The Use of Effective Dose as a Radiological Protection Quantity

IRPA Response to the ICRP Consultation on “The Use of Effective Dose as a Radiological Protection Quantity”

1 August 2018

IRPA welcomes the opportunity to provide comments on this draft ICRP report. For this consultation we invited the IRPA Associate Societies around the world to provide comment to IRPA, and we have also reviewed the draft report against IRPA’s current positions which relate to this topic. This response takes the form of a short note identifying the key issues arising from these considerations, together with a list of the full responses of those Associate Societies which responded to our request. The individual Associate Society responses will be forwarded separately to the ICRP consultation.

Headline Issues

Issue 1. We note that the document proposes that Equivalent Dose (H) is not required as a protection quantity, and that it is more appropriate for limits protecting against tissue reactions to be set in terms of Absorbed Dose (Gy) rather than Equivalent Dose (Sv). Whilst the attached annexes contain some detail suggestions for how to move forward in this direction, we did not receive any expressions of concern relating to the overall proposal.

Issue 2. A key proposal of the draft report is that Effective Dose may be considered as an approximate indicator of possible risk, all be it with the need also to give additional consideration of variation in risk with age, sex and populations group. This is described as ‘a pragmatic and unintended application’ of Effective Dose. Whilst in some ways this is perhaps an inevitable and natural step to take, IRPA is concerned that in formalising this approach ICRP is taking a quite major step in the overall context of the system of protection, which must be addressed more explicitly and carefully. The risk assessment method proposed is wholly based on LNT. To date there has been a clear acceptance that LNT is a model (or a hypothesis) purely for the purposes of protection, and it is not a scientific model of risk from radiation. This new formal proposal is therefore a significant step away from this historic approach, towards an inherent acceptance that LNT can be used as the basis for an approximate indicator of risk.

Of course the draft report presents several caveats to this approach – noting that there are many uncertainties associated with risk projection to low doses and low dose rates, and that the associated risks are increasingly uncertain at lower doses. However, in our recent report on IRPA’s consultation on the system of protection (ref below) we noted two very important factors of relevance to this proposal. We were concerned to ensure that as a profession we are always totally honest and accurate in statements concerning radiation risk, and that there is a concern that the system of protection relies too heavily on many detailed caveats, which we termed the ‘fine print’, which are absolutely essential to full understanding but which usually get lost in detailed application and discussions. Our concern is therefore that in practice the new proposals will result in many statements which quite definitively state that the risk (of such and such a procedure, etc) is X, without any visible caveat. Such statements would clearly be inaccurate and give a false impression of what we really know about radiation risk at low dose.

It is therefore essential that ICRP gives further consideration to the introduction of this significant change to the use of LNT, including how this new formal proposal can be framed to help ensure that the limitations of the approach are more clearly delineated and visible, and in particular how to make clearly visible the very significant uncertainty in risk estimates at low doses. It is important to minimise the opportunities for misuse of such risk estimations.

Issue 3. The draft report is a difficult read, with much repetition – both within the document itself and repetition of many prior ICRP statements. There is a need for significant editing which must lead to greater clarity, consistency and improved use of language.

Issue 4. A particular example of repetition is the discussion on collective dose, which is unnecessarily spread across several sections. The text includes several repetitions of previous ICRP statements, without adding any significant new material. This is a missed opportunity, and the report should include a clear ICRP position on use, and particularly the limitations, of collective dose. We draw attention to statements in section 3:

“Collective effective dose is not intended as a tool for epidemiological risk assessment …”: Given that this report is now endorsing the use of effective dose as an ‘approximate indicator of risk’ it would be helpful to make transparently clear the difference between the two types of risk assessments.

“The computation of cancer deaths based on collective effective doses involving trivial doses to large populations is not reasonable and should be avoided”: This gives an opportunity for ICRP to pull together a definitive statement on the reasons for the limitations, which has been lacking to date.

