Quality assurance of single isocentre multiple target stereotactic radiosurgery: Findings from long-term off-axis Winston-Lutz testing and machine performance checks

Abstract

Background: Single-isocenter multiple-target stereotactic radiosurgery (SIMT SRS) has emerged as an efficient treatment for multiple brain metastases. However, this technique demands exceptional geometric accuracy, particularly off-axis, to ensure optimal dose delivery while sparing healthy tissue. Traditional quality assurance (QA) methods require adaptation for SIMT SRS, highlighting the need for robust testing protocols. Purpose: This study aimed to evaluate the long-term off-axis targeting accuracy of three Varian TrueBeam linear accelerators using a dedicated off-axis Winston–Lutz test (OAWLT) and to investigate correlations with routine varian machine performance check (MPC) results. Materials & Methods: Weekly OAWLT measurements were performed over a 6-month period on three TrueBeam linacs using the Sun Nuclear StereoPHAN phantom and MultiMet-WL cube. The test delivered 6 MV flattening filter-free beams to multiple off-axis targets via eight beam orientations, simulating clinical SIMT SRS workflows. Concurrently, daily MPC assessments evaluated geometric parameters including isocenter size, kV isocenter offset, beam center, and couch radiation-induced offset. Results: Median OAWLT errors were 0.38, 0.44, and 0.59 mm for the three linacs included in this study, with maximum errors of 1.12, 1.08, and 1.54 mm. Notably, off-axis errors increased with target off-axis distance, with the worst performance observed at 7 cm off-axis. Conclusion: Routine OAWLT is an effective and sensitive QA tool for monitoring off-axis targeting accuracy in SIMT SRS. Despite the convenience of daily MPC, it cannot substitute for a dedicated OAWLT. The study supports implementing weekly OAWLT in clinical practice to ensure high-quality, consistent treatment delivery for SIMT SRS treatments.