ESTRO 2021 Abstract Book

S749

ESTRO 2021

magnetic field guides the beam along a circular track around the patient in a guiding field region. At a certain radius, the beam enters the region of the enclosed inner bending field, where a stronger magnetic field bends the beam to or near the isocenter. The patient is located in a field-free central region. The treatment angle is determined by the strengths of the arc-scanning magnets only. Changing the settings of those can be done very rapidly and a substantial arc can be covered within a fraction of a second. This opens the way to new fast delivery techniques, such as pencil beam scanning (PBS) arc therapy or recently proposed PMAT (Proton Monoenergetic Arc Therapy) to enhance the dose-averaged linear energy transfer (LETd) distribution in order to increase the relative biological effectiveness within the target volume. Our device enables non-standard sequences of dose delivery, such as applying an arc several times with a different energy and intensity profile. We also consider hypothetical conformal delivery with ultra-high dose rates (FLASH).

Figure 1: Layout of the device and its operation principle.

Results Our simulations have confirmed the feasibility of delivering pencil beams to the isocenter from all angular directions by changing the setting of scanning magnets only. The same magnets are used for pencil beam scanning. We identified and tested adequate solutions to address several beam-optics challenges. With a bending field of 2.1 T, the 1 m long device will have a radius of 4.4 m. By utilizing superconducting magnets and increasing the field to 3.1 T, the radius can be decreased to 2.6 m. We discuss how this device can be deployed for new delivery techniques considered in proton therapy. Conclusion The static device we proposed enables new delivery techniques. Fast arc proton therapy, especially with a new technique called PMAT, is feasible with this new device. Our design might be also a good candidate for hypothetical future FLASH treatments with protons. PD-0907 Modeling the first proton arc delivery sequence and investigating its efficiency improvement X. Ding 1 , G. Liu 1 , L. Zhao 1 , D. Yan 1 , R. Deraniyagala 1 , C. Stevens 1 , X. Li 1 1 Beaumont Health, Radiation Oncology, Royal Oak, USA Purpose or Objective Spot-scanning proton arc (SPArc) has been designed to improve the plan quality and delivery-efficiency for proton beam therapy. With the merging of dosimetric studies to demonstrate the needs of SPArc therapy, there are few investigations into the mechanical characteristics and treatment time due to the limited access to the prototype arc system. We a first experimental approach to model a precise prototype arc system and quantitatively assess its efficiency improvement in the routine proton clinical operation. Materials and Methods The SPArc delivery sequence model(DSM SPArc ) includes two kinds of parameters:(1) mechanical parameters (the maximum gantry velocity, acceleration, and deceleration speed). (2) irradiation parameters (tolerance window and buffer, spot scanning speed, energy layer switching time, and burst switching time). An independent gantry inclinometer was used to measure mechanical parameters. A series of test SPArc plans were designed and delivered using the prototype proton arc system where the machine log files were used to derive the irradiation parameters. The in-house DSM SPArc was established by fitting both mechanical and irradiation parameters. Eight SPArc plans from different disease sites (brain, headneck, lung, and liver cancer) were used to validate the model's accuracy. To quantitatively assess the treatment efficiency improvement compared to the clinical IMPT, a random clinical operation date of our proton center (total 21 cases on Jan 6 th 2021) was selected, and SPArc plans were generated for all the cases. The DSM SPArc was used to simulate the SPArc delivery sequence and treatment time compared to the clinical IMPT treatment logfiles. Results The relative difference of treatment time between log files and DSM SPArc ‘s prediction was 6.1%±3.9% on average, and the gantry angle vs. delivery time showed a good agreement between the DSM SPArc and log file (Figure 1). Additionally, the SPArc plan could effectively save two hours out of ten hours of clinical operation by simplifying the treatment workflow for a single room proton therapy center. The average treatment delivery time (including gantry rotation and irradiation) per patient was reduced to 226±149s using SPArc compared to 665±407s using IMPT (p<0.01). (Figure 2)