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Bidirectional Leaf Trajectory Optimization Approaches for Dynamic Delivery of Intensity-Modulated Radiotherapy Plans

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S Mirzapour

S Mirzapour*, E Salari , Wichita State University, Wichita, Kansas


WE-RAM1-GePD-T-2 (Wednesday, August 2, 2017) 9:30 AM - 10:00 AM Room: Therapy ePoster Lounge

Purpose: Traditionally, unidirectional leaf-sweeping schemes have been used for dynamic delivery of intensity-modulated radiotherapy (IMRT) plans. This research investigates the potential benefits of bidirectional MLC-leaf trajectories to delivery efficiency and beam-modulation quality particularly when the available beam-on time is limited.

Methods: This research relaxes the unidirectional leaf-motion restriction and develops exact and heuristic leaf-sequencing approaches to obtain optimal bidirectional leaf trajectories. In particular, the proposed exact and heuristic approaches allow the MLC leaves to start from an arbitrary or pre-specified initial position and move freely across the radiation field to modulate the desired fluence map. The exact approach employs mixed-integer programming techniques to solve the arising non-convex optimization problem to a guaranteed optimality gap. The heuristic methods exploit the special structure of the problem to obtain near-optimal leaf trajectories, while requiring a significantly smaller computational effort.

Results: The trade-off between the fluence modulation quality and the required beam-on time is quantified for fluence maps with different complexity levels using the new and traditional leaf trajectory optimization methods. The results show the superior performance of bidirectional leaf trajectories over unidirectional ones, where the largest gain is observed when the available beam-on time is very limited.

Conclusion: In this research, exact and heuristic leaf-sequencing approaches were developed and tested for dynamic IMRT delivery using bidirectional leaf-motion trajectories. The trade-off between beam-modulation quality and delivery efficiency for both unidirectional and bidirectional leaf-motion trajectories were quantified and compared. The results validate that bidirectional leaf trajectories may lead to significant improvement in fluence-modulation quality over unidirectional leaf trajectories particularly at short beam-on-time values.

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