Wider Matters

ICRP’s process of consultation on draft reports is a highly valued contribution to the development of the system of protection. It would be helpful if it is possible to allow somewhat more time for consultees to prepare their responses.

Reference

IRPA Consultation: is the system of protection ‘fit for purpose’ and can it be readily communicated? Views of the radiation protection professionals. R Coates and R Czarwinski. J. Radiol. Prot 38 (2018) 440-455.

Associate Society Responses

Argentinian Radiation Protection Society

Comments to the Draft by the SAR Working Group

 The Working Group welcomes the draft,  a much expected document on the use of effective dose  reflecting an important decision taken by the ICRP to revise and clarify concepts providing further guidance on specific aspects related to conceptual and practical applications in radiological protection following the Recommendations in Publication 103.

The draft remarks the relevance of Effective dose in optimization of protection and to verify compliance, addressing issues focused on the appropriate application in different medical diagnosis and treatment exposures.

A significant change is introduced related to Equivalent dose proposing that it is no longer  required as a protection quantity been replaced by absorbed dose in setting limits for tissue reaction.  It is apparent that a weighting methodology for the absorbed dose should be introduced for tissue reaction  assessment.

The draft reassures Collective Effective Dose as a valuable tool in optimization of occupational protection and remarks the need for careful use discouraging its application as an epidemiological tool or in predicting possible health effect for large population exposed to very low doses over long periods of time avoiding undue expectation on the number of future fatal cases. 

Never the less the WG considers that the use of collective effective dose in screening assessment evaluation and planning  of radiation protection actions are undoubtedly important potential application for which further clarification and guidance is recommended to beg include in the draft.

WG also feels that the writing of the draft should be more adjusted in style and focused as other document releases by ICRP with emphasis to Chap. 5 Medial Exposures.

The table shows comments, most of them of editorial and technical nature proposed by the WG  intended to improve the draft.

German-Swiss Society of Radiation Protection

It seems to be that the only thing that''s new here is the new treatment of the organ equivalent dose? Scientifically, it may be partly right, but the implementation is exaggerated, just because one could confuse the mSv of E with the mSv of H_T. For the user and especially for the layman, it is difficult to understand that we should now use two units, which are (except if w_R¹1) of the numerical value in about the same size. Why the whole thing? Indeed, a difference in numerical value is only where the w_R factor is different from 1, so it can only affect radiological events, e.g. thyroid doses or organ doses after incorporation after an incident. Normally, we are doing well with E, and we need neither the organ equivalent dose nor the absorbed energy. In addition, H_T seems problematic for the lungs unless we have a working dosimetric model.

Summing up, the whole thing is of a very sophisticated scientific level and it is not needed in practice. The definition of the dose quantities have ever been a work of scientists which have never been understood to the full by the practitioners. But this was never a problem for doing radiation protection. The level of occupational doses have reached a very low state. And we do not need to make the system more complicate or change the way it works due to scientific reasons.

Italian Association for Radiation Protection

The draft report appears to be a comprehensive review of the basis for the correct application of the concept effective dose. The report confirms that the system of radiological protection recommended by ICRP is basically sound and possible adjustments suggested by the report are of a rather theoretical nature and do not require an immediate impact on existing legislation and current procedures.

Specific points:

· Paragraph 75. In the formulation of the text, exposures due to naturally occurring radioactive materials in industry seem to be considered existing exposure situations, apart from single decisions to put under control the concerned activities. Actually, the international radiological protection frame regarding NORM activities has been reversed in the last years (IAEA BSS). NORM activities are generally managed as planned exposure situations, taking into account, nevertheless, the occurrence of possible exemption conditions. This perspective has already been adopted in International contexts, including EU BSS (Euratom Directive 2013/59).

· Table 2.5 caption: ‘…cases power per 100 per Gray…’

· Table 2.5 note: ‘…ERR/EAR of 100/0 for thyroid, 30/70 for lung, 0/100 for breast, 50:50 for all others)…’ Notation has to be standardised (in the text – lines 541 and 542 – the colon is used).

