Setup error and residual error analysis of ExacTrac X-ray image guidance system in stereotactic radiotherapy for brain metastases

This study aims to analyze the setup errors of the ExacTrac X-ray image guidance system in stereoscopic radiotherapy for brain metastases and discuss the necessity of setup error verification after bed angle movement. Methods: Twelve patients with brain metastases were treated with stereotactic radiotherapy. ExacTrac X-ray image guidance system was used to register and verify the image. Besides, the intra- and inter-fractional setup errors and residual errors were analyzed. Results: Twelve patients received head stereotactic radiotherapy 56 times. The inter-fractional setup errors and residual errors were x (1.04 ± 0.16) and (0.15 ± 0.05) mm in left and right directions, y (1.25 ± 0.28) and (0.18 ± 0.07) mm in head and foot direction, and z (0.93 ± 0.81) and (0.14 ± 0.13) mm in abdominal and dorsal direction, respectively; Rx (0.81 ± 0.24) and (0.24 ± 0.09) degree in the sagittal plane, Ry (0.95 ± 0.19), (0.22± 0.06) degree in the cross-section, and Rz (1.03 ± 0.26), (0.22 ± 0.06) in the coronal plane. The difference was statistically significant (P < 0.05). Only 19.6% of the intra-fractional non-coplanar arc setup verification passed without correction for the first time, and the others were significantly improved after correction, presenting statistical significance (P < 0.05). The inter-fractional setup errors and residual errors were x (0.82 ± 0.25) and (0.13 ± 0.04) mm in left and right directions, y (0.67 ± 0.23) and (0.18 ± 0.08) mm in head and foot direction, and z (0.74 ± 0.21) and (0.19 ± 0.08) mm in abdominal and dorsal direction, respectively; Rx (0.67 ± 0.13) and (0.20 ± 0.08) degree in the sagittal plane, Ry (0.68 ± 0.13) and (0.17 ± 0.10) degree in the cross-section, and Rz (0.60 ± 0.28) and (0.19 ± 0.07) degree in the coronal plane. The difference was statistically significant (P < 0.05). Conclusion: The ExacTrac image guidance system enhanced the accuracy of setup in stereoscopic radiotherapy for brain metastases and satisfied the clinical requirements. It is necessary to verify the non-coplanar arc setup