Clinical impact of enhanced leaf modelling on multi-metastatic intracranial SRT delivered on Halcyon: a modest but consistent dosimetric effect

Enhanced Leaf Modelling (ELM), introduced in Varian Eclipse v18, provides a refined representation of MLC transmission by explicitly modelling physical leaf geometry, interleaf leakage, and leaf-tip effects for Halcyon, replacing the traditional dosimetric leaf gap approximation. Unlike C-arm systems, Halcyon employs no collimator jaws, making out-of-field dose more exclusively dependent on MLC transmission characteristics and increasing the relative importance of accurate MLC modelling. Local commissioning supported the implementation of ELM on Halcyon and demonstrated improved agreement between calculated and measured dose distributions in the commissioning tests. These commissioning findings were separate form the present analysis, which characterises the dosimetric impact of ELM in clinically representative multi-metastatic stereotactic radiotherapy (MMSRT) plans rather than independently validating their physical accuracy.

Ten MMSRT patients (2-6 lesions) were replanned using two calculation models: the standard Acuros XBv16 and v18 (with ELM). For each, two optimisation strategies were compared, manual and automatic normal tissue optimisation (NTO), to assess robustness across planning methods. The low-dose valley, defined as [Brain-PTVs], represented normal brain between targets. Dose-volume metrics were extracted across V50% to V2% of prescription dose to evaluate low-dose spread. Dose fall-off was quantified using concentric 1 mm rings extending 1-7 mm from the PTV surface, recording mean, median, and maximum doses within each ring. Clinically relevant spread was further evaluated using V12 Gy, V20 Gy, and V24 Gy metrics. Statistical analysis was done using appropriate parametric or non-parametric testing (p<0.05).

Enhanced Leaf Modeling (ELM) primarily affected low- and intermediate-dose regions surrounding intracranial targets in MMSRT plans delivered on Halcyon. Statistically significant differences in normal brain volume were observed between ELM and the conventional MLC model (XB16), particularly beyond 3 mm from the PTV and at lower dose levels (V12-V24 Gy). ELM consistently reported higher low-dose volumes, with the largest effects observed for V20 Gy and V24 Gy within proximal rings, especially for manually optimised plans. Differences in extreme dose metrics were observed, with ELM associated with increased minimum dose and reduced maximum dose in the peri-PTV region, while mean dose remained largely unchanged. Despite statistical significance, all observed differences were small in magnitude and did not affect clinical acceptability.

Enhanced Leaf Modelling (ELM) introduces measurable but clinically small changes in dose distribution for MMSRT delivered on Halcyon treatment units, with effects confined predominantly to low-dose regions and steep dose gradients. These changes did not alter clinical acceptability, target coverage, or conformity in the plans assessed. ELM provides a more detailed physical representation of MLC transmission and leakage than the conventional DLG-based approach. The direction and consistency of the observed differences are compatible with previously reported commissioning and validation findings; however, the present study characterises model-dependent dosimetric changes and does not establish improved physical accuracy for the MMSRT plans analysed.