3075 - High-Speed Static Proton ARC Therapy Achieving Sub-100-Second Delivery in a Single-Room Compact System

Purpose/Objective(s):

Democratizing proton therapy requires reducing cost while increasing patient throughput. The MEVION S250-FIT system with upright radiation therapy represents one of the most compact clinical proton platforms, yet maximizing beam utilization in single-room facilities remains critical. We investigate static proton arc (ARC) delivery combined with in-house developed fast delivery strategies for the MEVION S250i and S250-FIT systems, targeting sub-100-second total ARC beam delivery.

Materials/Methods:

Treatment plans were generated in Ray Station 2024B using a validated MEVION S250-FIT beam model. A Phantom study comparing three-field IMPT and static ARC plans was followed by evaluation in five head-and-neck cases. Delivery time was reduced using three complementary strategies.

(1) Integrated Shoot-Through (ST) and Bragg-Peak (BP) beams: High-energy ST beams were placed at the tumor periphery, while conventional BP beams were used centrally to preserve depth conformity and sharpen the lateral penumbra. Plans were compared with adaptive-aperture BP plans using conformity index (CI), homogeneity index (HI), and organ-at-risk sparing metrics.

(2) Nearest-Neighbor (NN) scanning optimization: Conventional line-by-line spot sequencing was replaced with a dynamic nearest-neighbor algorithm selecting the closest undelivered spot within each energy layer. By minimizing large magnet transitions (>7.8 mm), in-layer scanning time was reduced by >80%.

(3) Two-pulse feedback dose control: A simplified regulation scheme replaced the standard four-step feedback process. An initial pulse delivers ~85% of the prescribed charge, followed by a corrective pulse, maintaining dose accuracy within ±2% while shortening beam-on time.

System log data from demonstration measurements at Mevion (Littleton, MA) quantified realistic timing parameters. These values were incorporated into a comprehensive delivery-time model including proton nozzle and upright positioning dynamics.

Results:

Across phantom and clinical cases, ARC plans showed improved CI, HI, and robustness compared with multi-field IMPT. The combined ST+BP strategy enabled complete ARC beam delivery within <100 s for all cases, corresponding to >90% time reduction relative to conventional IMPT (including field change time). NN scanning reduced lateral scanning time by ~80%, while two-pulse feedback preserved precise dose control. Estimated total delivery time, including energy switching, remained below 5 s per ARC angle.

Conclusion:

High-speed static proton ARC delivery within 100-seconds is feasible on existing clinical systems without hardware modification. This framework enables robust, precise, and cost-effective sub-100-second ARC delivery, substantially improving patient throughput and patient comfort. To our knowledge, this represents the fastest proton ARC delivery reported worldwide and demonstrates that fast arc therapy is achievable on compact single-room clinical systems.