Clinical Implementation and Utility of Triggered kV Imaging During Spine Stereotactic Body Radiotherapy for Intrafraction Motion Management

Abstract

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.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.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.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.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.