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An Integrated Quality Assurance Phantom for Frameless Single-Isocenter Multitarget Stereotactic Radiosurgery: A Multi-Institutional Trial

D Capaldi1*, T Nano2, L Skinner3, C Chuang4, P Dubrowski5, L Xing6, A Yu7, (1) Stanford University, Stanford, CA, (2) University of California, San Franisco, San Francisco, CA, (3) Stanford University Cancer Center, Stanford, CA, (4) Stanford, Stanford, CA, (5) Stanford University, San Jose, CA, (6) Stanford Univ School of Medicine, Stanford, CA, (7) Stanford University Cancer Center, Stanford, CA


(Thursday, 7/16/2020) 2:00 PM - 3:00 PM [Eastern Time (GMT-4)]

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.


3D, Stereotactic Radiosurgery, Quality Assurance


TH- External Beam- Photons: Quality Assurance - Linear accelerator

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