L relationship, from which values of wR but also, and perhaps more importantly, other parameters that facilitate the determination of Effective Dose may be derived.
Reference Radiation (Paragraphs 62-61).
It is respectfully suggested that the Commission clarify its recommendation that the value of the weighting factor, wR has the value 1 for all photons. There are energy limits beyond which this definition defies physical laws. The convention adopted by ICRP may be useful in the special but important case of low-energy photon irradiation. However, at energies above a few MeV, when incident photons have sufficient energy to induce nuclear interactions and produce secondary high-LET radiations within the body, the value of wR may have values significantly greater than unity because wR is an average value for the whole body
Radiation weighting factors for photons, electrons and muons (Paragraphs 63-67).
The caution given for photons also applies to this section.
Radiation weighting factors for neutrons (Paragraphs 68-74)
The Commission is to be complimented on its decision to recommend values of wR for neutrons as function of neutron energy and by reducing the value of wR for low-energy neutrons to 2.
However, there is little comfort to be gleaned from this section in its entirety because, among other deficiencies:
There are now three different methods by which the recommended values of wR are determined.
The relevant recommendations of ICRP Publication 92 are essentially ignored.
A large body of relevant published neutron data is ignored.
High-energies are ignored - despite their increasing importance to radiological protection.
The values of 5 and 2 recommended for wR for high-energy neutrons and high-energy protons are incongruent (see comments on the sub-section entitled Radiation weighting factors for protons).
If the primary radiation-weighting generator (convention or model) - the Q(L)-L relationship - represents best current judgement, then the laws of physics and mathematical logic will inevitably generate ‘correct’ values of Qaverage, qE, or wR under the irradiation conditions specified. It would then be expected that, if defined in an anthropomorphic phantom, that qE should be identical to wR:
wR(En) = q(En) (2)
Indeed, calculations for low and intermediate neutron energies show them to have values of qE near to 2, which are judged to be consistent with the relevant RBEs, and consequently appropriate reductions are recommended from the ICRP 60 value of 5.
However, near 1 MeV the calculated value of qE is about 13, considerably lower than the values of Q*(10) and of wR equal to about 20 recommended in ICRP 60. Apparently, this factor of about 1.6 at this energy is thought to be unacceptable to ICRP and the additional constraint
wR(1 MeV) = 20 (approx.) (3)
has been applied in making its recommendations.
If ICRP is to avoid ambiguity two determiners of radiation-weighting cannot be permitted. If ICRP selects the presently recommended Q(L)-L relationship to determine radiation-weighting factors, a value of qE and wR of about 13 at 1 MeV must be accepted. (We can do arithmetic and physics quite well these days). Conversely, if ICRP wishes a value of qE = 20 at 1 MeV, it must modify its currently recommended (ICRP60) Q(L)-L relationship. The degree to which the radiobiology permits this to be done is a matter for the judgement of ICRP. It may be significant that ICRP 92 makes no such recommendation.
Radiation weighting factors for protons (Paragraph 75.
The argument that there is considerable uncertainty in the values of the recommended values of wR is well taken. However physical considerations suggest that at high energies (hundred MeV region) the radiation fields produced in human tissues by neutrons and protons will be identical. This is an issue not of radiobiology but of energy deposition and track structure (physics) and it is therefore to be expected that, in this energy region, the RBEs (or wRs) for both particles will approach identical values.
In conflict with these physical constraints The Consultation Document recommends of values of wR of 5 for neutrons and 2 for protons respectively (see figure 1 and paragraphs 73 and 75)]. Paragraph 73 suggests that the lack of experimental data is the reason why no reduction in the value wR = 5 is recommended for high-energy neutrons. By contrast, in the case of protons, the results of calculations of RBE are referred to in justifying the reduction in the value of wR. Equally valid calculations for neutrons have been published in the scientific and support the views expressed here. The calculations and considerations of paragraph 75 should also therefore lead to the selection of a value of wR = 2 for high-energy neutrons.
3.5 Practical application in radiological protection (Draft Section 3.4)
3.5.1 Control of stochastic effects (paragraph 83)
In paragraph 83 the Commission continues to recommend the quantity ambient dose equivalent for area monitoring. With the greatest respect, the Commission might be able to avoid further controversy over the dual system of radiation protection quantities by softening this apparent imprimatur of the ambient dose equivalent to the exclusion of better alternatives now in general use.
While the ambient dose equivalent (ADE) may be of great utility for some aspects of dosimetry there are several published studies that reveal significant problems with the application of (ADE) to neutron dosimetry, particularly at high energies. The Commission might consider (ADE) be one weapon of an armoury of many alternatives that may be used as an operational quantity. Other techniques might, for example, include LET-spectrometry, and neutron and charged particle spectrometry.
An even better option would be to abandon the dual concept of protection and operational quantities altogether and define only protection quantities and leave it to the ingenuity of dosimetrists to deduce the means of measurement.
Ralph H. Thomas
Scientific Secretary ICRP Committee 3 1977-1985
Member ICRP Committee 2 1985-1996
Chairman Joint ICRP/ICRU Task Group 1991-1996
31 December 2004