Answer to Question #13504 Submitted to "Ask the Experts"

What is the standard uncertainty associated with the dose conversion factor at 0.6 MeV in, for example, Table A.1 of International Commission on Radiological Protection (ICRP) Publication 116? I have found values in ICRP Publication 51, ICRP Publication 21, and American National Standards Institute (ANSI) ANS 6.1.1 at this energy that differ (high to low) by as much as 40%. None of these documents indicate anywhere (that I could find) the usual +/- X found in scientific publications.

Your question is a good one, especially because it forces us to bring into focus the concepts and assumptions about the meaning and utility of the quantity, effective dose. I will say up front, consistent with ICRP’s recommendations, that there is zero uncertainty associated with the effective dose conversion constants that are presented in Table A.1 that you cite. The quantity, effective dose, as well as conversion constants, represent reference levels, and the pertinent statement from ICRP in Publication 116 reads as follows: "Dose limits, dose constraints, and reference levels specified in terms of effective dose serve a role in the contractual relationship between workers and the regulated licensee, as well as between licensees and the public within a regulatory framework. In this role, the coefficients are used in the process of assigning values of effective dose to individuals for a given exposure, and they have no uncertainties for this process."

To make sense of the rationale used we must recognize that the effective dose quantity was devised only for application in the field of radiation protection. We use effective dose limits or restrictions as a basis for judging whether our operations are being conducted in a manner sufficient to offer a high likelihood of adequate safety to workers and others who might be exposed to radiation as a consequence of our operations. The tissue weighting factors and the radiation weighting factors used in estimations of effective dose are independent of both sex and age, and no account is taken of personal physical variations of the individual from the reference computational phantoms that were used to determine the relationship between the effective dose and the irradiation conditions. As such, when a specific value of effective dose is calculated for assignment to an individual, it may be used for purposes of judging radiation protection effectiveness, but it must not and cannot be used to attempt to make any estimation of radiation-induced risk or potential health detriment to that individual.

Having said this, clearly the ICRP recognizes that there are computational uncertainties that accrue throughout the estimation of doses to individual tissues that are needed for the determination of effective dose. Many of these are discussed and estimates of their magnitudes presented in the document being considered. If you are interested in reviewing these, I suggest you do a search and find on the document using the word "uncertainties." You should find 40 instances of the word "uncertainties" and many will be related to estimations of the statistical uncertainty associated with the simulations. For example, for photon irradiations discussed in Section 4.1.2 the authors note that for large organs the relative statistical uncertainties were on the order of 0.2%, but for small organs the values were more like 2%, and for small organs at photon energies less than about 20 keV the uncertainties were on the order of 10%. For some other radiation types some statistical uncertainties were considerably greater.

Also recognized by ICRP, but not explicitly dealt with in the report, at least from a quantitative standpoint, is the fact that both tissue doses and effective dose to a specific individual are subject to much greater uncertainties than the simple statistical uncertainties associated with the Monte Carlo simulations, if one is considering actual doses in comparison to reference doses calculated from the use of reference phantoms under specified irradiation conditions. In the field of routine radiation protection, we do not typically attempt to account for such differences. However, if one were dealing with a case in which a high dose was received, and it became important to make as valid an estimate as possible of radiation health risk to the individual, possibly both stochastic and nonstochastic, then it might be necessary to perform more specific analyses. These could possibly involve simulations and related calculations, as well as physical mockups of the exposure scenario, to better account for the individual's personal characteristics as well as the actual field exposure conditions, such as orientation of the individual in the field. Depending on how such considerations differed from those used for establishing reference levels, large tissue and effective dose differences between the two could result.

Regarding your concern about differences in simulation results achieved by the same or different groups at different times, there are numerous factors that contribute to these. Among the most significant are changes in the sophistication of the computational phantoms used in the simulations; changes in physical data, such as tissue and/or radiation weighting factors, and interaction coefficients, and changes in the biophysical models used that affect distribution (this is especially true for internal uptakes of radionuclides). This may not be very satisfying since we are saying that a quantity that we specify by its definition as a reference quantity as having no uncertainty is subject to possible significant change as our ability to determine its magnitude improves with our growing knowledge base and improvements in technology. Of course, this is how much of science evolves. In general, we build enough conservatism into the values of important quantities, such as dose restrictions on workers and members of the public, so that future changes that might be warranted are not likely to reflect appreciable past significant harm or risk to individuals exposed under the assumption of the validity of earlier reference quantity values.

I hope this is helpful to you.

George Chabot, PhD, CHP