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- The Use of Effective Dose as a Radiological Protection Quantity
Draft Report for Consultation: The Use of Effective Dose as a Radiological Protection QuantityThe draft The Use of Effective Dose as a Radiological Protection Quantity is now available for public consultation. We welcome comments from individuals and organisations. The draft document can be downloaded from the ICRP website. Comments must be submitted through the ICRP website no later than August 3, 2018.
The concept of ‘effective dose’ (E) was developed by ICRP as a risk-adjusted dosimetric quantity for the management of protection against stochastic effects, principally cancer, enabling comparison of planned or received doses with dose limits, dose constraints, and reference levels expressed in the same quantity. Its use allows all radiation exposures 92 from external and internal sources to be considered together and summed, relying on the assumptions of a linear-non-threshold dose-response relationship, equivalence of acute and chronic exposures at low doses or low dose rates, and equivalence of external and internal exposures. Considering exposures incurred by patients during medical procedures, E is of practical value for comparing: doses from different diagnostic examinations and interventional procedures; the use of similar technologies and procedures in different hospitals and countries; and the use of different technologies for the same medical examination, provided that the representative patients or patient populations for which the effective doses are derived are similar with regard to age and sex. As stated in the 2007 Recommendations (ICRP, 2007a), “… risk assessment for medical diagnosis and treatment … is best evaluated using appropriate risk values for the individual tissues at risk and for the age and sex distribution of the individuals undergoing the medical procedures”. Publication 103 (ICRP, 2007a) provides detailed explanation of the purpose and use of E and of equivalent dose to individual organs and tissues. However, questions have arisen regarding practical applications, highlighting a clear need for further guidance on specific aspects. This publication draws on the explanations provided in Publication 103 and emphasises that E has proved a valuable and robust quantity for use in the optimisation of protection, to set dose criteria and verify compliance. Conclusions are drawn that: a) Equivalent dose (H) is not required as a protection quantity. It will be more appropriate for limits for the avoidance of tissue reactions for the hands and feet, lens of the eye, and skin, to be set in terms of absorbed dose (Gy) rather than equivalent dose (Sv). b) While risk assessments for individuals based on organ/tissue doses and specific dose-risk models make best use of scientific knowledge, E may be used as an approximate indicator of possible risk, recognising that this is a pragmatic, but unintended, application of effective dose. It is made clear in this report that while doses incurred at low levels of exposure may be measured or assessed with reasonable accuracy, the associated risks are increasingly uncertain at lower doses. However, bearing in mind the uncertainties associated with risk projection to low doses, E may be considered as an approximate indicator of possible risk, with the additional consideration of variation in risk with age, sex and population group. Use of E in this way is not a substitute for risk analysis using best estimates of organ/tissue doses, appropriate information on the relative effectiveness of different radiation types, and age-, sex- and population-specific risk factors, with consideration of uncertainties.