ESTRO 2021 Abstract Book

S748

ESTRO 2021

Fig 1: Instantaneous dose rate (right axis, in Gy/s)delivered versus time for one voxel in the irradiation field. The cumulated dose (left axis, in: Gy) along the time is also represented. Different sequences of spot scanning have been considered to cover the irradiation field size, using several parameters of the proton delivery system: the speed of the scanning magnet to move between spots, which is not uniform and varies depending on the direction of the axis of motion; the spot size, which is important to define the number of spots to be placed to cover the field while maintaining a Flash dose rate; The beam current available in the treatment room which will impact the spot delivery time; finally, the field size and shape representing some constraint to maintain the FLASH dose rate. Using open-source tools, MIROPT(http://openmiropt.org/), and MCSQUARE (http://www.openmcsquare.org/), we computed some treatment plans with realistic values of the machine parameters Results The results shows an impact of the scanning pattern and the proton therapy delivery system parameters on the dose and the dose rate delivered. Figure 2 illustrates a dose and dose rate map obtained with one specific scanning pattern., where we can see areas with very high dose rates (4cm wide), althoughthe limited number of spots lead to a non-uniform dose map. This figure shows an example in which the pattern is repeated twice with an overlapping area with lower dose rate.

Fig 2 : The figure overlay the dose and dose rate. The dose map (in Gy) is represented as continuous contour lines. The dose rate (Gy/sec) in overlaid as a color map with iso dose-rate line represented as dashed line.

Conclusion This work shows how the choice of the scanning pattern is critical for FLASH with PBS proton therapy by impacting the dose and dose rate distribution for certain given field sizes.

PD-0906 A static device for novel fast beam delivery techniques in proton therapy K. Nesteruk 1,2 , A. Lomax 3,4 , A. Bolsi 3 , D. Meer 3 , S. van de Water 3 , M. Schippers 2

1 Massachusetts General Hospital and Harvard Medical School, Department of Radiation Oncology, Boston, USA; 2 Paul Scherrer Institute, Division Large Research Facilities, Villigen, Switzerland; 3 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland; 4 ETH Zurich, Department of Physics, Zurich, Switzerland

Purpose or Objective To enable new fast proton beam delivery techniques by means of a static device. Materials and Methods

We conceived a new beam delivery device, which consists of a system of fixed and static magnets to deliver the beam to the isocenter from a continuous range of angular directions. In contrast to conventional gantries, there is no need to move large magnets to choose the desired treatment angle (figure 1). Instead, a static