Room: Track 6
Purpose: Brain stereotactic-radiosurgery (SRS) treatments require multiple quality-assurance (QA) procedures to ensure accurate and precise treatment delivery. As single-isocenter multitarget SRS treatments become more popular, quantifying off-axis accuracy of linear-accelerator is crucial. In this study, a novel brain SRS integrated phantom was developed and validated, at multiple-institutions, to enable SRS QA with a single-phantom to facilitate implementation of a frameless single-isocenter, multitarget SRS program. This phantom combines independent verification of positioning systems, Winston-Lutz test, off-axis accuracy evaluation (i.e. off-axis Winston-Lutz), and dosimetric accuracy utilizing both point-dose-measurements as well as film-measurement in a static-phantom.
Methods: A novel 3D-printed phantom, coined OneIso, was designed with a movable insert which can switch between Winston-Lutz test targets and dose measurement without moving the phantom itself. For dose verification, three brain SRS clinical-treatment-plans with 10MV flattening-filter-free (FFF) beams were delivered on three Varian TrueBeams with high-definition-multi-leaf-collimator (HD-MLC) at three independent cancer-centers. Radiochromic film and pinpoint ion chamber comparison measurements were obtained with OneIso. For off-axis Winston-Lutz measurements, a row of off-axis ball-bearings (BBs) was integrated into the OneIso. To quantify the spatial accuracy versus distance from isocenter, two-dimensional displacements were calculated between the planned and delivered BB locations relative to their respective MLC defined field-boarders.
Results: OneIso phantom measurements across all three centers were within 2% agreement, for both film and point-dose measurements. OneIso identified a reduction in spatial accuracy further away from isocenter for all three SRS machines. Differences increased as distance from isocenter increased exceeding recommended SRS accuracy tolerances (<1mm) at different distances for each machine (3-5cm), indicating that the tolerance are machine-dependent.
Conclusion: OneIso provides a streamlined, single-setup workflow for single-isocenter multitarget frameless linac-based SRS QA that can be easily translated to multiple-institutions. Additionally, quantifying off-axis spatial-discrepancies allows for determination of maximum distance between targets that meet single-isocenter multitarget SRS program recommendations.
Funding Support, Disclosures, and Conflict of Interest: Dr. Capaldi receives funding support from the American Association of Physicist in Medicine (AAPM) Seed Grant Award.