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.