Comments from a German group of experts on the July 2017 draft of a joint ICRU/ICRP report on operational quantities for external radiation exposure.
November 2017
Dear colleges,
let us first thank you for publishing a draft of the planned ICRU/ICRP report on operational quantities for external radiation exposure and calling for comments. We want to emphasize our common responsibility for the best possible solutions in matters of practical radiation protection, in this case for the workplace and environmental monitoring of external exposures, and we acknowledge the engagement and effort of ICRU and ICRP working groups in reviewing the operational quantities for external radiation exposure. There is a tradition of German cooperation in the ICRU and ICRP since 90 years, and in writing this critical statement, we will not leave this old basis of cooperation.
We urgently recommend to reconsider the rationale of changing the operational quantity for external exposure: A) The draft report does not consider the technical and non-technical pro’s and contra’s of its central aim that already the operational quantity should provide a “close estimate” of the protection quantity, the effective dose of the exposed person. Particularly it does not discuss the possible undesired effect that the existing two-barrier system constituted by the operational quantity and the protection quantity would be lost. B) It does not realize that the direct approach just by calibration of an area-monitoring instrument in terms of effective dose does not work in many practical situations due to the physical properties of the external radiation field, and it does not mention the existing additional methods of assessing the effective dose to the exposed person. C) The reasoning based on the over- or underestimate of the effective dose of the exposed person by the ambient dose equivalent at low or very high photon energies appears as biased as it does not realize the existing alternative methods of assessing the effective dose, whose development was already recommended in ICRU 39. D) We have some concluding remarks on particular issues.
Critical review of the reasons for a change in the operational quantities
With regard to area monitoring in the case of external whole-body exposure, the reference to a 30 cm diameter, spherical soft tissue-equivalent phantom now exists throughout ICRU reports 19 (1971), 20, 39, 43, 47, 51, 66 and 84 (2010), and includes the work of ISO on reference fields and area dosemeter calibration up to the recent F-DIS of ISO 4037. It has been a matter of 46 years of endeavor and practical probation. The reason to introduce the spherical phantom was clearly marked e.g. in ICRU 39, Fig. 3, namely that the operational quantity previously used for area monitoring, the air kerma, did not reflect a relatively strong physical effect, namely photon backscattering from the body in the photon energy region around 100 keV. ICRU 39 also comprises a critical numerical assessment of the ratio HE/H*(10) for photons and neutrons up to 10 MeV (Figs.1 and 2) and shows that the ambient dose equivalent at 10 mm depth is a rough, but conservative estimate of the effective dose equivalent. It is pointed out that for very high and low energy photons some modifications may be useful. In our previous letter to the ICRU of April 2016, we have pointed out on the basis of Dr. Endo’s data that a modification of H* at energies beyond about 4 MeV, such as the introduction of H*(150), would be sufficient to correct for the underestimation by H*(10). In addition, the role played by other methods such as tissue-equivalent proportional counters and radiation transport simulation (ICRU 84) has to be accounted for. The idea of changing the worldwide system of the operational quantities therefore appears as an overresponse to the present underestimation of the effective dose by H*(10) in the region of high photon energy.
The other concern is the overestimation of the effective dose by H*(10) at photon energies below about 70 keV which appears as practically important before the background that the largest man-made exposure is due to X-ray imaging. We may remind of the statement in ICRU 39, that for photons at low energies “more complex measurements will be needed” (page 5). In fact, e.g. in X-ray computed tomography, the exposure of the patient is usually determined from the “dose-length product” along the tomographic axis, and at single-beam diagnostic X-ray units the “dose-area product” serves the same purpose. These methods have been standardized, e.g., in the German standard DIN 6809-3. An often applied solution for the derivation of the effective dose and the organ doses from measured values of the ambient dose equivalent is publication 43 of the German Radiation Protection Commission (3rd ed. 2017). An example of the recommended conversion factors is given in the attached Fig. 1. We are concerned that the present attempt to change the worldwide system of the operational quantities appears to have its roots in the underestimation of these long existing solutions for the assessment of medical external exposure.
