Purpose: Tracking intrafractional motion is a critical component in paraspinal stereotactic body radiotherapy (SBRT) to ensure the treatment effectiveness. We developed a quantitative software application that uses orthogonal mega-voltage (MV) and kilo-voltage (KV) images to track such motion for paraspinal SBRT. This work reports our pilot study results of this software on clinical patients.
Methods: Simultaneous MV/KV image pairs were acquired on a conventional linac platform during beam delivery. To improve the field-of-view of MV images, an enhanced synthetic MV (ESMV) image was generated by combining multiple frames of MV images after corrections for scatters and frame intensity fluctuation. Our software registers ESMV/KV images to their corresponding references, either CBCT projections acquired right before treatment or digitally reconstructed radiograph (DRR), based on the highest cross-correlation, and reports the 3D intrafractional motion in real-time. This software was used during paraspinal SBRT treatments for 32 clinical patients with sites covering four spine regions, all treated with 9-field IMRT plans. Offline manual registration was performed retrospectively to verify the software’s automatic registration accuracy.
Results: The percentage of patients whose 3D motion, in at least one field, exceeded 1 mm, 2 mm, and 3 mm is 46.9%, 9.4%, and 3.1%, respectively. The percentage of patients whose average 3D motion of all fields exceed 1 mm, 2 mm, and 3 mm is 15.6%, 3.1%, and 0%, respectively. For patients with at least one field exceeding 1 mm, motion magnitude increased by 32% on average in the second half of the treatment when compared to the first half.
Conclusion: Despite immobilization, significant intrafractional motion exists in certain paraspinal SBRT patients. Motion tends to increase as treatment progresses. Our MV-KV software provides a 3D quantitative solution for tracking such motion. It can be seamlessly integrated with the conventional Linac platform and offers definitive guidance for treatment intervention.
Funding Support, Disclosures, and Conflict of Interest: Memorial Sloan-Kettering Cancer Center has a research agreement with Varian Medical Systems. This research was partially supported by the MSK Cancer Center Support Grant/Core Grant (P30 CA008748).
Image Guidance, Patient Movement, Quantitative Imaging