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The author is uncompromising in his criticisms of the quantity "effective dose" and proposes its replacement with a quantity termed "effective risk". The uninformed use and misapplication of effective dose is a recognised problem [1–4]. However, used as intended within the protection system recommended by the International Commission on Radiological Protection (ICRP) [1, 5], it provides a risk-related quantity for the control of exposures and the optimisation of protection. In response to the article by Brenner [6], we would like to take the opportunity to explain the purpose of effective dose and answer some specific criticisms. We agree with Brenner to the extent that an alternative approach is required in situations for which effective dose was not intended, but conclude that risk calculations should be made for clearly defined purposes, not to provide a general replacement scheme.
The ICRP protection quantities — equivalent and effective dose — enable the summation of doses from internal emitters and from external sources to provide a single number for comparison with dose limits, constraints and reference levels that relate to stochastic risks of whole-body radiation exposure.
Thus, the primary application of effective dose is in the planning and demonstration of compliance in various situations of exposure of workers and members of the public. Doses are calculated using defined dosimetric models, including reference anatomical data for the organs and tissues of the human body. Hence, effective dose applies to a reference person and is not intended to provide a measure of risk to individuals. Radiation-weighting factors are used to adjust for the different effectiveness of different radiation types (e.g. photons, neutrons and 
particles) per unit absorbed dose (Gy) in causing stochastic effects at low doses and dose rates. Tissue-weighting factors are used to take account of the contribution of individual organs and tissues to the overall detriment from cancer and hereditary effects, providing a simple set of rounded values chosen on the basis of age- and sex-averaged values of relative detriment. While the definition of absorbed dose has the scientific rigour required of a basic physical quantity, the same is not true of the ICRP protection quantities — equivalent and effective dose (i.e. quantities measured in sieverts (Sv)). The ICRP quantities are intended for practical application in radiological protection and the choice of radiation- and tissue-weighting factors used in their calculation involves simplifying assumptions regarded as acceptable only for this purpose. Effective dose is established worldwide as the principal quantity used in the regulatory control of radiation exposures.
Some of the specific criticisms of effective dose made by Brenner concern (i) the ICRP choice of tissue-weighting factors (wT values), (ii) the lack of age-dependence in wT values, (iii) the inclusion of hereditary effects with cancer in the choice of wT values, and (iv) confusion between absorbed, equivalent and effective dose.
Tissue-weighting factors
Effective dose is defined with a single set of tissue-weighting factors based on estimates of the different radiosensitivities of the organs and tissues of the human body in terms of stochastic risks at low doses. New wT values have been given by ICRP in the 2007 recommendations to replace those given in the 1990 recommendations. A detailed explanation of the derivation of wT values is given in Annex A of the new ICRP recommendations [1]. The main source of data on cancer risks is the follow-up studies of the Japanese atomic bomb survivors, used to derive risk coefficients averaged over seven Western and Asian populations with different background cancer rates [1]. The new wT values are based on more reliable cancer incidence data rather than fatality data, and are adjusted for lethality, loss of quality of life and years of life lost. Weighting for hereditary effects is now based on a more detailed analysis, estimating disease in the first two generations rather than at theoretical equilibrium. The main changes in wT values in the new recommendations are an increase for breast (from 0.05 to 0.12), a decrease for gonads (from 0.2 to 0.08) and inclusion of more organs and tissues in a larger "remainder" (from 0.05 to 0.12). In our opinion, it is not a valid criticism that weighting factors change every decade or so; on the contrary, the ICRP would be open to criticism if relevant scientific advances were not taken into account.
Age at exposure
A single set of wT values is used in the calculation of effective dose, although there is good evidence that risks are generally greater at younger ages and there are also differences between males and females [1, 7, 8]. Similarly, the radiation-weighting factors used by the ICRP do not take account of some known differences between radiation types [4, 9]. These approximations are considered to be acceptable for the purposes of practical radiation protection in which estimates of effective dose are compared with limits, constraints, etc., which distinguish between situations of exposure but not between men, women and children.
