Evaluation of EPID-Based Transmission and DLG Correction Methods for Dosimetric Verification in HyperArc Single-Isocenter Multiple Target Radiosurgery

Background/Objectives: Accurate verification of single-isocenter multiple-target (SIMT) stereotactic radiosurgery is challenging owing to the complexity of multi-lesion delivery and the sensitivity of stereotactic dose gradients. We aimed to evaluate the efficacy of electronic portal imaging device (EPID)-based multileaf collimator (MLC) transmission and dosimetric leaf gap (DLG) correction for improving portal-dose prediction agreement in SIMT stereotactic radiosurgery (SRS) and to propose an exploratory target count-based institutional action level. Methods: This retrospective analysis included 112 consecutive patients treated with HyperArc™-based SRS (1–13 targets). Treatment plans were calculated using the Acuros XB algorithm for 6 MV flattening filter-free (FFF) beams. Two sets of MLC parameters were compared for portal-dose image prediction (PDIP): (1) standard parameters measured using an ion chamber; (2) EPID-derived corrected parameters. Portal-dose accuracy was evaluated using gamma index analysis with a 95% pass rate threshold. Results: The uncorrected method showed a strong negative correlation between target number and gamma passing rates, with complex plans (≥9 targets) dropping as low as 70%. EPID-based correction substantially improved dose agreement, yielding consistent passing rates above 98%, regardless of target number. Although uncorrected parameters remained within tolerance for plans with one to two targets, accuracy declined markedly starting at three targets. Conclusions: Our findings indicate that the EPID-based correction of MLC transmission and DLG mitigated cumulative modeling discrepancies in complex SIMT SRS. Thus, we propose the exploratory institutional observation of three targets, beyond which EPID-based correction may be considered to achieve optimal portal-dose prediction agreement in HyperArc-based stereotactic radiosurgery.