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Category: Radiation Fundamentals — Doses and Dose Calculations

The following question was answered by an expert in the appropriate field:

Q

Is the concept of effective dose accurate? For example 80 mSv to the lungs carries with it the same risk as 10 mSv to the whole body?

A

The short answer to your question is "Yes, we believe the concept of effective dose provides an acceptably accurate estimate of risk-related dose, even though it is subject to considerable uncertainty with respect to specific individual doses."

While there is much we might say about the degree of accuracy that might be represented by a given evaluated value of effective dose, I believe the critical point to bear in mind is that the quantity effective dose is intended for use as a reasonable tool for implementing and judging the effectiveness of radiation protection programs in terms of ensuring that workers are not subjected to excessive levels of dose and associated risk. The types of effects that are included in the effective dose estimation are so-called stochastic effects – those whose probability of occurrence are assumed to be proportional to dose (effects such as fatal and nonfatal cancer, genetic effects, and life span shortening fall in this category). I shall attempt to provide some clarification regarding the intended use of effective dose and to provide some consideration as to your concern with accuracy.

The quantity effective dose, as you know, represents the summation over all significantly irradiated tissues/organs of the products of the individual tissue absorbed dose, the radiation weighting factor, and the tissue weighting factor. The tissue weighting factor for a given tissue represents the fractional risk of significant stochastic effects relative to what would be experienced if all tissues of the body were irradiated equally. Thus, the sum of all the weighting factors for all tissues considered in establishing limits is 1.00 and a tissue weighting factor for the lungs of 0.12 implies that when the entire body is irradiated uniformly, the stochastic-based risk to the lungs represents 12% of the total stochastic-based risk to all tissues. Using this system, it then is possible to judge the significance of a given dose. In this light, it is appropriate to say that, from a radiation protection perspective, yes, the 10 mSv whole-body dose that you cite is equivalent to the 80 mSv lung dose in terms of potential stochastic-based risk—i.e., (80 mSv)(0.12) ˜ 10 mSv. We should note that the assumption that is made of uniform irradiation of the whole body is often not a reality in many radiation fields to which workers are exposed, and this is one source of uncertainty in effective dose estimations.

Regarding the accuracy of any individual determination, we must further recognize that the basis for the estimation of tissue doses and the modifying tissue weighting factors rely on a number of assumptions. The first is that the tissue weighting factors themselves are relative risk factors based on epidemiological evidence from large populations of persons exposed to significant radiation doses. The effective average values generated from this process may, in fact, be different from the actual weighting factors that apply to any specific individual; there is nothing we can do to account for such possible differences. Additionally, the estimation of effective dose relies on evaluations that have been made for an assumed characteristic individual with a particular body configuration, who is a mixture of both sexes and has no specific age. The actual worker may have many deviations from the assumed reference phantom used in establishing standards. He or she may be much different in size and shape, his or her organs and selected tissues may have different sizes and positions in the body compared to the reference phantom, individual radiation sensitivity may be different, etc. All of these considerations may result in estimates of effective dose to an individual that may be in considerable error.

The system we use of accepting a single model as representative of every radiation worker has the great advantage that we employ the same system for all, and this makes for easy comparisons among workers and with dose limits for purposes of judging compliance with legal and recommended dose values. This comes at the possible expense of accuracy in the dose assessment for any given individual. When doses are near to or in excess of limits, we may opt to attempt more sophisticated analyses to get a more realistic estimate of the actual effective dose for an individual. This may be of more concern in certain legal actions in which a radiation worker may be associating dose(s) received with particular physical ailments that he or she is experiencing. Such individual dose evaluations may require performing extensive and expensive studies possibly involving such things as physical mockups of the exposure scenario, more extensive measurements, and possibly detailed Monte Carlo simulations to estimate the actual absorbed doses to the significant tissues of the involved individual to better establish the effective dose.

It would be impractical and probably not very useful, from the point of view of improving radiation safety, to require such an approach for every exposed individual. There are often exposure conditions that we would not adequately be able to simulate if we attempted to take account of variations in exposure conditions. Things such as the orientation of the worker in the radiation fields to which he/she was exposed, the energy distributions and directionality of radiations involved, and the intensities of specific radiation fields are often not readily determined; such is especially true for routine external exposures that may occur over an extended period such as one month, a common wear interval for external radiation dosimeters by workers who may be exposed in multiple radiation fields.

The dose limits we presently used are believed to have sufficient conservatism built into them to allow for the individual body variations that are a reality among radiation workers (as well as among members of the public) and for the uncertainties in the radiation field properties that affect interpreted dose without presenting unacceptable excess risks to exposed individuals who function within those limits.

So, yes, there may be considerable uncertainties/inaccuracies in estimated values of effective dose, but the principles invoked in applying the effective dose limitation methodology are sound, and provide our current best tool for providing a reasonable and appropriately safe dose-restricting system for use in radiation protection.