As discussed by the ICRP [1], the use of effective dose in medical applications may be compromised if, for example, comparisons are being made between procedures for which the age distribution of the patients is substantially different. It may be that, for some applications, it would be most appropriate to calculate the risks for specific age groups, using absorbed or equivalent doses to organs and tissues and age-related risk factors such as those tabulated by Brenner. This is consistent with ICRP [1] advice, not as a replacement for effective dose but as an alternative approach for specific circumstances of medical exposure. However, such risk estimates cannot be regarded as precise, particularly at low levels of exposure.
Risk coefficients based on cancer incidence only
Brenner proposes that tissue-specific risk factors should relate to cancer incidence only. While recent assessments of radiation-induced hereditary risk [1, 10] are substantially lower than previous estimates [5], we see no reason to discount hereditary disease in the protection system. We believe that this would be a backward step that the public and radiation protection professionals would not understand.
The proposal to restrict consideration of cancer risk to cancer incidence also seems unacceptable. It ignores large differences in the success rates for treatment of different cancer types. For example, the wT value for skin would be much larger on the basis of incidence alone, skewing its relative importance. Future developments in cancer treatment are likely to impact further on wT values. An open question on the basis of current epidemiological and experimental evidence is whether future changes in risk estimates and wT values will need also to take account of non-cancer effects such as cardiovascular disease [8].
Confusion between quantities
We agree that equivalent dose and effective dose have often been misused and not clearly distinguished. However, confusion between quantities is an inevitable consequence of specifying only units without also referring to the quantity being used. This problem cannot be solved by applying a different unit to each quantity. The special name, sievert (Sv), is used for the unit joule kg–1 for all operational dose quantities for external exposure, for effective dose and for equivalent doses in more than 20 different organs and tissues [1]. This is not a unique situation in physics and the only solution to avoid confusion is to always specify the quantity being used.
An important argument against the "effective risk" quantity proposed by Brenner concerns the dose (risk?) range to which the new quantity would principally be applied. In radiation protection practice, the effective doses recorded are mostly far below the main limit of 20 mSv per year for occupationally exposed persons, often down to 10 µSv or even less. Dose measurements and assessment are well established in this dose range and, particularly for external exposures, quality assurance programmes guarantee reliable measurements even at such low doses. In addition, the operational dose quantities defined for external exposure and used for calibrating dosemeters are taken to provide sufficiently accurate assessments of effective dose for the purposes of radiation monitoring and dose recording. However, while doses can be measured with some precision at such low levels, the corresponding risks are uncertain or even unknown.
The protection system and the calculation of effective dose are reliant on the assumption of a linear non-threshold (LNT) relationship between doses and stochastic risks. The LNT assumption is implicit in the addition of external and internal doses of different magnitudes, with different temporal and spatial patterns of delivery. However, a clear distinction should be drawn between the necessary and justifiable reliance on LNT for protection purposes and its scientific validity extending to very low doses and all stochastic risks. The LNT dose response remains controversial, with arguments for supra-linear low-dose responses and for thresholds and/or hormetic effects [1, 11–14]. The ICRP [1] concludes that the true validity of the LNT model may prove to be beyond definitive resolution for the foreseeable future. Hence, individual "risk monitoring" at low doses would be, at best, misleading.
While the calculation of risks cannot be regarded as a sensible substitute for effective dose for the majority of radiation protection applications, there are situations in which risk estimation is appropriate. Such situations include, for example, the assessment of doses received by astronauts and the assessment of risks to individual patients from medical imaging procedures. Furthermore, as discussed in ICRP recommendations [1, 5], assessments of an exposure received by an individual worker, as might be required if dose limits are exceeded, will always need to take account of all available information to provide best estimates of risk. Similarly, estimates of risk to population groups should properly be based on the best available data. The new ICRP recommendations [1] provide an explanation of the intended application of effective dose. Further guidance will be provided in a forthcoming ICRP report that will also discuss approaches to assessments in situations, including medical applications, for which effective dose was not intended.
Yours etc.,
Günther Dietze, Paracelsusstr. 7, D-38116 Braunschweig, Germany, E-mail: guenther.dietze{at}t-online.de
Correspondence: John Harrison, HPA, RPD, Chilton, Didcot, Oxon OX11 0RQ, UK. E-mail: gill.fisher{at}hpa.org.uk
Footnotes
Chairman1 and members of Committee 2 of the International Commission on Radiological Protection) 
Received for publication October 31, 2008. Accepted for publication November 11, 2008.
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