[Improvement of CT Imaging with a Metal Artifact Reduction Technique for Radiation Treatment Planning-Fundamental Study of Structure Delineation and Dose Calculation].

In radiotherapy planning, CT images are widely used to delineate the gross tumor volume (GTV) and the organs at risks (OARs), which allows for the calculation of the dose distribution to each structure. The delineated contours of the GTV and OARs may become inaccurate, and subsequently result in the inaccurate derivation of the dose distribution, if there are metal artifacts present in the CT image. The metal artifact reduction technique, single energy metal artifact reduction (SEMAR), installed on the CT system (Aquilion ONE ^TM Vision Edition, Toshiba Medical Systems Corporation) could potentially reduce metal artifacts. Therefore, we investigated whether SEMAR can improve the accuracy of delineation, and subsequently the dosimetric accuracy, in the treatment planning process. Using an acrylonitrile-butadiene-styrene resin phantom (RT-3000-New, R-Tech. Inc, Tokyo, Japan), titanium bars were inserted on both the left and right sides, and four types of electron density inserts (rods) were separately inserted in the middle. The electron densities of the rods were 0.90, 0.96, 1.07, and 1.09. After CT images were acquired, SEMAR-ON (when applying the SEMAR correction) images were generated. On both SEMAR-ON and SEMAR-OFF (when not applying the SEMAR correction) images, the rod contours were delineated automatically, using a CT value threshold. This threshold was selected so that the area of the automatically delineated contour was 615.4 mm ² . The difference in the contour area of SEMAR-ON, SEMAR-OFF, and no metal artifact images were compared using the dice coefficient. When SEMAR was used, the dice coefficient improved by 57.4%. Therefore, SEMAR was considered to be useful in improving the accuracy of GTV and OAR delineation.