ESTRO 2020 Abstract book

S768 ESTRO 2020

every map of dose, the mean signal from 1x1 cm ROI was obtained. For one dose map, mean signal from 1x1 cm ROI was read ten times. Results The mean differences between the two film irradiated simultaneously in phantom measurement was 0,2%±0,9%. During treatment sessions mean differences between two films were: 0.8% ±1.3%. The mean difference between measured and calculated with TPS doses for all patients was 2.6%±6%, 4.8%±4.7%, 4.6% ±7,7%, 5.1±4,3% in A, B, C, D respectively. For single patients, the mean difference between measured and calculated dose in A, B, C, D points was in range from -15.5%±12.3% up to 25,5%±8.3%. To access the repeatability of treatment, the normalized value of the mean dose for every patient was used. Results fiveteen on eighty eight cases exceed 3SD total measurement uncertainty. Results for point located on the central axis (B) are presented in Fig 1.

computational head models in a rapid manner that is robust even when MRI image quality is restricted. Material and Methods We created our patient models using a detailed head model of a healthy person as a deformable template. After pre-processing (denoising and reducing background noise, if needed: super-resolution algorithms), we manually segmented and masked the tumor to leave only healthy tissue in the MRI. This is then registered to the template space transforming the patient space to template space. In the next step, the template is deformed into the patient space by inverse transformation before placing back the tumor to create the full patient model. Landmarks on the patient’s head are automatically identified for positioning of the transducer arrays to include them into the model. We then simulated field distribution with the Finite Elements Method (Sim4Life V3.0, ZMT-Zurich). Results We simulated distribution in 340 TTFields-treated patients of the EF-14 trial that led to the therapy approval in GBM. Our method enables to accurately contour tissues known to have a great impact on the electric field distribution such as the skull, scalp, CSF, or ventricles, a fundamental prerequisite for the subsequent study investigating correlation of TTFields spatial distribution and patient outcome. Conclusion Our method for fast patient-specific model creation enabled us to rapidly create patient-specific head models even when the quality of the available MRI images was low. This ultimately prepared the ground for investigations of correlations between spatial electric field distribution and disease progression. 1 Maria Sklodowska-Curie Institute – Oncology Center, Medical Physics Department, Warsaw, Poland ; 2 Erasnus MC- University Medical Center, Department of Radiation Oncology, Rotterdam, The Netherlands Purpose or Objective In the new protocol, recently implemented in our clinic, unresectable and partially resectable soft tissue sarcomas (STS) were treated with neoadjuvant chemotherapy with hypofractionated radiotherapy before surgery. Usually, STS is located under the skin which is often included in the PTV. That is why in many cases boluses are used. The purpose of this work was to investigate the possibility of using Gafchromic EBT-3 films (EBT-3) for measuring the skin dose and verification of treatment repeatability. Material and Methods Twenty-two patients with STS were treated with IMRT and VMAT plans (Eclipse v.13.6, Varian, Palo Alto). The dose of 25 Gy was delivered in five fractions with 6X or 15X photon beams. Due to shallow tumor location and skin infiltration all patients were irradiated with individual bolus. In-vivo measurements were done with EBT-3 at four points on the skin surface, all located under the bolus: (B: central axis, A, C: shifted in and opposite to gantry direction, respectively and D: shifted right or left from B). For every session, two pieces of 2x2 cm EBT-3 were placed together, one on another, in previously described points., The films were laid on skin before the bolus was placed and CBCT acquisition was performed. Films were scanned using EPSON V750PRO flatbed scanner. The multichannel method was used. The average signal in the 1x1 cm region of interest (ROI) was read. Measured dose values were compared with dose from adequate ROI calculated in TPS . To assess total uncertainty phantom measurements were performed . Ten 2x2cm films (5 pair) were irradiated 5 Gy dose. All films with blank and calibration film were scanned (next day). Map of doses was obtain using homemade software. It was repeated 10 times. From PO‐1358 In vivo measurements in sarcoma radiotherapy A. Walewska 1 , M. Gizynska 2 , P. Kukolowicz 1

Conclusion Repeatability of dose reading was checked for EBT-3 in phantom measurements, confirming usability for in-vivo dosimetry. EBT-3 in vivo measurements allowed to verify patient skin dose and indirectly also treatment repeatability . Influence of difference between irradiated and planned dose in regions that underwent resection in further treatment should be investigated. PO‐1359 Evaluation of a Beam Model for Stereotactic Radiotherapy using a 2D Semiconductor Array A. Roeser 1 , E. Ulmer 1 1 Helios Universitätsklinikum Wuppertal, Klinik für Strahlentherapie, Wuppertal, Germany Purpose or Objective Stereotactic irradiation requires high precision in both imaging and dosimetry. For quality assurance of these requirements Sun Nuclear has developed the chamber array SRS MapCHECK together with the phantom StereoPHAN. The StereoPHAN is a multipurpose Phantom made of PMMA, which includes various inserts for determining geometric and dosimetric accuracy. The agreement between the planned dose using a special stereotactic beam model for the TPS and the measured dose with the SRS MapCHECK inside the StereoPHAN was investigated. Material and Methods The delivered dose was measured using the SRS MapCHECK, which is a 2D array of 1013 semiconductor detectors. Absolute dose and profile measurements of quadratic fields (same as for measuring output factors) as well as for different MLC field configuration at fixed jaws were performed. In addition, conformal-arc plans and VMAT plans for phantom based PTV as well as for real patients were verified. Results The measurements of the output factors for quadratic fields (1x1cm2 to 8x8cm2) showed a high agreement