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Submitted by Carl Ross, Retired
   Commenting as an individual
Document Operational Quantities for External Radiation Exposure
 


 


Comments on the ICRU draft report entitled “Operational Quantities for External Radiation Exposure”


(Carl Ross, October, 2017)


I have good knowledge of radiation physics and radiation detectors but I am not an expert on radiation protection dosimetry. I am aware of the basic principles but feel somewhat uncertain when one begins to probe more deeply than simply using a survey meter or personal dosimeter.


The subject is complicated, partly because the impact of ionizing radiation on living tissue is complicated, but also because of the way the field has developed over several decades.  The possibility for confusion is perhaps best exemplified by the myriad uses of the noun “dose” combined with different adjectives. The book by Simmons and Watt, “Radiation Protection Dosimetry: A Radical Reappraisal” provides a good history of the subject and a review of some of the remaining challenges.


The idea that the protection quantities are not measureable is a difficult concept for anyone trained in the physical sciences. It is counter intuitive that the health of workers and the public is based on quantities that are not subject to experimental testing. In order to circumvent this dilemma, operational quantities are defined which are intended to be conservative estimates of the protection quantities.


Prior to this Report, the operational quantities (e.g., ambient dose equivalent) were distinct from the protection quantities (e.g., effective dose).  The idea was that the ambient dose equivalent, defined for the ICRU sphere, was a reasonable estimate for the effective dose, calculated for an anthropomorphic phantom. Now, the operational quantity is called the “ambient dose” and is an estimate of the effective dose. In this new approach, it is not clear to me why both “protection” and “operational” quantities are needed. The “ambient dose” is an estimate of the “effective dose” so there seems no need for both quantities. In support of this perspective, the authors state at L. 280: “The determination of an operational quantity should give a value that is a close estimate of the value of the protection quantity.”


The authors introduce new concepts and terms and deprecate others.  It is probable that these changes will do little to improve the health and safety of radiation workers or the general public. In particular, only a very small subset of society will ever have to worry about the radiation hazard of pions or similar exotic particles. Most dosimetry requirements will continue to be for ortho- and mega-voltage photons, neutrons and beta-rays. In these areas, the authors state that any changes are expected to be modest. I wonder if it might not have been preferable to leave this area largely unchanged but to have added sections for what might be called exotic radiation protection. In short, introducing yet another “dose” quantity with a modifying adjective is sure to lead to more confusion among the user community.


A surprising weakness of the report is that there is no discussion at all of the uncertainty of the conversion coefficients. The only uncertainty mentioned is the statistical uncertainty related to the Monte Carlo calculations. Checking the companion report, ICRP 116, one finds the statement: “Within the scope of their intended use, the coefficients as issued by an international authority have no uncertainties and, as such, are considered by ICRP and ICRU to be reference data, in accordance with the guidance of the Joint Committee for Guides in Metrology (JCGM, 2008).” This is not an acceptable approach for a modern scientific report. It essentially states that the results have no uncertainties because they were established by an august body. I have not discovered how JCGM, 2008 can be used to justify this statement. ICRU 90 on key data has recently been published and this document provides uncertainty estimates for all its recommended values.


The authors must say SOMETHING about uncertainties – they had no time to evaluate them, they did not know how to evaluate them, they will be evaluated in a forthcoming document. Of course, the preferred option is that realistic uncertainties are provided. Simmons and Watt draw attention to the quality factor data and question how the data were obtained. The authors need to address how the uncertainties on these data impact their results.


The book by Simmons and Watt is not referenced at all in this report. It deserves at least a few sentences if for no other reason than it provides a concise history of radiation protection dosimetry. Simmons and Watt also propose an alternative approach to radiation safety. The Report Committee can at least acknowledge their work even if they chose not to pass judgement. I note a recent presentation (http://www.icrp.org/docs/2015tokyo/12%20-%20Sato.pdf) that seems to develop several of the points made by Simmons and Watt although there is no reference to their work.


The term “local skin” does not seem to be defined. Why is the adjective “local” required?


I do not find Emax defined. I suppose it is the maximum effective energy among the different phantom orientations but it should be clarified.


In general, it is the effective dose or equivalent dose, i.e., including a quality factor, that is calculated. However, for the eye and skin it is simply the absorbed dose that is determined. Why?


There is a distinction made between deterministic and stochastic effects and it is claimed that Sv cannot be used for deterministic effects. This is confusing because there will still be a quality factor involved no matter what the level of the delivered absorbed dose. I note that the Wikipedia article on the sievert (https://en.wikipedia.org/wiki/Sievert) uses Sv to describe health effects up to 64 Sv.


Section 6 is very short, poorly written and not very helpful. This is an important subject because instrument calibration is the first step in a reliable determination of the effective dose. The authors should consider representative cases, e.g., 137Cs and pion beams, and provide drawings on how survey meters and personal dosimeters are to be calibrated. There are ISO and IAEA documents related to instrument calibration. Highlighting any necessary changes to these procedures would be helpful.


The Conclusions are not as detailed as I would like. There are big changes (the authors mention up to a factor of 90) in some of the conversion coefficients. How will workers in these areas be affected? If your exposure has been overestimated by a factor of 90, will you now be permitted higher exposure levels? What happens if your exposure has been underestimated?


The report is already very long. Appendix C is labelled as “informative” but is not discussed in the report. It should either be left out or discussed in the text and shown to be relevant.


The report has the feel of being drafted by a committee and needs editorial help. I have identified a few issues below:


-          L. 89: Nice to give his age.


