Heavy-ion Radiotherapy Rotating Gantry Calibration for Improving Patient Positioning Accuracy

Abstract

In heavy-ion radiotherapy, accurate patient positioning is necessary for treatment accuracy. For this purpose, calibration of the X-ray imaging device position is indispensable. However, because positional repeatability of a calibration phantom in the rotating gantry affects the calibration result, it is not possible to calibrate using known 3-D positions of iron balls. Hence, instead of using the positions of the iron balls, we developed an X-ray tube position calibration method using the position and rotation of the FPD measured by a laser tracker. In the proposed method, the positions of the X-ray tube and iron balls are adjusted so that the distance between the position to be projected of each iron ball and the detected position (reprojection error) is minimized. In the rotating gantry calibration, multiple images are taken while changing the gantry angle, but since the position of the calibration phantom during imaging is unchanged, it is possible to represent the 3-D iron ball position with common parameters among the images. As described above, bundle adjustment can be cited as a method for calculating the position of the 3-D point and the imaging system parameter from the corresponding point between the multiple view images. The principle is to adjust all the unknown parameters so that the projected iron ball position on the image by the estimated parameters matches as closely as possible the iron ball position detected from the image. In this paper, we propose a rotating gantry calibration method based on bundle adjustment. First, by approximating the position of the X-ray tube at which the reprojection error becomes small as a function of the iron ball position of the calibration phantom, the number of parameters is reduced and the position of the iron ball is optimized. Next, the positions of the iron balls are fixed and the position of the X-ray tube is optimized. At this time, by analytically calculating the solution of two axes out of three axes where reprojection error becomes the minimum, efficient search is executed as a 1-D optimization problem. A head phantom was imaged while rotating the gantry in increments of 15 degrees, patient positioning was performed at each angle, and how the result changes with or without calibration was evaluated. For the head phantom study, the Euclidean distance patient positioning error of each angle with respect to the positioning result at the reference angle was 0.62 ± 0.21 mm (mean ± SD) and the maximum was about 1.0 mm without calibration, but with calibration, these values became 0.09 ± 0.03 mm and 0.2 mm. We proposed an automatic calibration method for rotating gantry heavy-ion radiotherapy. By using the method, the patient positioning accuracy of the rotating gantry was improved.Abstract 3625; Table 1Patient positioning error with / without calibrationPatient positioning error [mm]Maximum error [mm]without calibration0.62(±0.21)0.96with calibration0.09(±0.03)0.20 Open table in a new tab