Also, we must criticize the clearly subjective opinion layed down in the draft report (line 281) that “The determination of an operational quantity should give a value that is a close estimate of the value of the protection quantity”. In reality, whatever the definition of the operational quantity, this will unavoidably remain a rough estimate of the protection quantity. One of the reasons is that the conversion coefficients from fluence or kerma towards the protection and operational quantities refer to idealized conditions such as broad parallel beams, whereas in practice there may be gamma-ray or X-ray sources near the body, emitting strongly divergent beams. Whereas the broad-beam condition is acceptable and widely used (ISO 4037) for the calibration of area monitoring instruments, other methods are generally applied when it comes to the numerical determination of the protection quantities in critical situations. For instance according to the UNSCEAR Fukushima report “Methodology for the assessment of doses from external exposure and inhalation of radionuclides (2013)”, the air kerma has been used as the operational quantity, but the decisive numerical determination of protection quantities involved the particular conversion coefficients valid for exposure from gamma-ray sources in the soil or in a plume. In the draft ICRU/ICRP report itself (310 ff) there is an enumeration of the various influence factors. The German legal regulations clearly distinguish between the operational quantities, which are regarded as conservative, but rough estimates in the sense of a “first barrier” or a “screening parameter”, whereas in all situations of suspect the “second barrier” has to be activated, which means the assessment of the protection quantities under full consideration of all parameters of the actual exposure, e.g. by means of the already mentioned publication 43 of the German Radiation Protection Commission.
Let us also point out that the difference respectively the close agreement between the numerical values of the protection and the operational quantities is not only a matter of measurement or conversion factor uncertainty. People seeing the microsieverts per hour on their hand-hold portable area-monitoring instruments while in a scenario near radiation sources with divergent radiation fields should not be mislead to think that they directly measure the legally limited protection quantity. After a long service in radiation protection, dealing with the consequences of the Chernobyl accident, with radioactive waste disposal, with natural radiation sources and with radiation protection in medicine, we are sorry to predict considerable non-technical difficulties arising from the wrong expectation that portable area-monitoring instruments directly indicate the protection quantities. This could in practice mean the loss of the two-barrier system. With all respect we must state that the draft ICRU/ICRP report fails to analyze and reflect the potential non-technical consequences of the idea of the “close estimate of the protection quantities”.
Concluding remarks
The proposed names “ambient dose” and “personal dose” of the operational quantities sound nicely. Taking this together with the simultaneous use by ICRP of the terms “effective dose” and “organ dose”, the reader recognizes the systematic elimination of the word “equivalent”. There is a risk that the quantities ambient dose and personal dose will no longer be perceived as dose equivalent quantities. While this might be a charming solution to the old conflict between “dose equivalent” and “equivalent dose”, it should not be “smuggled in” but clearly explained, and the “type of quantity” of the “ambient dose” and “personal dose”, namely a weighted sum of absorbed doses, should be clarified. Moreover, if it is intended to make an end to the difference between Q(L) and wR, this should clearly be explained. A mistake in the world of quantities and unities seems to be the attempt to “define” the effective dose and the ambient dose by their conversion factors from fluence or air kerma; rather it goes the other way round, namely that the conversion factors are defined as the quotients between these doses and the fluence or air kerma. Taking of terminology , we also want to express that a placative term such as “clean break” (lines 1263/1264) should be avoided wherever possible; rather we should acknowledge that (not only the “fictive”, but also some hardware-constructed) spherical ICRU phantoms have served radiation protection over almost half a century.
In order to keep the deadline, these comments are today only signed by the four of us, Klemens Zink, Dietrich Harder, Hans-Michael Kramer and Dieter Regulla, but we will later update the list of signers.
With kindest regards, hoping on further common progress, Klemens Zink.
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Fig. 1 Example of the conversion factors used to derived the effective dose from the ambient dose equivalent for photons below 10 MeV. The blue curve holds for anterior-posterior incidence of a parallel beam. From Publications of the German Radiation Protection Commission, Vol 43 (3rd ed. 2017) |