-          L. 95: rather than “strongly”, “he was a major contributor …”


-          L. 97: “absorbed dose” or perhaps better, “dosimetry for medical …”


-          L. 100: comma after “led”


-          L. 146: Confused title. Could be “Comparison with Previous Recommendations”


-          L. 183: I don’t understand the word “optimization” here. It is not a very good sentence. The ICRP recommends protection quantities and maximum permissible values for these quantities to limit harmful effects of ionizing radiation.


-          L. 185: “the ICRU”. ICRU should probably be defined here.


-          L. 191: perhaps “maximum absorbed dose equivalent” (MADE)


-          L. 207: “of” should probably be “due to”. This sentence uses both “dose” and “exposure” rather loosely and should be recast. “assessments” should be “quantities”?


-          L. 209: the sentence beginning “The set of ICRU …” seems out of place or irrelevant here.


-          L. 218: “of” missing. Poor sentence. What does “and the combination of different depths” mean?


-          L. 233: “for” should be “because of”


-          L. 234: period after Section 2. Then “The definitions of the relevant radiometric, dosimetric, protection and operational quantities are given in Section 3.”


-          L. 239: “is given” is repeated.


-          L. 245: Rework as “Section 7 gives some general conclusions regarding the improvements expected by using the new operational quantities compared to the limitations inherent in the previous quantities.” I see the last part of this sentence as being one of the conclusions.


-          L. 256: “to” to “for”. This sentence does not summarize well what is in Appendix C.


-          L. 275, 278: there is confusion here over “current” and “previous”. I think the authors are referring to the same set of quantities.


-          L. 285: “on” should be “of”


-          L. 299: commas around “effective dose”


-          L. 303: 100 mSv is confusing. A radiation worker is allowed more over a career and skin dose can be higher.


-          L. 309: “in common practice” may be confusing here. I think what is meant is that, normally, the protection quantities are taken to be equivalent to the operational quantities.


-          L. 320: “the particles” is not needed here.


-          L. 323: “incident” and “incidence” are repetitive.


-          L. 333: Not sure why this sentence is here. The measurement of fluence is at a point (or close to it). But the result is multiplied by a coefficient that assumes a broad-beam, plane-parallel irradiation of a human.


-          L. 375: This paragraph may seem confusing. Yes, there were previous conversion coefficients to ambient dose equivalent and the like. The new operational quantity is ambient dose so it is not obvious that one should simply “replace” data already in use.


-          L. 415: should be Ep


-          L. 422: “energy radiance” should be simply “radiance”.


-          L. 425: The subscript “ptr” probably means “particle” and “transfer”, but it might help to say so.


-          L. 471: Sentence needs work. It mixes up the identification of the organs and the reason they are being singled out.


-          L. 476: “of density …” should be moved after “of skin …”


-          L. 474: This paragraph talks of absorbed dose. Should this not be an equivalent dose, i.e., involving a quality factor?


-          L. 493: Define Emax.


-          L. 512 and following: Explain why there is no quality factor or RBE involved with these quantities.


-          L. 579: For which phantom orientation is E calculated?


-          L. 638: Not a good sentence. Leave out “irradiation of” and after “skin” write “using broad parallel radiation beams in vacuo”.


-          L. 642: a drawing would help to clarify what is meant by this description of reference systems.


-          L. 662: the description of the angular ranges is confusing.


-          L. 684: sentence fragment.


-          L. 689: The report is already long and complicated. Explain why Appendix C is needed.


-          L. 803: I don’t understand what “allows” means.


-          L. 1083: does “lens of both lenses” mean lens of both eyes?


-          L. 1086: Why is this sentence about nuclear medicine here? It is no doubt true but is it relevant to this report?


-          L. 1091: “of the” is repeated.


-          L. 1169: Haven’t natural sources of radiation, e.g., uranium mines, always been part of ICRP recommendations?


-          L. 1176: Isn’t this paragraph better suited for Section 6?


-          L. 1181: The phrase “which can be well realized by a standard field quantity at the point of test” is confusing and can be left out.


-          L. 1182: “to an operational quantity”. This is confusing.  Does it mean “to obtain an operational quantity”?


-          L. 1184: I would leave this sentence out. A standards lab would not consider these conversion coefficients as part of their metrological chain because they cannot be measured.


-          L. 1185: This statement cannot be right. If it were true there would be no point of giving conversion coefficients for, e.g., pions. It is true that no NMI can calibrate a pion fluence meter but this does not mean that the pion fluence measurement is not traceable to SI standards.


-          L. 1201: I don’t see why this paragraph is part of a reference document. It points towards important work that needs to be done but doesn’t help in clarifying the operational quantities.


-          L 1212: Bad sentence. Far too long.


-          L. 1220: This whole paragraph is confusing. A diagram might help to clarify what is intended.


-          L. 1269: Remove “case”


-          L. 1273: Be more specific as to what is meant by “very close”.


-          L. 1277: Singular and plural references are not correct in this sentence.


-          L. 1282: This paragraph is confusing. It says the new results are an improvement but no changes are necessary.


-          L. 1291: “for ambient” should be “the ambient”


-          Line numbering starts anew with the appendices. Why is the title page and contents repeated here?


-          L. 668: Table and Figure missing


-          L. 1953: space missing


-          L. 2033: Table and Figure missing


-          L. 2087 “area”


-          L. 2211, 2219, 2246, 2358, 2389 “personal communication” is not useful as a reference. Attribution can be included in the text, e.g., “calculations of g were carried out by Seltzer of NIST”.


-          PENELOPE seems to have been used in at least one data set (Fig. 4.16) but it is not described in the MC codes.