ESTRO 2022 - Abstract Book

S640

Abstract book

ESTRO 2022

Conclusion We proposed an approach to adapt the ViTAT technique using a 6MV FFF beam on Halcyon. The 20° beam delivery angle originally set for the ViTAT is not sufficient to achieve clinically acceptable coverage of the PTV. Increasing the delivery angle to 30°/40° resulted in optimal PTV coverage, maintaining similar sparing of the OARs. The clinical implementation of fully automatic plan optimization for R whole breast with Halcyon by this adapted ViTAT is ongoing.

PD-0729 Variable RBE for a proton therapy class solution in a randomised trial for high-risk prostate cancer

K. Lie Aas 1 , R. Klitgaard 1 , A. Vestergaard 1 , H. S. Rønde 1 , S. Tilbæk 1 , L. Stolarczyk 1 , T. Stagaard Johansen 1 , S. E. Petersen 1 , L. Bentzen 2 , L. P Muren 1 1 Department of Clinical Medicine, Aarhus University, Danish Center for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark; 2 Department of Clinical Medicine, Aarhus University, Department of Oncology, Aarhus University Hospital, Aarhus, Denmark Purpose or Objective Pelvic lymph node irradiation is recommended in patients with high-risk prostate cancer. Due to the large treatment volumes, these patients could benefit from the normal tissue sparing that might be achieved with proton therapy. However, the influence of the proton relative biological effectiveness (RBE) on normal tissue doses for this treatment site has not previously been studied. We are launching a multi-centre randomised clinical trial to compare front-line photon-based radiotherapy with pencil-beam scanning (PBS) proton therapy for high-risk prostate cancer. The aim of this study was to explore the implications of potentially variable RBE for the proton therapy class solution that will be used in the trial. Materials and Methods The proton therapy class solution consisted of four PBS beams, including two lateral beams tilted slightly posterior to avoid the femoral heads (gantry angles 100°/260°) and two posterior beams angled slightly laterally to avoid the rectum (gantry angles of 170°/190°). The two nearly lateral beams covered the high-dose target volume and in addition the lymph nodes on either side. The two nearly posterior beams covered the entire target with prostate, seminal vesicles and lymph nodes on both sides. Treatment planning of eight test cases was done in Eclipse, while dose and linear-energy transfer (LET) distributions were re-calculated using Monte Carlo simulations (TOPAS v3.5/Geant4 v.10.6) for our beamline (Fig. 1). Biological doses with published variable RBE models (Carabe, Wedenberg, McNamara) as well as the LET-weighted dose (LWD) (Fig. 1c) were compared to the clinically applied constant RBE (RBE=1.1). The analysis focused on the bowel, bladder and rectum, also taking the uncertainty in the model input parameter α / β into account (±2 Gy around the point estimates). The differences between different RBE models and between different α / β values were assessed at dose levels corresponding to the dose/volume constraints of the trial. Results With both LWD and even more so with the LET-based variable RBE models, the biological doses in the bowel, bladder and rectum increased compared to constant RBE (Fig. 2a). The Wedenberg and Carabe RBE models resulted in the highest doses. At the dose levels of the normal tissue constraints, volume increases of 0.3-8% were seen, largest for the bowel. For each