CT Patient Risk: NCRP 160 and UNSCEAR 2008
A. Calzado
Medical Physics Group. Radiology Department. Complutense University Madrid
Synopsis of the presentation
We begin with a review of data from the NCRP Report No. 160 and the 2008 UNSCEAR Report on the frequency of computed tomography (CT) examinations and their contribution to collective effective dose in the U.S. and other health care level I countries. The following is an analysis of the risks descriptors of induction of stochastic effects in patients undergoing CT examinations. The LNT model is recognized as being the hypothesis most accepted by the specialized agencies (ICRP, BEIR Committee and Committee UNSCEAR, among others) to explain the dose-risk relationship at low doses (<100 mSv). The risk dependence on patient age and sex of the patient, as well as on the organs or tissues receiving highest doses from the examination are discussed. Some remarks about the existence of alternatives to the LNT hypothesis are mentioned; risks of cancer induction by CT scans are compared with other risks in everyday life. Emphasis is placed on the risks in pediatric CT examinations (higher radiosensitivity and large lifetime background risk of cancer mortality). In addition, risk of deterministic effects, as stated in ICRP Report 102, can arise when the patient undergoes more than one radiological procedure where the radiation beam remains for an extended time over the anatomic region (e.g., perfusion or interventional CT), so that although such effects occur in very special cases, their possibility cannot be excluded.
Finally we conclude that in the vast majority of cases, the diagnostic benefit of CT scans clearly outweighs the risks of induction of cancer and propose optimization (ALARA principle) as the way to decrease them. The need for adapting the examination settings to the size and morphology in pediatric patients is stressed. In addition, other particular measures, such as the use of dose-saving options provided by modern scanners or the consideration of individual risks for younger or middle aged patients suffering from curable diseases and undergoing multiple CT examinations, are also recommended. In addition to all these considerations, some information about a proposal, introduced in the draft version of a new European Directive, requiring patient dose value to be displayed in the console of the equipment and to be transferred to the patient's history, is given.