LAUSR

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, the quantification of off-axis accuracy of the linear-accelerator is crucial. In this study, a novel brain SRS integrated phantom was developed and validated to enable SRS QA with a single phantom to facilitate implementation of a frameless single-isocenter, multitarget SRS program. This phantom combines the independent verification of each positioning system, the Winston-Lutz, off-axis accuracy evaluation (i.e. off-axis Winston-Lutz), and the dosimetric accuracy utilizing both point-dose-measurements as well as film-measurement, without moving the phantom.Methods and Materials:A novel 3D-printed phantom, coinedOneIso, was designed with a movable insert which can switch between the Winston-Lutz test target and dose measurement without moving the phantom itself. For dose verification, eight brain SRS clinical-treatment-plans with 10MV Flattening-Filter-Free (FFF) beams were delivered on a Varian TrueBeam with a high-definition-multi-leaf-collimator (HD-MLC). Radiochromic film and pinpoint ion chamber comparison measurements were made between the OneIso and solid water (SW) phantom setups. For the 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 border.Results:OneIso and the SW phantoms agree within 1%, for both film and point-dose measurements. OneIso identified a reduction in spatial accuracy further away from isocenter. Differences increased as distance from isocenter increased exceeding recommended SRS accuracy tolerances at 3-4cm away from isocenter.Conclusions:OneIso provides a streamlined, single-setup workflow for single-isocenter multitarget frameless linac-based SRS QA. Additionally, with the ability to quantify off-axis spatial-discrepancies, we can determine limitations on the maximum distance between targets to ensure a single-isocenter multitarget SRS program meets recommended guidelines.