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Purpose/objective(s): To monitor intrafraction motion during spine stereotactic radiosurgery (SRS) treatment delivery with readily available device, we implemented triggered kV imaging using the on-board imager of the medical linear accelerator advanced imaging package.Materials/methods: Triggered kV imaging technique was clinically implemented and applied to 32 spine SBS patients (75 fx, 20 T-spine tumors, 12 L-spine tumors) during VMAT delivery. Currently available trigger options in the medical linear accelerator software package are MU, gantry angle, time, and respiratory gating. Time trigger was selected to acquire kV images every 15 seconds, in an effort to balance temporal resolution with reasonable time for image evaluation. Upon each image acquisition, a 2D projection of the 3D image-guided radiotherapy (IGRT) structures was automatically calculated and updated at arbitrary angles as a qualitative visual guide for patient alignment during treatment delivery. For IGRT structures, patient anatomy (vertebra and spinous process) and orthopedic hardware if present were contoured, with a 1 mm expansion. As the technique depends on visual inspection, decisions could vary depending on interobserver variability. Therefore, a survey of 10 trained physicists (4) and physicians (6) was conducted to evaluate efficiency and accuracy of clinical decision with kV images collected using an anthropomorphic phantom with different CT slice thicknesses and shifts introduced. The survey consisted of 192 sample images. The participants were asked to determine if a shift requiring intervention was observed. Average time per decision was recorded.Results: The robustness of our current patient immobilization system was validated in our clinical implementation. Based on visual inspection of projected IGRT contours on planar kV images, appreciable movement was detected in only one fraction (1.3%). However, in this instance, motion was well beyond PTV/PRV margins (2.4 mm) and treatment was stopped to take a CBCT and correct the displacement, which otherwise would have not been detected. From the survey, the average time spent per decision was 7.5 seconds. In all shift directions, detectability was proportional to the shift size and superior detectability was observed with contours drawn using CT images with 1 mm slice thickness. Shifts smaller than 1 mm are less noticeable (12.6%) compared to 2 mm shifts (42.6%). Participant detection capability was dependent on the level of training.Conclusion: This approach provides an effective, non-invasive intrafraction motion monitoring solution, using the built in triggered-imaging technique of the medical linear accelerator. One of the benefits this technique offers is that it can serve as a quality assurance approach to verify adequacy of a patient immobilization system. An important limitation is that the monitoring result is qualitative and depends on observer visual detection capability. Enhanced precision and utility could be achieved if an automated quantitative evaluation feature is implemented.Author Disclosure: J. Koo: None. L.Nardella: None. M.Degnan: None. J.Andreozzi: None. H.M. Yu: Honoraria; UpToDate. Speaker's Bureau; BrainLab. Advisory Board; Novocure, AbbVie. J.A.Penagaricano: None. P.A. Johnstone: None. D.E. Oliver: None. K.A. Ahmed: Research Grant; Bristol-Myers Squibb, Genentech. S.Rosenburg: Consulting; Novocure. E.J.Wuthrick: Research Grant; BMS. Honoraria; Regeneron/Sanofi, Varian. Consultant; Regeneron/Sanofi, Varian. R. Diaz: Honoraria; Zeiss; Global Health Outreach. V.Feygelman: None. E.G. Moros: Research Grant; Varian Medical Systems. Promote research in the AAPM; American Association of Physicists in Medicine. Promoting research in the AAPM; American Association of Physicists in Medicine. G.Redler: None. [ABSTRACT FROM AUTHOR]

MRgRT is used to treat UC lung lesions. Adaption is triggered if the original plan does not meet criteria of target coverage or organs at risk (OAR) constraints on recontoured anatomy of the day. Current MRgRT software does not provide composite dose delivered over the complete course, partly because daily anatomy changes make accurate spatial dose accumulation difficult. Previous approaches used DVH point averages to approximate doses from full course. We aimed to more accurately quantify true delivered dose via rigid fusion (RF) of fractional doses and hypothesize that treatment with adaptive MRgRT has superior cumulative doses than non-adaptive treatment. We conducted a single institution analysis of patients receiving 60 Gy in 8 fractions with adaptive MRgRT to UC lesions (per HILUS) between 2/2020 – 1/2022. We created composite doses from 'delivered' treatment plans and 'predicted' treatment plans. For the 'delivered' plans, dose for clinically treated plan (adapted or non-adapted based on daily plan quality) was exported for each fraction with MR contoured anatomy of the day. Using a third-party image registration software, we performed RF for each adaptive fraction focused on aligning two regions: (1) tumor and (2) proximal airway, to separately determine local cumulative dose to those sites. For fractions that were not adapted, the initial plan was fused to daily MR. For the 'predicted' treatment plan, initial plan dose was RF to primary tumor location for each MR contoured anatomy of the day and then summed for composite dose. Eleven consecutive patients with UC lesions were treated with online adaptive MRgRT. Eight (73%) patients had primary lung tumors. Median distance from proximal airway: 0 mm (0-6). Median number of adapted fractions was 7 (6-8). Differences between cumulative delivered and predicted plan dose on daily anatomy GTV D100%, PTV D95%, and Proximal Airway D0.03cc (PA0.03cc) were all statistically significant (p<0.01). The delivered dose had improvement in GTV D100% versus the initial plan (p<0.01), PTV D95% and PA0.03cc were not significant. Median follow up was 7.4 months. No grade 3+ acute or late toxicity was reported. Online adaptive MRgRT resulted in composite delivered doses from treatment on par with initial plan dosimetric quality and significantly outperforms doses delivered with non-adapted treatment. The adapted plan coverage superiority over the non-adapted plan is explained by the 'feathering' of dose that creates a 60 Gy isodose line while respecting OAR, which is underappreciated with just simple summing of changes via adaptive plan DVH values. This is the first ever plan evaluation study of its kind for MRgRT and aids in establishing dosimetric superiority of daily online adaptation. [ABSTRACT FROM AUTHOR]