Feasibility of Using Machine Performance Check-enhanced Couch Check as a Daily Surrogate for Multisphere Winston–Lutz Testing in Single-isocenter Multitarget Stereotactic Radiosurgery

Single-isocenter multitarget (SIMT) stereotactic radiosurgery (SRS) requires submillimeter geometric accuracy. Although Winston–Lutz (WL) testing remains the stereotactic standard, it is time-intensive. This study evaluated whether Varian’s machine performance check (MPC)-enhanced couch check can serve as a surrogate for WL by comparing geometric agreement across routine clinical use. WL tests were performed on 23 SRS/SRT treatment days using a multisphere VisionRT phantom. MPC-enhanced couch check data were extracted for the same dates, including maximum positioning error (MPC-max) and rotation-induced couch shift (MPC-shift). Agreement between WL and MPC was evaluated using Pearson correlation and Bland–Altman analysis. WL values ranged from 0.27 to 0.67 mm. MPC-max and MPC-shift ranged from 0.37 to 0.44 mm and 0.33 to 0.49 mm, respectively. Correlation between WL and both MPC metrics was high ( r > 0.85). Bland–Altman analysis showed minimal bias for MPC-max (+0.033 mm) and MPC-shift (+0.037 mm). The 95% limits of agreement (LOA) were −0.176 to + 0.243 mm (MPC-max) and −0.160 to +0.235 mm (MPC-shift), all within SIMT SRS geometric tolerances. MPC-enhanced couch check demonstrated strong agreement with WL measurements, with sub-0.05-mm bias and LOA within ± 0.25 mm. These findings support the use of MPC as a practical surrogate for WL in daily or high-frequency isocenter and couch-rotation verification within SIMT SRS workflows.