Author: Evan Barber, Gloria P. Beyer, Matthew J. Daniels, Callum Hartley, Jordi Saez, Philip Wheeler 👨🔬
Affiliation: TrueNorth Medical Physics, Department of Radiotherapy Physics, Velindre University NHS Trust, Department of Radiation Oncology, Hospital Clinic de Barcelona, Medical Physics Services, LLC 🌍
Purpose: To quantify inter-system and inter-layer dosimetric leaf gap (DLG) variations in Halcyon/Ethos multileaf collimators (MLCs) using custom sweeping gap tests, and to evaluate implications for cross-system dosimetric matching in clinics utilizing multiple Halcyon units.
Methods: Three Halcyon systems and two Ethos were evaluated. Vendor-supplied DLG test patterns are limited to the distal MLC layer; we addressed this constraint by generating Python-based DICOM plans to characterize proximal and distal layers independently. Sweeping gaps (2, 5, 10 and 20 mm) were delivered to a 10 cm solid water phantom at 90 cm SSD, with dose measured using a Farmer-type ionization chamber. DLG values were derived from linear regression of dose versus gap width. Average leaf transmission was also measured for static closed fields.
Results: Significant system- and layer-specific DLG variations were observed. Distal layers exhibited DLG values of 0.22, 0.29, 0.32, 0.22 and 0.28 mm, while proximal layers showed greater variability: 0.35, 0.46, 0.41, 0.48 and 0.43 mm. These align with reported DLG ranges (Med.Phys.48(7):3413-3424, 2021) of 0.12–0.38 mm (distal) and 0.36–0.48 mm (proximal) across five Halcyons. The observed mean inter-system DLG difference was 0.1 mm for both layers, exceeding typical MLC calibration tolerances. Average leaf transmission was 0.34% (distal) and 0.32% (proximal), with minimal variations (0.01%). Despite these differences, all systems share a single, non-adjustable TPS model in Eclipse, with identical DLG values across machines. Furthermore, service-mode tools also lack user access to layer-specific offsets, preventing dosimetric harmonization between systems.
Conclusion: Our work demonstrates measurable DLG variations between Halcyon/Ethos systems, potentially impacting dose delivery consistency when transferring plans between machines. The inability to the introduce a user-defined offset in service mode limits cross-system dosimetric matching, particularly for clinics relying on multiple Halcyons/Ethos. Enabling user-configurable DLG offsets in service mode—akin to conventional linacs—would improve agreement between systems.