ESTRO 2023 - Abstract Book

S189

Saturday 13 May

ESTRO 2023

Three concurrent effects were observed: (a) lateral beam deflection of all spots, (b) asymmetric trapezoidal deformation of the radiation field (Figure 1), and (c) deformation and rotation of individual dose spots. The lateral deflection was energy-dependent and consistent for both field s within the uncertainty of the measurements (Table 1). The field deformation was more pronounced for the 30 × 20 cm ² field than for the 15 × 15 cm ² field, indicating a field dependence. At P_isoc for the 15 × 15 cm ² field 100 MeV beams the σ _max decreased by up to 3.66%, while σ _min increased by a maximum of 2.15%. The dose spot eccentricity underwent minor changes with a maximum decrease and increase in ε of 0.08 and 0.02, respectively. The spot orientation changed by a maximum θ of 5.39°. Similar effects were observed at the higher proton energies but to a lesser extent.

Conclusion For the first time, the 2D dosimetric impact of scanned proton pencil beams traversing the B0 imaging and fringe field of an in-beam MR prototype on the proton beam path, radiation field shape, and dose spot form has been measured in air. The results demonstrate the complex energy- and position-dependent transport behaviour of the pencil beams that requires the 3D B0 field to be taken into account by future MRiPT treatment planning systems. Further investigations are mandatory to assess the dosimetric effects of the B0 field on proton beams delivered with range shifters positioned inside the B0 field and on beams delivered in homogeneous and inhomogeneous target volume media. PD-0253 Ultra-compact and highly efficient proton therapy: Design considerations and clinical simulations V. Maradia 1,2 , D. Meer 1 , S. van de Water 3 , W. Verbakel 3 , D.C. Weber 1,4,5 , A.J. Lomax 1,2 , S. Psoroulas 1 1 Paul Scherrer Institute, Center for Proton Therapy, Villigen PSI, Switzerland; 2 ETH Zurich, Physics, Zurich, Switzerland; 3 Amsterdam UMC, Vrije Universiteit Amsterdam, Cancer Center Amsterdam, Department of Radiation Oncology, Amsterdam, The Netherlands; 4 University Hospital Zurich, Department of Radiation Oncology, Zurich, Switzerland; 5 University Hospital Bern, University of Bern, Department of Radiation Oncology, Bern, Switzerland Purpose or Objective Proton therapy may lower the chance of causing secondary tumors or impairing white blood cells and the immune system compared to conventional radiation therapy. Despite this, the use and expansion of proton therapy is controversial, primarily due to the substantial costs associated with the technique. To reduce the cost of proton therapy, we propose a solution of a gantry less and ultra-compact proton facility. Here, we describe the detailed design and dosimetric performance of two ultra-compact and highly efficient solutions for cyclotron-based, gantry-less proton therapy facilities. Materials and Methods