CTシステムコットトップの高精度品質管理方法：高度な3D座標測定機を用いた治療計画

Quality control (QC) methods for computed tomography (CT) systems used in treatment planning have not been updated since the release of the Task Group (TG) 66 report by the American Association of Physicists in Medicine (AAPM) in 2003. Conventional QC methods for CT systems fail to fulfill the requirements of high-precision radiation therapy. Moreover, because the geometric accuracy of CT systems can affect the accuracy of radiation therapy, which is particularly critical in high-precision radiation therapy, a highly accurate QC method is required.

This study aimed to develop a high-accuracy QC method for CT system couch tops suitable for high-precision radiation therapy by utilizing a wide-area three-dimensional (3D) coordinate measuring machine (3D-CMM), a type of laser tracker.

We used a 3D-CMM, which includes a wireless probe and a camera unit, focusing on a SOMATOM go.Open Pro CT system. The system was set up in accordance with the reference method outlined in the AAPM TG66 report guidelines. The initial phase verified the accuracy of 3D-CMM measurement within a CT room using a micrometer. Subsequently, a novel continuous measurement method was developed to enable the real-time tracking of the couch-top displacement during travel. This new method was evaluated against the standard manual measurement method. Measurements were performed at 0.1 s intervals at various index positions on the couch top, with and without added weight to simulate the presence of a patient. The gathered data were analyzed to assess the couch-top displacement, horizontality and orthogonality relative to the imaging plane, providing a comprehensive evaluation of the stability and alignment of the couch top.

The difference between the micrometer and measured shifts peaked at 0.08 mm. The continuous measurements agreed with the standard measurements within one standard deviation of the three measurements. The maximum displacements of the couch top were 5.23 and 2.00 mm in the vertical axis, with and without a weight load, respectively. There were differences in the displacement at each measurement point. In the lateral axis, the maximum displacements were 2.05 and 2.09 mm with and without a weight load, respectively. The maximum displacement in the imaging plane was observed at approximately half the distance traveled by the couch top. As the couch top traveled, the horizontal angle in the imaging plane of the couch top varied from 0.06° to 0.07° and 0.25° without a weight load and from 0.09° to 0.21° and 0.34° with a weight. The orthogonal angles of the couch top varied from 0.07° to 0.08° and 0.14° without a weight load and from 0.05° to 0.23° and 0.46° with a weight.

The developed QC method for the couch tops of CT systems can evaluate the displacement of the couch top and its horizontality and orthogonality to the imaging plane with detailed submillimeter and subdegree accuracy levels. The high-accuracy QC of CT systems can improve the accuracy of irradiation in radiation therapy.