Develop a novel non‐coplanar trajectory optimization algorithm for an efficient 4pi spot‐scanning proton Arc (SPArc ‐4pi ) treatment delivery

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Abstract Background The degree of freedom has been recognized as a key factor in particle beam therapy, such as improving the dose conformity and linear energy transfer distributions. Therefore, proton arc therapy has drawn significant interest in the Society of Radiation Oncology as a potentially efficient and optimal treatment option for cancer patients. However, state‐of‐the‐art spot‐scanning proton arc therapy (SPArc) still relies on single or dual‐ co‐planar arc trajectories, which limits its capacity to advance the treatment outcome further. There is an urgent need to explore particle beam therapy's full potential via 4pi. Purpose This study aims to develop the first 4pi arc optimization algorithm that searches for efficient arc trajectories in 4pi space, explores the potential dosimetric improvements, and demonstrates its feasibility through simulation. Method Dynamic Programming, originally from control theory, was translated into this new concept of the 4pi SPArc optimization algorithm (SPArc ‐4pi ) for the trajectory search and route decision‐making. It breaks down the complicated and high‐computational‐demand main problem into a series of small sub‐problems and searches for delivery‐efficient 4pi arc trajectories through an iterative approach. Five different disease sites, e.g., head & neck, partial brain, clival brain chordoma, lung and pancreatic cancer, were used for testing purposes. Conventional Intensity Modulated Proton Therapy (IMPT) and 2D co‐planar arc (SPArc ‐2d ) plans were generated as a benchmark. Treatment delivery efficiency was evaluated through a published and validated dynamic arc system controller. Plan quality was assessed through target coverage and organ at risk (OAR) sparing. Result The new SPArc ‐4pi demonstrated superior dosimetric performance across various evaluation metrics for all five disease sites. The simulation result shows that SPArc ‐4pi is able to be delivered within a reasonable time of around 5–11 mins, which is comparable to the SPArc ‐2d plans Conclusion The study introduced the first optimization algorithm for SPArc ‐4pi technique. It not only showed SPArc ‐4pi ‘s potential to improve the plan quality via a greater degree of freedom compared to the conventional IMPT and state‐of‐art 2D co‐planar SPArc‐2d technique but also demonstrated its feasibility for future clinical implementation based on the current proton beam therapy system's machine characteristics.

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