Background: Stereotactic radiosurgery (SRS) is one of the main approaches to managing brain metastases. Single iso multi-target (SIMT) treatment emerges as a new branch of SRS. Standard daily clinic quality assurance of SIMT is in the need to assure the treatment with high precision. Purpose: This study reports a three years’ longitudinal experience of performing daily (the day of SRS/SBRT/SIMT treatment) Off-Isocenter Winston-Lutz (OWL) test on one VitalBeam linear accelerator (Varian Medical Systems, Palo Alto, CA). A practical workflow and detailed radiation-mechanical spatial discrepancy information associated with six ball bearings (BBs) inside MMWL phantom (Sun Nuclear, Melbourne, FL) are reported for the first time in routine clinical practice. Method: The daily Off-iso Winston-Lutz (OWL) test was performed on one VitalBeam Linac which is equipped with a 120-leaf Millennium Multileaf Collimator (MLC) and a standard IGRT couch. Imaging capabilities include onboard kV and MV system that supports both 2D radiographic and 3D CBCT acquisitions. The MultiMet phantom and QA software (v2.1) are from Sun Nuclear Inc. The “Excluding 45？ Couch Kicks” 10-beam plan was adopted in Aria 16.1 R&V and Eclipse system (v16.1). The plan utilizes eight distinct Gantry-Couch-Collimator (GCC) angle configurations. An institutional workflow of off-iso Winston-Lutz test was established and implemented to ensure high-precision measurements with minimal impact on the daily treatment schedule. Results: The three years (May 2023-May 2026) OWL test were conducted on eight gantry, couch and collimator (GCC) angle sets. A total of 81 valid and formatted reports were collected and analyzed. Initial phantom positioning offset and final radiation-mechanical field center offset are reported. From GCC (0, 0, 0), GCC (0, 90, 0) and GCC (0, 0, 270) three years’ trend, it clearly showed that the further the target is from iso center, the larger radiation-mechanical spatial discrepancy is. From the GCC (90, 0, 90), GCC (180, 0, 90) and GCC (270, 0, 90) three years’ trend, it suggests us the gantry sagging as a contribution factor to affect the discrepancy in this VitalBeam linear accelerator. Conclusions: Three years of daily implementation off-iso Winston-Lutz test experience confirm that the OWL test is a feasible and highly valuable routine QA practice. The rich spatial discrepancy information provides actionable insights for the plan designer to optimize the beam placement and refine isocenter selection for
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