(Monday, 7/15/2019) 3:45 PM - 4:15 PM
Room: Exhibit Hall | Forum 3
Purpose: The Halcyon is a straight-through linear accelerator offering a single 6X FFF beam with a jawless design and new dual-layer MLC system. Treatment planning system (TPS) beam data is pre-configured and not user customizable. This study investigated several new dosimetric characteristics for the dual-layer MLC system and compared them with pre-configured beam data.
Methods: Transmission, dosimetric leaf gap (DLG), and tongue and groove effects (TnG) were measured for each MLC layer and combined. Ionization chambers, diode arrays, and an electronic portal imaging device (EPID) were used to measure various MLC characteristics. Measured results were compared to TPS values. Patient specific QA results were analyzed for a number of treatment sites to validate pre-configured MLC characteristics in clinical settings.
Results: Transmission through both MLC banks was found to be 0.008%, while the individual banks had transmission values of 0.4%, compared to TPS data of 0.47%. The penumbra was slightly sharper for fields using only the distal MLC bank, but largely independent of leaf position. The DLG was measured for the proximal and distal MLC banks both individually and combined and found to have values of -0.22mm, -0.23mm, and 0.964mm, respectively, compared to -0.10mm in TPS. Measurements of DLG were also performed at the leaf edge and midline and found to indicate effects of leaf corner rounding. TPS data over-estimates TnG effect. Difference between measured and TPS calculated TnG is smaller (~5%) for the distal layer than proximal (~19%). Patient-specific QA showed >94% gamma analysis passing rate for clinical cases with 3%/3mm criterion.
Conclusion: The new dual-layer MLC system exhibited improved transmission and penumbra, but more pronounced TnG. Current TPS modeling of these characteristics were found to be within a reasonable range from measured data. Dosimetric accuracy for clinical VMAT and IMRT plans were acceptable based on our patient-specific QA.
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