Comparison of the Motion Accuracy of a Six Degrees of Freedom Radiotherapy Couch with and without Weights

In this study, we compared the motion accuracy of six degrees of freedom (6D) couch for precision radiotherapy with or without weights attached to the couch. Two digital cameras were focused on the iso-center of a linear accelerator. Images of a needle which had been fixed to the 6D couch were obtained using the cameras when the couch moved in translation and rotation around each axis. The three-dimensional (3D) coordinates of the needle were calculated from coordinate values in the images. A coordinate error of the needle position relative to the theoretical position was calculated. The errors were obtained with or without a 60 kg weight attached to the 6D couch, and these errors were compared with each other. The mean distance of the 3D error vectors for the weighted test was 0.21 ± 0.11 mm, and ˃0.16 ± 0.09 mm for the non-weighted test (p < 0.05). However, the difference of two values was 0.06 mm which is smaller than the minimum distance the 6D couch system can move correctly. The variance of 0.16 mm for the Y coordinate errors for the weighted test only was larger than that for the non-weighted test, which was 0.06 mm (p < 0.05). We found that a total weight of 60 kg did not affect the accuracy of the 6D couch clinically. However, the variance of the Y coordinate errors was increased. This might suggest that the addition of this weight increase the uncertainty of the motion of the 6D couch.