· Lines 645-647: Effective dose is a weighted average of equivalent doses to organs and tissues, used as a measure of whole-body dose (see also line 954). This is at variance with ICRP 103, whose definition of effective dose is: The tissue-weighted sum of the equivalent doses…

· In the paragraph 4.1 there are at least two apparent out of topic related to the measurements methods (1301-1306) and to the exposure protection (1372-1373). These aspects are very interesting but should require specific reports or recommendations, here they appear to be improperly placed.

Prepared by AIRP-International Committee

Japan Health Physics Society

GENERAL COMMEMTS

  We can understand the concept of changing the settings of dose limit from equivalent doses to absorbed doses to prevent tissue reaction. However, practical guidance on how to actually use absorbed dose limit to avoid non-cancer effects should be prepared until new main recommendations are issued. It has not been clarified yet how we should use the absorbed dose including RBE, particularly for high-LET radiation. Pub. 118 did not provided updated knowledge on high-LET radiation. For skin and extremities, wR can be used for absorbed dose limits for skin and extremities, because RBEM and RBEm have been compared. However, for the lens of the eye, such comparison has not been made, and RBE will thus be more appropriate.

  The summary of the October 2012 Main Commission meeting states “the use of the special name sievert without proper context can cause confusion, and therefore recommends that the quantity (effective dose or equivalent dose) always be specified. Furthermore, when used for equivalent dose the relevant organ must also be specified”. This statement may be explicitly described in this report, i.e., noting that “when used for absorbed dose the relevant organ must also be specified”.

  This document is a supplementary guideline for the use of effective dose described in Publ.103. So, ICRP should give us a possible scientific evidence for the items mentioned in this document as a reference. For example, in this draft document para. (46), it is mentioned that “for doses in excess of 100mSv delivered at high dose rate, a DDREF of two applied in determining solid cancer risk at low dose /dose rates will not apply”. ICRP should identify the value of high dose rate above which a DDREF of two is not applicable to evaluate radiation risk at an actual field while showing the scientific evidence.

  According to the report of TG84 they mentioned that the lack of a formal quantity for a radiation-weighted dose for high doses was an issue in the current radiological protection system. We think that ICRP should add the comment to this issue in this draft document.

SPECIFIC COMMENTS

Main Points lines 241-242 Section 3.7 (63)

  As described in this draft document (63) the use of committed doses introduces conservatism into calculation of doses from annual intakes for the radionuclide with long half-lives and long retention time, which should be reconsidered to avoid irrational anxiety or excess concern about internal exposure among the public and workers.

Main Points lines 221-223 Section 6 (126) line 2020-2022

  “its use exceptionally in emergency exposure situations at acute doses in the range up to around 1 Sv (or the order of several 100 mSv) is reasonable” ß For managing short term exposure of the Fukushima emergency workers (age >40 with low risk for radiogenic thyroid cancer), the equivalent dose to the thyroid was added to effective dose (in the same way as for the planned exposure situation). There is the possibility that this impeded emergency responses or led emergency workers to increase their anxiety. As such, for management of dose to emergency workers, other index may be considered, taking into account mortality, lethality or severity of health effects.

Section 6 (122) lines 1963-1973

  ICRP is required to clarify the following points in advance. In updating the dose limit associated with non-cancer effects, ICRP should consider the establishment of a factor substituting the radiation weighting factor so that radiation risks can be easily evaluated even in a mixed radiation field.

・Concept of setting the dose limit (how are the effects of high LET radiation and the distribution of tissue doses incorporated in setting the dose limit)

・Evaluation of doses associated with non-cancer effects (a specific way of evaluating doses in the work place exposed to mixed low and high LET radiation)

Section 6 (126) lines 2025-2028

  The paragraph(126) describes the thyroid tissue reaction; if there was a significant contribution to the effective dose from radionuclides concentrated in particular organs (e.g., iodine-131 in the thyroid, inhaled insoluble radionuclides in the lung),tissue damage could occur. Notable for 131I, for example, an effective dose of 250 mSv could correspond to a thyroid dose of >6 Gy. On the other hand, ICRP Publ. 41 notes that if the whole thyroid is exposed to approximately 25～30 Gy x-ray doses in fractions for 30 days, it could result in serious functional damage. It is necessary to clearly describe the effects that may be caused by the difference in the thyroid tissue reaction from Publ. 41 description.

