ESTRO 2020 Abstract book

S103 ESTRO 2020

investigated the absorbed dose calculated with Acuros XB (AXB) and Geant4/GATE 4 Monte Carlo (MC) in terms of dose-to-medium and dose-to-water. Material and Methods The study was carried out on heterogeneous phantoms constituted of a multilayer slab of low (lung) and high- density media (bone and PTFE). The AXB and GATE calculations were investigated and compared with experimental measurements using Gafchromic EBT3 (External Beam Therapy 3) films. These latter were chosen because they were supposed to generate low fluence perturbation due to their small thickness. Film measurements were also compared to D w obtained by applying the method conversion suggested by Andreo 1,2 and Reynaert et al 3 . Results For D m , a good agreement was found between AXB and MC for all configurations (±1%). For D w , AXB agreed with MC in lung (0.7%) whereas significant differences were observed in both bone (4.5%) and PTFE (3.5%). D w calculated by AXB and MC were greater than film measurements in high- density media: average discrepancies of 9% and 5% were observed in bone for AXB and MC, respectively. These differences decreased significantly (<1%) by applying the conversion method suggested by Andreo 1,2 or Reynaert et al 3 . 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.

mean dose difference between calculated and measured output in the beam central axis, for rectangular fields with one side at 0.5 cm, and the other varying from 1 to 3 cm in a number of different spot size configurations.

The configuration with effective spot size X and Y set to 1.5 and 0 mm is recommended from the vendor. A better compromise in the here presented extreme conditions would suggest to set an X spot size smaller than the recommendation. Conclusion The effective spot size has a strong impact on the MU calculation for very small fields shaped by the MLC. The adjustment of those parameters according to accurate measurements is advisable. PD-0187 Converting absorbed dose-to-medium to dose-to-water in heterogeneous media A. Delbaere 1 , T. Younes 2 , M. Chauvin 1 , L. Simon 1,2 , C. Khamphan 3 , L. Vieillevigne 1,2 1 Centre de Recherche et de Cancérologie de Toulouse, UMR1037 INSERM - Université Toulouse 3 - ERL5294 CNRS, Toulouse, France ; 2 Institut Claudius Regaud - Institut Universitaire du Cancer de Toulouse, Department of Medical Physics, Toulouse, France ; 3 Institut Sainte Catherine, Department of Medical Physics, Avignon, France Purpose or Objective The conversion from absorbed dose-to-medium (D m ) to dose-to-water (D w ) usually based on the Bragg-Gray cavity theory has been recently discussed. Andreo 1,2 proposed to modify this theory by adding a correction factor to take into account the difference of the electron fluence between water and the various media. In Reynaert et al 3 , it was proposed to use average mass energy absorption coefficient ratios instead of average mass collisional stopping power ratios for bone. In this context, we

Poster discussion: PH: Treatment planning: applications 1

PD-0188 Artificial intelligence-assisted full-process solution for rectal cancer radiotherapy

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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