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Dosimetric Leaf Gap and Pre-Treatment Plan Verification for Halcyon Linacs

M Constantin1*, J Mai2 , (1) Varian Medical Systems Inc, Palo Alto, California, (2) Duke University, Durham, NC


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: The newest Varian’s multi-leaf-collimator (MLC) on Halcyon linacs has a dual-stack design with proximal and distal leaves staggered by 5 mm. The purpose of this study is to present a series of dosimetric leaf gap (DLG) measurements on two Halcyon MLC systems using different methods to assess the DLG variation (across machines and across the maximum treatable field) and to investigate the feasibility of having a fixed DLG parameter in the Treatment Planning System (TPS).  

Methods: The setups for DLG measurements were: 1) standard solid water block and external ionization chamber (IBA CC13, PTW Farmer); 2) 1-dimensional setup using a cylindrical phantom (SNC ArcCheck) and external ionization chamber (CC13); 3) 2-dimensional setup utilizing Octavius1500 (PTW) chamber arrays. Static gantry sliding window plans were used for the flat phantoms. For cylindrical phantoms, sliding window plans were converted to a partial arc delivery. The DLG plans for Halcyon were created for distal and proximal leaves (i.e. single stack configuration).

Results: The DLG for setup-1 and 2 was measured to be -0.323 mm ± 0.053 mm (distal) and -0.243 mm ± 0.043 mm (proximal). For setup-3, Matlab code was developed to extract the DLG variation versus x/y-offsets based on zero-charge extrapolation method. The values across the detector area showed small deviations, -0.471 mm ± 0.072 mm (distal) and -0.364 mm ± 0.065 mm (proximal). Machine-to-machine variations were found to be <0.05mm. Setting the DLG to 100 microns in TPS (pre-release Eclipse v15.5) showed 2%/2mm/global gamma passing rates of better than 97% for multiple-field IMRT plans (using PTW Verisoft-6.2 pre-treatment plan verification gamma analysis).

Conclusion: This study consolidates the pre-configured DLG value used in Eclipse for the Halcyon MLCs. The robust leaf design with small (<0.5%) leaf transmission, large leaf-end radius, and negligible machine-to-machine variations justify the small DLG value used in Eclipse.

Funding Support, Disclosures, and Conflict of Interest: Varian Medical Systems Inc


MLC, Modeling, Treatment Planning


TH- External beam- photons: Development (new technology and techniques)

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