Section 3.7 (64) lines 1209-1213

  Dose coefficient for internal exposure due to radiopharmaceuticals has been provided for age groups, but the control of public or occupational exposures does not include medical exposure. This statement may need to be added.

Section 4.2 (88) lines 1522-1524

  “It is important to balance the reduction in doses with any deleterious effects of the action and a cautious assessment of doses could lead to unnecessary actions with adverse consequences for the affected population.” ß For optimization purposes, a balance between the degree of caution and efforts for dose reduction is important in any exposure situations. To clarify this, the following revision of the sentences is proposed. ”It is important to balance the reduction in doses with any effort or resource to reduce dose in any exposure situation. Particularly in an emergency exposure situation, a too cautious assessment of doses could lead to unnecessary actions with adverse consequences for the affected population.”

Section 5.4 Table 5.2

  It needs to be explicitly stated that this table is only dedicated to risk communications for medical exposures of adults. From the viewpoint of risk communications, it is important to touch on epidemiological observability as was done in Table 1 of Publication 96. In the second column from the left, it needs to be stated that risk at <0.1, 0.1-1, 1-10 mSv is also based on LNT in addition to that at 10-100 mSv. The label for the second column from the right may be changed from “Proposed term for dose level” to “Proposed term for dose level in medical exposure of adults”

Section 1 (3) lines 327-328

  The need to define the concrete level of low dose rate is unclear, and it may be better if only the order of magnitude is described while describing the relevant numbers: e.g.,

Section 1 (6) lines 368-370

  “The use of equivalent dose in setting limits for the avoidance of tissue reactions in the cases of irradiation of the hands and feet, lens of the eye, and skin; that is, limits set below thresholds for the occurrence of acute damage to organs and tissues.” ß “acute” needs to be deleted. Take cataracts and circulatory effects for instance: these thresholds are set for effects occurring at >20 years and >10 years after exposure, respectively.

Section 1 (6) lines 370-372

  “In general, smaller differences in effects per Gy are observed between radiation types in relation to tissue reactions than stochastic effects (ICRP, 2003b).” ß This does not apply to the lens because the lens does not develop tumors.

Section 2.2 (10) lines 440-443

  “However, the available data indicate that differences between radiation types (e.g. alpha particles and neutrons relative to gamma rays) in their effectiveness per Gy in causing tissue reactions are smaller than differences in their effectiveness in relation to cancer induction (ICRP, 1990, 2003b).” ß This does not apply to the lens of the eye, because the lens does not develop tumors.

Section 5.1 (97)

  Dose conversion coefficient for effective dose due to external exposure and dose coefficient for committed effective dose due to internal exposure should distinctively be described.

Section 5.4 (116) lines 1853-1854

  “Depending on the risk projection models used, there are also differences between populations.” ß Risk can differ among populations, but population transfer is used for simplification.

Section 6 (122) lines 1970-1973

  “This change to the use of absorbed dose rather than equivalent dose would not require changes to the numerical values of dose limits for tissue reactions and will be considered by the Commission when new general recommendations are formulated.” ß It is unclear what the Commission will consider upon formulation of the new general recommendations.

Section 6 (125) lines 1997-2000

  “It has been argued that this approach does not adequately protect women and younger children and that differences between males and females and greater risks at younger ages should be reflected more explicitly in the ICRP system, including the use of different detriment values and wT values.” ß Detriment is used only for deriving wT. As such, age-, sex-dependent detriments have no specific functions in the system of radiological protection as indicated in Table A.4.18 of Publication 103.

