Room: Exhibit Hall | Forum 5
Purpose: To quantify the variability between observers of intra-fraction motion evaluated via MV images during intracranial stereotactic radiosurgery (SRS) using HyperArcáµ€á´¹ (Varian Medical Systems, Palo Alto, CA).
Methods: HyperArcáµ€á´¹ has the option to use mid-treatment MV images registered to the treatment planning CT to evaluate intra-fraction motion. For each of ten patients, a set of mid-treatment images was selected. For each set, the first image, obtained immediately after CBCT-guided setup, and the second image, acquired prior to the first non-coplanar beam, were retrospectively evaluated by four observers. A rectangular region-of-interest (ROI) was selected for each image, which was then registered to the planning CT using automated registration to obtain lateral and longitudinal shifts.
Results: The magnitude of the lateral and longitudinal shifts for the first MV image ranged from 0 mm to 1.1 mm and 0 mm to 1.0 mm, respectively while both the lateral and longitudinal shifts ranged from 0 mm to 1.2 mm for the second MV image. The mean of the 2-dimensional shift magnitude for the first and second MV images was 0.6 mm Â± 0.3 mm and 0.7 mm Â± 0.3 mm, respectively. The median 2-dimensional shift magnitude was 0.5 mm for the first MV image and 0.6 mm for the second MV image.
Conclusion: This retrospective study displayed a small inter-observer variance in motion during intracranial SRS treatments as assessed by MV imaging. Differences between observers are attributed to variations in selected ROIs. Work is ongoing to better assess the accuracy of MV imaging for intra-fraction motion assessment. Until the accuracy of MV imaging is established, it should be used only as a screening tool and should not be used for mid-treatment repositioning of patients.
Funding Support, Disclosures, and Conflict of Interest: UAB receives research funding from Varian Medical Systems. R Popple has received speaking honoraria from Varian Medical Systems.