ESTRO 2020 Abstract Book

S93 ESTRO 2020

Conclusion The conversion implemented in AXB and MC provides different D w in high-density media which might lead to several issues in clinical context. In addition, our experimental measurements showed that film measurements appear to be unable to fulfill the conditions of the Bragg-Gray cavity theory in high-density media but agree with the Andreo 1,2 and Reynaert et al 3 conversion proposals. 1. Andreo P. 2014 Dose to ‘water-like’ media or dose to tissue in MV photons radiotherapy treatment planning: still a matter of debate Phys. Med. Biol. 60 309–37 Andreo P. 2018 Monte Carlo simulations in radiotherapy dosimetry Radiat. Oncol. 13 121 Reynaert N. et al. 2018 On the conversion of dose to bone to dose to water in radiotherapy treatment planning systems Phys. Imaging Radiat. Oncol. 5 26–30 Sarrut D. et al. 2014 A review of the use and potential of the Gate Monte Carlo simulation code for radiation therapy and dosimetry applications Med Phys. 41 064301 2. 3. 4. PD-0189 Individualized Beam Angle Selection for MR- Linac Treatment of Rectal Cancer Patients R. Bijman 1 , L. Rossi 1 , T. Janssen 2 , P. De Ruiter 2 , B. Van Triest 2 , S. Breedveld 1 , J. Sonke 2 , B. Heijmen 1 1 Erasmus MC Cancer Center, Radiation Oncology, Rotterdam, The Netherlands ; 2 the Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The Unity MR-Linac (MRL; Elekta AB, Stockholm, Sweden) offers high accuracy MR-guided dose delivery to rectal cancer patients, suffering from large day-to-day anatomical variations. However, the unavailability of VMAT requires beam angle selection for static IMRT. In this study we used an in-house system for fully automated, integrated optimization of IMRT profiles and beam geometries to investigate the dosimetrical impact of beam arrangement in the treatment of rectal cancer. Material and Methods The TPS for automated planning was configured for rectal MRL treatments with reduction of the D mean in a composite OAR (small-bowl-bag + bladder excluding PTV overlap) as main OAR objective, after PTV coverage. For 23 rectal cancer patients, treated with 25 fractions of 2 Gy, three types of plans were automatically generated: 1) BAO L : plans with L=7-12 individualized beam angles, generated with computerized selection from 56 coplanar, equiangular candidate beams, 2) EQUI M : plans with M=9- 56 fixed (non-patient specific)equiangular beam angles, 3) CLASS N : plans with fixed (non-patient specific) beam angle class solutions with N=7-12 beams. The class solutions were established by analyzing the BAO L beam angles of 10 training patients. For all plans, beams passing through the MRL cryostat pipe and the high attenuation parts of the couch were avoided. Prior to dosimetric comparisons, all plans were scaled for equal PTV coverage (PTV V 95% = 99%). The two sided Wilcoxon signed rank test was used for statistical significance testing (p<0.05). Results Poster discussion: PH: Treatment planning: applications 1 PD-0188 Artificial intelligence-assisted full-process solution for rectal cancer radiotherapy Abstract withdrawn

All 437 automatically generated MRL plans fulfilled all clinical requirements. Clinically relevant differences between plans were observed for the D mean of the composite OAR, as shown in Figure 1. Detailed comparisons between plans regarding OAR D mean are presented in Table 1. For all planning approaches (BAO, EQUI and CLASS), large improvements in plan quality were observed for increasing beam numbers; an OAR D mean reduction off 5.6 Gy (31%) between BAO 7 and BAO 12 . Figure 1 demonstrates clear advantages of BAO and CLASS over EQUI, e.g. an average reduction of 2.6 Gy (15%) ([1.0, 8.5], p<0.01) in OAR D mean was observed for the patient-specific 9-beam plans (BAO 9 ) compared to the non-patient-specific equiangular 9 beam plans (EQUI 9 ). Less well-selected beams lead to similar plan quality, e.g. BAO 12 and EQUI 24 had equal OAR D mean (similar for BAO 10 vs EQUI 15 , BAO 11 vs EQUI 19 ). Figure 1 shows equivalence of BAO N and CLASS N plans.

Conclusion Fully automated, integrated optimization of IMRT profiles and beam angles was used to robustly investigate the impact of beam directions and beam numbers on plan quality in rectal treatments with a Unity MRL. With optimized beam angles, plan quality is substantially enhanced compared to equiangular treatment with equal number of beams. Beam angle class solutions were developed that could replace individualized beam angle selection without plan quality loss, avoiding time- consuming individualized beam angle optimization . PD-0190 Update on yesterday’s dose – Use of delivery log files to refine the daily proton delivery M. Matter 1,2 , L. Nenoff 1,2 , L. Marc 3 , A.J. Lomax 1,2 , D.C. Weber 1,4,5 , F. Albertitni 1 1 Paul Scherrer Institute, Center for Proton Therapy, Villigen PSI, Switzerland ; 2 ETH Zürich, Departement of Physics, Zürich, Switzerland ; 3 EPF Lausanne, Departement of Physics, Lausanne, Switzerland ;