Section 6 (126) lines 2028-2033

  “A secondary consideration is that for doses in excess of 100 mSv (or more precisely, absorbed doses to organs and tissues > 100 mGy) delivered at high dose rate, the DDREF of 2 applied in determining solid cancer risk at low doses will not apply, so that risks may be somewhat greater than might be assumed on the basis of Publication 103 (ICRP, 2007a) nominal risk coefficients.” ßThe risk posed by acute exposure to >100 mSv can directly be estimated based on epidemiological evidence, rather than with the nominal risk coefficients.

EDITORIAL COMMENTS

Section 2.6 (21) lines 687-688

  “Risk of Exposure-Induced InCidence (REIC)” may be changed to “Risk of Exposure-Induced Cancer”

Section 3.2 (34) line 841

  "clinical radiology” may be changed to “radiology, radiation oncology”.

Section 3.5 Fig 3.2

  "reference phantoms” in the title may be changed to “adult reference phantoms”.

Section 3.6 (54) lines 1085-1087

  “The standard approach to the calculation of skin doses is to determine the average dose to the most exposed 1 cm2 at a depth of 70 μm (ICRP, 1991a, 2007a)” ß The local skin dose is defined by the mean equivalent dose in 0.07 mm depth averaged over any 1 cm2 of the skin, regardless of the area exposed.

Section 3.7 (58) lines 1143-1147

  “the increased importance of the lens of the eye with the reduction in the dose limit to 20 mSv per year (ICRP, 2012a) has led to a re-evaluation of its application (ICRP, 2010a; Bolch et al., 2015).” ß “20 mSv per year” may be changed to “20 mSv/year year, averaged over defined periods of 5 years, with no single year exceeding 50 mSv”.

Korean Association for Radiation Protection

Hak Jae Kim, Professor, Seoul National University Hospital, Korea

The values in Table 5.3 and 5.4 should be re-checked. In Table 5.3, which shows the total lifetime risks for the ICRP Euro-American composite population, there is no significant difference in risk between males and females for the examination of Head (AP+PA+Lat), while the risk is quite predominant in males for the examination of CT Head.

On the other hand, in Table 5.4, which shows the total lifetime risks for the ICRP Asian population, the relationship of risks is reversed; that is, in Table 5.4, the risk is predominant in females with the examination of Head (AP+PA+Lat), while there is no significant difference in risk between males and females for the examination of CT Head.

It should be noted that in Wall et al. (2011, Radiation Risks from Medical X-ray Examinations as a Function of Age and Sex of the Patient. HPA Report HPA-CRCE-028. Chilton), the risk of cancer incidence is predominant in both males and females.

 Jeong In Kim, Researcher, KHNP Radiation Health Institute, Korea

The collective dose should be eliminated or should be recommended with a specific lower bound for dose integration. In the latter case, ICRP should provide the value of the lower bound for integration, preferentially in this report, to address the problem that the collective dose is misused to predict effects on health in large populations, large areas, and over extended time periods.

Please note that paragraph (69) concludes that the estimates of the individual effective dose and collective effective dose can be used to optimize the protection. However, the necessity of collective effective dose is not clearly demonstrated in the report. It is believed that the distribution of individual effective dose values provides enough information to optimize protection.

Paragraph (92) argues the usefulness of collective effective dose in situations of severe nuclear accidents. In this case, the collective effective dose could be used with a lower bound for integration. However, again, the value of the lower bound should be provided by ICRP.

Romanian Society for Radiological Protection

1.- We have nothing against to the idea that effective dose can be used as a rough indicator of risk in some circumstances. Many reasons for estimation of the effective dose as a "rough indicator" are included in this draft report itself.

2.- Regarding the proposal to stop the use of equivalent dose as an important radiological protection quantity we have some doubts. All we know that any organ/tissues radiation effect is strongly determined by the local absorbed dose and on the LET of the radiation involved. Particularly, in the very frequent medical situation of  non-uniform spatial distribution of a mixed radiation field , the use of dose limits expressed in equivalent dose may be more appropriate to be "continued", than to be "discontinued". We may agree only on that the use as a distinct protection quantity is not required, but its use still is essential in individual monitoring, external dosimetry of lens of eye, skin and extremities and in internal dosimetry.

