Room: Exhibit Hall | Forum 4
Purpose: The Halcyonâ„¢ linac has a single flattening-filter-free (FFF) photon beam. For simple beam geometries (such as single fields, parallel opposed, or four-field box geometries) an approach similar to a dynamic wedge is proposed (i.e. pre-defined MLC sequences which flatten the beam with the proximal layer and shape the aperture with the distal layer) to produce â€œ3Dâ€?-like plan geometries which do not require inverse planning. This study presents results of measurements and Eclipseâ„¢ treatment planning system (TPS) predictions for dynamically flattened beams (DFB) on Halcyonâ„¢ linac.
Methods: A pre-clinical Halcyon 2.0 linac was used to deliver the pre-defined leaf sequences for DFB plans ranging from 4x4 to 26x26 cmÂ². The DFB plans were generated with a pre-release Eclipse TPS (v15.6). The flatness (i.e. maximum dose variation within 80% of the field) is defined at 100 cm source-to-surface distance and 10 cm depth in water. The DFB profiles were measured using Octavius1500 2D panel array (PTW) with 9.2 cm solid water buildup. Verisoft 6.2 has been used for data acquisition and gamma analysis.
Results: The predicted DFB flatness was within 2.5% for all fields considered in this study. The passing rates for the 2%/2mm/global gamma (with 10% dose threshold) were greater than 96% for fields ranging from 10x10 to 26x26 cmÂ² and above 93% for fields from 4x4 to 10x10 cmÂ². The DFB flatness was considered clinically acceptable.
Conclusion: The flatness achieved dynamically on Halcyonâ„¢ linacs using a sweeping leaf sequence involving one stack of the dual multi leaf collimator was found comparable with the flatness achieved on conventional linacs that have a flattening filter in the beam-line. This approach reduces the need for inverse planning for simple parallel opposed beam geometries commonly used in palliation and urgent treatments.