Jodie A. Christner PhD, Presenter: Grant, Siemens AG

James M. Kofler PhD, Abstract Co-Author: Nothing to Disclose

Cynthia H. McCollough PhD, Abstract Co-Author: Research grant, Siemens AG Research grant, RTI Electronics AB Research grant, Bayer AG

To determine how well DLP-based methods for estimating effective dose (E) agree with organ-dose-based calculations made using 1) the revised ICRP 103 tissue-weighting factors published in 2007 and 2) dual-energy CT scan protocols.

Using ICRP 60 tissue-weighting coefficients and organ-dose coefficients published by the NRPB, a widely used method of estimating E based on the dose-length-product (DLP) was published for CT exams. ICRP 103 recommends revised coefficients for several organs, although retaining the name “effective dose.” An Excel-based tool from ImPACT and kV-dependent organ-dose coefficients were used to calculate E for CT exams of the thorax (Tx), abdomen (A), abdomen & pelvis (AP) and coronary arteries (CTA) using routine clinical protocols for 1) single-kV (120 kV) and dual-energy CT protocols (80/140 kV typical, 100/140 kV CTA) and 2) ICRP 60 and 103 tissue-weighting coefficients. Estimates of E were also made using the ICRP 60-specific DLP method (coefficients of 0.015 mSv/mGy-cm for A or AP and 0.14 mSv/mGy-cm for Tx or CTA). All calculations assumed a dual-source CT system (Siemens Definition DS).

ICRP 103 values of E calculated using organ dose estimates differed by -6.8, 4.8, 12.9, and 37.6% relative to ICRP 60 values for Tx, A, AP and CTA exams, respectively. DLP-based estimates of E (using ICRP 60-based conversion coefficients) differed by -17, -10, -23, & -49% relative to organ-dose-based estimates for ICRP 60 and -11, -14, -32, & -29% relative to organ-dose-based estimates for ICRP 103 for Tx, A, AP, and CTA exams, respectively. These results were essentially energy-independent. The use of ICRP 103 tissue-weighting factors decreased estimates of E for AP CT exams by 7%, but increased estimates of E by 5 – 38% for CT scans including the thorax, indicating a need to reassess E/DLP conversion factors. For the evaluated scanner, E/DLP conversion factors were relatively energy independent and underestimated E relative to organ-dose based calculations.

The published E/DLP conversion factors will require modification, according to the scanned anatomic regions, if the ICRP 103 recommendations are to be adopted, particularly for scans over the thorax.

This paper examines effects of new recommendations, ICRP 103, and dual source CT exams on estimating effective dose in patient populations.

Christner, J, Kofler, J, McCollough, C, The Effect of ICRP Report No. 103 and Dual-Energy CT on DLP-based Effective Dose Estimates.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6010388.html