Assessment of Patient Dose in CT

This report summarises work undertaken in support of a European Commission (EC) Concerted Action on CT that has included both a review and further development of the framework for CT dosimetry. The system previously established by the EC for reference doses in CT (with the quantities weighted CTDI (CTDIw) and dose-length product (DLP)) has been expanded to include subsequent developments in CT technology, such as multi-slice scanners and the recommendation for display of volume CTDI (CTDIvol) on the scanner console. New Monte Carlo calculations for CT have been carried out to supplement the relative lack of normalised organ dose data available for paediatric patients. Series of simulations have been completed for 3 particular models of scanner and 6 mathematical phantoms representing ages newborn, 1y, 5y, 10y, 15y and adult. Analysis of these results has confirmed the trends for an enhancement of the doses to small children relative to those to adults under similar conditions of CT exposure, by factors up to 2.6 in the case of the newborn patient, depending on scanner model and region of scan. A new comprehensive framework is proposed for assessing organ doses in CT based on coefficients relating them to corresponding values of CTDIw in the standard (head or body) CT dosimetry phantoms. This approach would potentially provide a single set of dose coefficients for each standard patient age for universal application to all scanner models. Some initial values are presented of effective dose normalised both to CTDIw and DLP for combinations of standard examination and phantom age. This study was partially funded by the European Commission under Contract No FIGM-CT2000-2078. © National Radiological Protection Board Chilton Didcot Oxon OX11 0RQ Approval: March 2004 Publication: March 2004 This NRPB report reflects understanding and evaluation of the current scientific evidence as presented and referenced in this document. ASSESSMENT OF PATIENT DOSE IN CT