Recent controversy in radiation dosimetry

Abstract

The correspondence between Drs. Macleod and Kemp [1] and Drs. Johansson et al. [2] in this issue is an example of the controversy that has arisen over use of the quantities effective dose equivalent (EDE) [3] and effective dose (E) [4]. Johansson et al. correctly point out the interpretations of these quantities, as well as their intended applications. The EDE was originally defined for use in radiation protection programs for situations in which nonstochastic effects do not occur. The adaptation to medical uses of radiation, including radiology and nuclear medicine, has been made by authors other than Johansson et al. [5, 6, 7] and advocated by the ICRP in ICRP Publication 53 [8]. Nonetheless, some people remain opposed to the use of the unit in medicine. The traditional approach to the characterization of radiation dosimetry in medical uses of radiation has been to report individual organ absorbed doses and a quantity called "total body" (or "whole body ") dose. Thus, radiation use committees, regulatory agencies, physicians, manufacturers, and others have become focused on calculation of "critical organ" dose and "total body" dose. As noted by Johansson et al., one accepted way to evaluate a patient's risk from stochastic effects would be to consider the average doses and radiation sensitivities of all organs. In principle, the EDE permits this evaluation to be made for different radiopharmaceuticals, as well as different modalities of ionizing radiation exposure, with the results being directly comparable. Furthermore, the EDE is designed to represent the uniform wholebody exposure that would carry the same risk as the actual nonuniform exposure received. The quantity "total body" dose, on the other hand, is usually calculated by dividing all of the energy absorbed in all tissues from a given exposure by the total body mass. This quantity is completely meaningless when a nonuniform distribution of radioactivity occurs. "Total body" dose is clearly inappropriate for evaluating the risk from nuclear medicine examinations and should not be the basis for decision-making or comparison of agents or procedures. The question becomes whether an effective dose equivalent or effective dose can be a suitable substitute for "total body" dose (or, more appropriately, "total body" dose equivalent). The effective dose contains by definition two sets of so-called "weighting factors," which are introduced through multiplication. The first are the "radiation weighting factors" (formerly quality factors), which express the relative effectiveness of different types of radiations at producing biological damage. Multiplication of a tissue absorbed dose (DT,R) (Gy, mGy, rad, etc.) by the appropriate radiation weighting factor (WR) (including all radiations) produces the tissue equivalent dose (HT) (formerly called dose equivalent) (Sv, mSv, rem, etc.):