Review of draft document The Use of Effective Dose as a Radiological Protection Quantity
Submitted by Chris J. Huyskens as in individual
Part A; about effective dose
The draft document gives, at various places, descriptions of the quantity effective dose, however not always precise a/o consistent in wording. For better understanding, I advise to clarifying the original conceptual intention of the construct effective dose as a virtual total-body-dose: it reflects the weighted mean dose equivalent for total body. This became obscure since abandoning the initially given name ‘effective dose equivalent’.
Effective dose is a virtual total-body-dose quantity that accounts for the dose distribution within the body in conjunction with the stochastic health detriment resulting from exposure to ionizing radiation. The concept is applicable for total body and partial body exposures due to external radiation sources as well as from intakes of radionuclides.
The assessment of effective dose is based on calculations, which incorporate gender- and age averaging, and include numerical conventions on tissue-weighting factors for a ‘hypothetical referent individual’
The principle application of effective dose is for the purposes of radiological protection: particularly for (1) planning and optimization of protection and for (2) demonstration of compliance with standards and regulations for ALARA & dose limitation.
[ad-1] The concept provides a useful yardstick for evaluation and inter comparison of the dose consequences resulting from various types of radiation, from various radiation sources, in various exposure conditions and exposure geometries.
[ad-2] The concept is an intrinsic additive quantity because it refers to a single receptor (total body) and therefore allows for addition of separate contributions from various origin & sources, either simultaneously or subsequent in time and place.
NB. Incidentally in the text, as in previous ICRP publications, the method for calculating effective dose is wrongly explained as a summation of doses over different organs/tissues; this gives the false impression of mutual additivity of doses in various organs/tissues.
As was clearly identified by TG 84 [Gonzalez ea, 2011] gross difficulties in communicating radiological information to non-expert decision makers & press & public originate from misunderstanding, misinterpretation or even misuse of the wisdom in ICRP publications.
A particular problem in this respect is that quite many caveats against unintended use of ICRP concepts and quantities were insufficiently clarified in respective recommendations, or hidden in the texts so that they are easily missed -as in the fine print clauses of insurance policy-. The draft documents seems to suffer from the same flaw and needs improvement in editing so that the important caveats against misuse of effective dose will clearly be addressed and explained. Clear guidance with supportive arguments is needed about the use and non-use of effective dose including that it:
- is a rough indicator for radiation dose; numerical values cannot imply high precision.
- is based on rounded figures for weighting factors for radiation quality & organ weighting factors.
- is not intended for assessment of health detriment for specific individuals or a-typical sub-populations.
- is not to be used for epidemiological evaluations,
- is not a predictor nor an indicator for individual cancer risk from radiation*
- is not a directly measurable dose quantity
- is not defined for the high dose range where deterministic tissue effects are predominant.
* NB. I express specific criticism against the proposed tolerance -for the sake of pragmatism-.
[lines 267-297] to open the door for the use as an approximate indicator for possible risk.
Part B; about replacement of equivalent dose by absorbed dose for dose limitation
From conceptual viewpoint it would be correct and understandable to discontinue the use of the quantity equivalent dose for defining individual dose limits for separate organs/tissue in occupational and public exposures in planned exposure situations. The quantity absorbed dose for an organ/tissue would serve just as good or sometimes better for this purpose and is conceptually correct because tissue reactions relate directly to the relevant absorbed dose, provided that the relevant RBE is known.
However, the draft document seems to suggest (as in some review comments) to entirely abandon the use of equivalent dose. This seems to me an ad hoc proposal and ill-considered. It gives me a flavor of ‘throwing away the baby with the bathwater’. In brief, I emphasize that the concept of equivalent dose in an organ or tissue (i) incorporates the radiation weighting factor as an approximation different relative radiobiological effectiveness for different types of ionizing radiations and (ii) holds a key role in the definition of the effective dose.
Part C; avoid misunderstanding of reported dose values
Experience in the aftermath of the Chernobyl and Fukushima reactor accidents revealed great difficulties in communicating radiological dose information to non-expert decision makers and press and the public. Misunderstandings about the quantities and units for reported doses lead to untoward difficulties, incorrect interpretations of potential consequences and incorrect decisions, especially in the hectic situations.
Remedies for improving the situation in the short term are:
- avoid reporting on dose values and numbers, without clear statement of the dose quantity and units;
- avoid the use of equivalent dose and absorbed dose without specifying the organ or tissue concerned;
- promote and foster the use of ‘millisievert’ [mSv] as the preferred and most practical unit for reporting values for effective dose;
- avoid the use of small sub-units [µSv & nSv] in combination with meaningless impressive big numbers;
- introduction of a special unit name for effective dose might be helpful.
NB. The editing of the draft is far from optimal. Some topics suffer from repetition at different places in the text, be it here and there in inconsistent wording.
NB. In particular, chapter 5 will benefit from scrutinizing of the medical terminology.
Please note my support for the review submitted on behalf of American Association of Physicists in Medicine regarding chapter 5.
Moreover, in the spirit of Cato Major, I am of the opinion that
1) the symbol E for effective dose must be destroyed.
2) values for effective dose should always an only be expressed in the unit millisievert.