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Initial Evaluation of the Multileaf Collimator Motion Tracking System On a TrueBeam Accelerator

L Zhang

L Zhang*, T LoSasso , P Zhang , M Hunt , G Mageras , G Tang , Memorial Sloan-Kettering Cancer Center, New York, NY


SU-K-605-14 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 605

Purpose: Real-time motion adaptation through MLC tracking is a promising method to compensate for target motion during treatment. It has the potential to improve dose conformity without introducing frequent beam interruption that often results in lengthening of treatment time. This study evaluates the characteristics of the MLC motion tracking system installed on a Varian TrueBeam accelerator in Developer Mode.

Methods: Phantom experiments facilitated with MV/kV imaging were designed to investigate the tracking latency and accuracy during treatment. A respiratory motion simulation platform was used to drive an anthropomorphic phantom with embedded gold markers and reproduce programmed one-dimensional periodic motion. The RPM optical marker block was placed on top of the phantom as the surrogate for tracking. A static and a dynamic plan were delivered with MLC tracking using different tracking speed and dose conformity tolerance. Machine trajectory logs were obtained for all deliveries. Continuous MV and kV image pairs were acquired throughout the treatment. Images were sorted retrospectively and registered to the corresponding reference fiducial template images to determine the instantaneous phantom position. Reconstructed Motion Traces were then compared to the instantaneous leaf position in the machine trajectory log.

Results: Analysis on Reconstructed Motion Traces and machine trajectory logs showed that the MLC was able to continuously track and compensate for target motion with speed up to 2.5 cm/s parallel to leaf motion. The tracking latency was quantified as the time delay between the response of MLC and any target motion. An average latency of 95 ms was found at any time point when tracking 1-D periodic target motion.

Conclusion: This work demonstrated the capability of real-time motion tracking through MLC compensation on a TrueBeam. The tracking performance was evaluated with image-based phantom studies using clinically relevant inputs. Further evaluation using patient prostatic or lung motion traces is warranted.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by Varian Medical Systems

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