Purpose: To quantify the errors in daily alignment of Thoracic patients on CT-on-Rails unit with standard and slow CT protocols.
Methods: Patients and a QUASAR-motion phantom were simulated using 4DCT protocol. MIP was used to delineate iGTV, and the AVG was used for both treatment planning and as reference scan. We also measured the alignments using a phantom with motion amplitudes of 5, 10 and 15 mm with two breathing frequencies using 10 scans for each protocol. The rotation and pitch of the standard and slow protocols were 0.5, 1.75:1 and 1s, 0.562:1 respectively. The rotation, pitch and scan times of the standard and slow protocols were 0.5s, 1.75:1, 9s and 1s, 0.562:1, 59.5s. The difference in vertebral body/GTV alignment (used external contour for the phantom) to determine the systematic error. The bone alignment was done using auto align feature, but the GTV was aligned manually to the center of iGTV.
Results: The mean (standard deviations) of Bone-GTV longitudinal alignments standard protocol were -0.03 (0.58), 0.08 (0.37), 0.25 (0.42), 0.07 (0.28), 0.05 (0.13) and 0.14 (0.14) mm respectively for the phantom with motion parameters of 15mm-8BPM, 15mm-20BPM, 10mm-8BPM, 10mm-20BPM, 5mm-8BPM and 5mm-20BPM. These deviations for slow CT protocol were -.13 (0.29), 0.07 (0.06), 0.08 (0.27), 0.12 (0.15), 0.13 (0.08) and 0.16 (0.09) respectively. Our phantom results indicated that slow protocol was able to reduce random errors in the motion direction. The retrospective analysis of 13 patients’ data showed a systematic error of -0.17 ±0.23 (0.27to-0.49) mm in the longitudinal direction.
Conclusion: Our analysis indicates that the slow CT protocol was able to converge to the mean tumor position better than the fast CT protocol. For a given motion amplitude, slower breathing rate had larger variation in alignment for both protocols.