Towards the ICRP's new recommendations - problems in dosimetry

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Abstract
Dear Sir I was most interested to read in the June issue of the Journal the latest ICRP Memorandum [1] reporting on the progress made in new thinking since the publication of a proposal for possible changes to the system of protection, put forward by Roger Clarke in 1999 [2]. In going from a population-based to an individual-based philosophy, a number of fundamental issues arise, such as the implications of the abandonment of the concept of collective dose [3]. Clearly, there is some way to go, but here I should like to draw particular attention to the underlying questions concerning dosimetry. In his invited editorial, also published in the June issue [4], although pleading for minimal changes generally (`If it ain't broke, don't fix it'), Geoffrey Webb refers to `sorting out the dosimetry quantities' as `one of the things that need to be done'. Indeed, the ICRP Memorandum itself confirms that this issue `causes concern'. In the Memorandum, mention is made of the `views of metrologists' who, it is stated, have different objectives from those of ICRP. Essentially, metrologists `correctly provided an unequivocal definition of the absorbed dose at a point', which is measurable, whereas ICRP's protection quantities are concerned with the `average tissue dose', which is not. It is stated that in the next recommendations `it will be necessary to clarify the difference' between these two types of quantities. Indeed, the difference between point quantities, including dose equivalent and the operational quantities such as ambient dose equivalent, and mean value quantities such as equivalent dose and effective dose, is discussed in detail in Report 51 of the International Commission on Radiation Units and Measurements entitled `Quantities and Units in Radiation Protection Dosimetry' [5]. It was on the grounds that in radiation protection `complexity is not justified', and with the laudable aim of simplifying proceedings, that the ICRP introduced the above named mean-value quantities for limiting purposes [6]. However, as these quantities were not subject to `conventional' metrology, and there was still a need for measurement, the ICRP introduced them in parallel with the ICRU point quantities, resulting in the present dual approach. This unsatisfactory situation was compounded by the confusion in terminology, e.g. dose equivalent and equivalent dose. In the ICRP Memorandum [1], mention is made of the possibility of redefining the protection quantities, albeit with `simpler' weighting factors. It should be recalled that it was the introduction by the ICRP in 1991 of simpler radiation weighting factors, applicable to incident beams only irrespective of the size, shape or composition of the medium concerned, which gave rise to some of the present problems. If, as hoped, the `persistent difference of view' between the respective approaches is `to be removed in the next recommendations', it would seem that close cooperation between the two commissions is not only desirable, but essential. The joint objective should be to develop a system of dosimetry which is compatible with the needs of the known biological effects of radiation, hopefully both simpler and meaningful (therein lies the `root of the conflict'), subject to conventional metrology, by the year 2005 as proposed. Yours faithfully,