Effective dose: practice, purpose and pitfalls for nuclear medicine

Abstract

Effective dose (E) is the only comparatively simple dose quantity that is related to healthdetriment for stochastic effects from exposure to ionising radiation. As such,E has found wide application for medical exposures, as it allowscomparisons with doses from different examinations and other sources.E is derived from the weighted sum of doses to tissues known to be sensitive toradiation from epidemiological studies and contains inherent approximations.Thus it is not a scientific quantity, but a practical one that the InternationalCommission on Radiological Protection (ICRP) has created for use in thecalculation of reference doses for protection purposes. In the application ofE to medical exposures, there has been a tendency to attribute a greater accuracy to values ofE than is justified by its derivation. Recognising thatE is strictly not subject to uncertainties, an analysis has been undertaken of potential uncertainties inE for different nuclear medicine examinations to enable users to judge its reliability asa comparator of relative risk. Assessments have been based on the consideredaccuracy of the component parts and indicate that the uncertainties in the values ofE as a relative indicator of harm for nuclear medicine procedures for a reference patient are about ± 50%. These are larger than those for radiology procedures, because of the tendencyfor doses to single organs, especially the bladder, to form a substantial part ofE for someprocedures. Revision of the tissue weighting factors in 2007 produced a 10% decrease in the mean valueof E for nuclear medicine examinations. Estimations of cancer risk based onE for an individual could vary by one or two orders of magnitude.E fulfils an important role as a health-related dose quantity that can be used in justificationof nuclear medicine examinations, but physicians should be aware of its limitations.General terminology should be used in conveying risks to patients and medicalprofessionals.