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Session: Tracking Strategies [Return to Session]

Simultaneous Temporal and Dose Optimization for MLC Tracking: Cutting Dose Errors in Half

L Mejnertsen1*, E Hewson1, J Booth2,3, P Keall1, (1) ACRF Image X Institute, University of Sydney Medical School, Sydney, NSW, Australia (2) Northern Sydney Cancer Centre, Royal North Shore Hospital, Sydney, NSW, Australia (3) School of Physics, University of Sydney, Sydney, NSW, Australia


MO-H345-IePD-F5-1 (Monday, 7/11/2022) 3:45 PM - 4:15 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 5

Purpose: Intrafraction motion causes a reduction in dose delivered to the target, while increasing dose to healthy tissue. Multi-Leaf Collimator (MLC) tracking has been clinically implemented to adapt for intrafraction motion. Previous advances in MLC tracking include temporal optimization to overcome the greedy heuristic nature of current algorithms, and dose optimization to directly adapt the plan based on the most clinically relevant metric. In this work, we demonstrate and investigate simultaneous temporal and dose optimization for MLC tracking.

Methods: A new method has been developed wherein the sequence of MLC apertures ahead of the current treatment time is optimized to account for intrafraction motion. The sequence is optimized based on previously delivered dose and dose to be delivered within the sequence, computed using a simplified pencil beam kernel. As treatment progresses, new apertures from the treatment plan are added to the optimization, while delivered apertures are removed. In this work, we investigate the impact of increasing the number of apertures to optimize, with each aperture spanning 100ms. The method is applied to a prostate VMAT plan, with a static shift (5mm SI) to the dose volume. The method is quantitatively assessed using dose error and compared to the case of geometry-based MLC tracking and no tracking.

Results: Temporal dose optimization achieved the lowest dose error of 3.3% or less. When only considering 1 to 4 apertures (100-400ms ahead of time), the dose error was 3.2%-3.3%. However, 8 apertures (800ms) produced the lowest error of 2.4%. These values are smaller than using geometric tracking (7.8%) and without tracking (31.5%).

Conclusion: Simultaneous temporal and dose optimization for MLC tracking has been demonstrated. The results show that the new method halves the dose error between the delivered and planned dose when compared with a recent clinically implemented method.

Funding Support, Disclosures, and Conflict of Interest: The authors acknowledge funding provided from a Cancer Council NSW Project Grant RG19-10


DMLC, Dose, Radiation Therapy


Not Applicable / None Entered.

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