Effective dose: a flawed concept that could and should be replaced. Comments on a paper by D J Brenner (Br J Radiol 2008;81:521-3).

The effective dose is designed to provide a single number proportional to the radiobiological "detriment" from a particular, often inhomogeneous, radiation exposure, with detriment representing a balance between carcinogenesis, life shortening and hereditary effects. It is commonly used to allow a comparison of the risks associated with different spatial dose distributions produced by different imaging techniques. The effective dose represents questionable science: two of the most important reasons for this are that the tissue-specific weighting factors used to calculate effective dose are a subjective mix of different endpoints, and that the marked and differing age dependencies for different endpoints are not taken into account. Importantly, the effective dose is prone to misuse, with widespread confusion between effective dose, equivalent dose and absorbed dose. It is suggested here that effective dose could and should be replaced by a new quantity that does not have these problems. An appropriate new quantity could be "effective risk", which, like effective dose, is a weighted sum of equivalent doses to different tissues; unlike effective dose, where the tissue-dependent weighting factors are a set of subjective committee-defined numbers, the weighting factors for effective risk would simply be evaluated tissue-specific lifetime cancer risks per unit equivalent dose. The resulting quantity would perform the same comparative role as effective dose; it would have the potential to be age- and, if desired, gender-specific, just as easy to estimate, less prone to misuse, more directly interpretable, and based on more defensible science.