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Conference Proceeding

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Medical Physics


Purpose: It was observed on a VersaHD that clinical SRS deliveries created in the Elements treatment planning system (TPS) slowed gantry rotation and dose rate to allow parked MLC leaves to move into position for use in an upcoming control point. Gantry rotation slowed suddenly, visibly backtracking to previous gantry angles. The purpose of this work is to study the effect of parked MLC positioning parameters in Elements on cranial SRS delivery speed and gantry motion on a VersaHD. Methods: To study the effect of parked MLC leaf positions on VersaHD deliverability, cranial SRS plans were generated with different TPS parameters and subsequently delivered on the VersaHD, while monitoring gantry speed and motion. Specifically, the preferred parked leaf position parameter was changed from the clinical ‘left’ position to an experimental ‘middle’ position. The experimental model was used to replan 5 clinical CVMAT and 5 clinical MME plans with identical arc lengths, geometries, and optimization parameters. Patient-specific QA was performed on the CVMAT cases to verify clinically-acceptable deliverability. VersaHD delivery logfiles were used to evaluate dose rate, instances of gantry backtracking, and delivery speed. Results: Reductions in delivery time per arc were statistically significant for both CVMAT (p<0.01) and MME (p<0.05) plans. Reductions in gantry backtracking instances per arc was statistically significant for CVMAT arcs (p<0.01), falling from 73 total instances to 10, and for MME arcs (p<0.05), which fell from 29 to 15. Only one experimental arc had multiple instances of gantry backtracking, as opposed to 27 clinically-delivered arcs. Conclusion: The default parked leaf position in Elements on a VersaHD can have a clinically significant impact on treatment time, and can cause gantry backtracking, compared to more ideal parked positions. It is recommended to set preferred MLC parked position to “middle” in Elements for the VersaHD, especially for CVMAT cases.





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American Association of Physicists in Medicine 65th Annual Meeting & Exhibition, July 23-27, 2023, Houston, TX