Re: ICRP XXX (2012) Occupational Intakes of Radionuclides
Dear collegues,
this is a proposal for two corrections of the terminology used in the draft ICRP report XXX (2012) on Occupational Intakes of Radionuclides. The proposal comes from the members of the German Committee of Radiological Standards (Normenausschuss Radiologie, NAR), Dr. Ambrosi, Dr. Kapsch, Prof. Kramer and myself, and is supported by Prof. Breckow, president of the German-Swiss Society of Radiation Protection (Fachverband für Strahlenschutz), and by Prof. Dietze, member of ICRP Committee 2. We express our high estimation of the ICRP and want to give our advice in the tradition of friendly relationships that has always guided the German members of the ICRP main commission, subcommittees and task groups.
Our concern is the long-term stability of ICRP terminology in an important definition of radiation protection. We are referring to the basic definition
(formula - see ICRP 60 pg, ICRP 103 p 23) (1)
(ICRP 60, page 5; ICRP 103, page 23) which expresses that the "equivalent dose in a tissue or organ, HT" is the weighted sum of the mean absorbed doses DT,R in the tissue or organ T, with radiation weighting factor wR. This definition, introduced in ICRP 60 and maintained in ICRP 103, has been entered into international legislation, standards and textbooks, and it has achieved particular importance by its worldwide use to define the effective dose, the wT-weighted sum of the equivalent doses to the tissues or organs, the quantity in terms of which the fundamental radiation protection dose limits for workers, the population and emergency cases are expressed. However, in the draft ICRP XXX (2012) on Occupational Intakes of Radionuclides, this definition has been changed to read
(formula - see draft report, page 11) (2)
(ICRP XXX, page 11) where rT is a "target region" in the Reference Adult Male or Female used for dose calculations by application of the absorbed fraction concept, or a region in a voxel phantom applied to calculate equivalent doses from external exposure. Moreover, H(rT) is here sometimes called "equivalent dose", while the specifier "to a tissue or organ" is lost.
a) Proposal concerning the maintenance of the symbol HT: We acknowledge that the symbol rT is an instrument of transparency, declaring what has really been done, namely calculations in virtual phantoms substituting the human body. However - as we will see in a moment - this declaration can be performed by other means. On the other hand, there is a considerable disadvantage of writing the definition of HT in the way of eq. (2) instead of eq. (1), namely the loss of stability of ICRP terminology. To provide long-term reliability, throughout the development of radiation protection recommendations, has been an approved concept for many years of ICRP work, and we are convinced that ICRP members have already noticed the conflict of the new eq. (2) with this concept. Furthermore in the form of eq. (2), the notation deviates from the convention of writing mathematical relationships because a) rT has been written in the bracket behind DR, which in normal writing of functions would be the place for variables such as the space or time coordinates and not for a topographic symbol, b) the index T has lost its conventional position as a subscript of DT,R.
The undesirable effects that would follow ICRP's change from the previously recommended eq. (1) towards the recommendation of eq. (2), namely a) of giving the impression of some instability of the ICRP definitions, b) the need of rewriting this equation in numerous laws, regulations, textbooks and public informations and c) the need to prohibit the foreseeable misunderstandings following from this change, could be avoided by positioning the transition from the rT to the T notation at an earlier stage in the evolution of the ICRP XXX (2012) equations, namely at the key equation
(formula - see draft report, page 98) (3)
(ICRP XXX, page 98) which expresses , the time-dependent equivalent dose rate in an organ or tissue T, by A(rs,t), the time-dependent activity in the source regionr rS, and S(rT←rS), the conversion factor based on the absorbed fraction in the target region rT and other factors including wR. Eq. (3) would then describe the typical approach of committed organ equivalent dose rate calculations, and in all equations following from eq. (3) - starting with the expression for the committed equivalent dose HT(TD) as the time integral of - the index T could then be applied in the hitherto conventional way. The advantage of introducing index T already at this stage is that eq. (3) is adressing the specialists using it, who understand the details and who will have anyway to anticipate formal changes in the present stage at which the ICRP and MIRD terminologies are merged, whereas eq. (1) is for international use in laws, regulations and textbooks, i.e. for a worldwide readership.
b) Proposal concerning the maintenance of the specifier "to a tissue or organ" in the name of quantity HT: The importance of stating the name of the organ or tissue for which values of quantity HT are communicated is well known and has recently been confirmed when reports about committed doses to the thyroid expected to occur in the aftermath of the Fukushima accident could not be deciphered because the name of the organ was not communicated (Report of ICRP TG 84). We are therefore advising against the tendency to call HT simply the "equivalent dose", thereby omitting the specifier "to a tissue or organ", which has happened not always, but in the list of terms of the draft ICRP report XXX (2012), lines 341 and 342. Furthermore we are convinced, that experienced members of the ICRP will feel that using the term "equivalent dose" without the specifier "to a tissue or organ" would bring the conflict with ICRU's "dose equivalent" to culmination and cannot do any good in worldwide radiation protection, where is trust and hope that this lack of coordination will soon have an end.
In summary, our proposals to ICRP are a) to continue recommending the long-standing definition of the "equivalent dose to a tissue or organ" in the form of eq. (1) and to write eq. (3) in the indicated way, thereby introducing the term HT already at the technical stage of the sequence of formulae, and b) to maintain the specifier "to a tissue or organ" for the sake of clarity. Hoping on your understanding and with all good wishes for the success of ICRP,
yours sincerely
Dietrich Harder
Former member of the ICRU,
Former chairman of the German Radiation Protection Commission.