3.- In chapter 5 regarding MEDICAL EXPOSURE, the given examples for supporting effective dose as an important protection quantity are valid only in diagnostic radiology practice. Particularly, in nuclear medicine and in radiotherapy the equivalent dose in specific organ/tissue may be more significant than the effective dose. Even in diagnostic radiology, for optimisation of radiation protection of an individual patient, the use of DRLs in terms of measurable quantities is  more effective than the proposed effective dose.

3.- We fully agree that:

- Several confusions between the equivalent dose and efective dose arised from the use of a same name for the unit  (Sievert).  This may be simply solved by given (by ICRU/ICRP) of a separate name to one of them. Additionally, we may add confusions between the equivalent dose and the operational quantity dose equivalent, and that between "reference" person and "representative" person.

- There is a real need to develop in the future appropriate tissue weighting factors depending on age, sex and cancer type.

- The prudent approach in the draft report for some declared assumptions in the context of practical implementation of the radiological protection system, particularly regarding the use of collective dose in the context of risk projection should be appreciated, but we consider that no important changes are presented here, in comparison with the recommendations already available in ICRP the 103. Also, the same  known  statements from ICRP 103 are unuseful repeated in  several chapters of the  draft report and this should be avoided in the final version.

Constantin MILU, Ph.D.

RSRP President

Bucharest, 13 July 2018

Spanish Radiation Protection Association

UK Society for Radiological Protection

SRP believes that the proposed changes in the ICRP document are reasonable and pragmatic from an occupational dosimetry perspective.

One issue that has not been addressed is the retrospective calculation of organ/tissue ‘doses’ for compensation scheme / litigation purposes (and possibly also relevant for epidemiology studies).  In such cases the end point is often to derive a dose relationship to a diagnosed cancer in a specific organ/tissue; in such cases the use of Absorbed Dose would not be appropriate, and Equivalent Dose (or some similar quantity which retains a radiation-weighting factor) would still be more appropriate (ie the incidence of biological/clinical effect is of a stochastic from rather than deterministic).

SRP note that some believe that over the years ICRP has sometimes been guilty of change for change sake.  With average annual occupational doses being steadily reduced (typically less than 2 mSv) is a change to Gray really necessary and might it undermine public confidence in the system of radiological protection?

The main advantages of the use of effective dose and Sieverts were that it allowed the addition of assessed doses from various sources eg internal, external and individual organ doses in some sort of logical fashion, and that it allowed some account to be taken of risk and lifetime risk.  The main disadvantage being that you can''t measure Sieverts directly in metrology terms.

 There is of course always the option for regulatory authorities to continue to set dose limits in Sieverts if ICRP choose to recommend constraints or similar in Gray?

With regard to eye lens SRP notes that the cause and effect to the eye lens has to be primarily a Deterministic effect. Although arguments are made to apply dose as Stochastic for the eye lens this is primarily based on use or application to Occupational Exposure. ICRP should make this argument transparent. This would help with control for occupational dose limits as now applied in Legislation for Work with Ionising Radiation. Noting there is no weighting factor for the eye lens.  ICRP then need to comment on a difference between occupational exposure and emergency exposures in relation to setting dose limits and the appropriate quantity to use in both situations. Clearly should there be an emergency exposure (to an operator, responder or member of the public) this is most likely a one off lifetime occurrence (unlike that for occupational exposure).  ICRP would help to comment on appropriate dose quantity and limits to use for the eye lens in an emergency to differentiate between what are two different types of situations and consequences to those exposed to ionising radiations where the dose to the eye lens could be used for both Deterministic and Stochastic parameters.

We note the detailed additional comments from Sellafied Limited and our member Graham Smith.

Additionally we endorse the comments of the IRPA.