ESTRO 2020 Abstract book

S775 ESTRO 2020

(7.74 ± 0.51) mm, specification < 8 mm. Initial MR-to-MV checks indicated isocenter shifts of up to Δx = 0.37 mm, Δy = 0.10 mm and Δz = 0.80 mm, indicating loose fixation of the EPID panel. After re-fixation of the panel holder, the shifts were reduced to Δx = (0.05 ± 0.02) mm, Δy = (0.04 ± 0.04) mm and Δz = (0.02 ± 0.01) mm. The deviation of the central EPID pixel was found to be (0.04 ± 0.06) pixel and (0.11 ± 0.09) pixel, respectively. The radius of the isocenter circle was (0.300 ± 0.137) mm (tolerance < 0.500 mm). Dose deviations concerning the reference measurements for all field sizes were below 1%. An overview of the geometric accuracy of the MRI is shown in Fig. 2.

GI 2%/2m m Global

GI 3%/2m m Global

GI 3%/3m m Global

GI 2%/2m m Local

GI 3%/2m m Local

GI 3%/3m m Local

Mea n

92.5

97.4

96

99.2

98.2

99.6

SD 6.6

4

4.9

2

3

1.5

Conclusion RayStation can be used for the creation of clinical tomotherapy treatment plans. The optimized model produces high values for pre-treatment patient specific QA, also using tighter GI than the ones suggested by TG- 218. If compared with data reported in TG-218, the collected GI 3%G/2mm outperforms previous published results. Based on the presented results the implementation in clinical routine can be considered feasible and safe. PO‐1369 Stability of machine QA parameters for a 1.5 T MR‐Linac for the first year after installation J. Winter 1,2 , D. Mönnich 1,2,3 , M. Nachbar 1 , L.A. Künzel 1 , D. Zips 2,3 , O.S. Dohm 2 , D. Thorwarth 1,3 1 Section for Biomedical Physics. Department of Radiation Oncology, University Hospital and Medical Faculty. Eberhard Karls University Tübingen, Tübingen, Germany ; 2 Department of Radiation Oncology, University Hospital and Medical Faculty. Eberhard Karls University Tübingen, Tübingen, Germany ; 3 German Cancer Consortium DKTK- partner site Tübingen, and German Cancer Research Center DKFZ, Heidelberg, Germany Purpose or Objective To assess long-term stability of machine QA results for the hybrid 1.5T MR-Linac (MRL) during the first year of clinical treatments. For a hybrid system as the Unity MR-Linac (Elekta AB, Stockholm, Sweden), checks regarding accelerator and imaging characteristics have to be monitored in dedicated time intervals. Material and Methods Routine machine QA at the 1.5T MR-Linac (MRL) was performed using the following protocol. Dose calibration in water (Stationary Water Phantom MR, PTW-Freiburg, Germany) using a Farmer type chamber (PTW 30013), beam quality factor Q (ratio of dose measured in 20 cm vs. 10 cm water depth) and a Periodic Image Quality Test (PIQT) with a special MR phantom (Philips Oy, Vantaa, Finland) for basic MR system performance, including Signal-to-Noise Ratio (SNR) and slice profile measurements evaluating image intensity with respect to Full Width at Tenth Maximum (FWTM), are checked weekly. The accordance of MR and accelerator isocenters (MR-to-MV) using a dedicated phantom (Elekta), a check of electronic portal imaging device (EPID) central pixel position to map the MV isocenter stability (MV Alignment Phantom, Elekta), dose profile measurements for field sizes of 2x2 cm 2 , 4x4 cm 2 , 10x10 cm 2 , 30x20 cm 2 and 40x22 cm 2 (STARCHECK maxi, PTW-Freiburg) and a test for geometric accuracy of MR images (3D Geometry QA Phantom, Philips), are verified monthly. The radius of the MV isocenter circle is calculated with a Winston-Lutz-test quarterly. In this work, the QA results for weekly, monthly and quarterly checks over the first year of operation are analyzed and presented. Results Fig. 1 shows an overwiev of the weekly dose calibration with majority of values within a range of ± 1%. Absolute dose checks resulted in a mean dose difference with respect to the dose defined during calibration of (0.21 ± 0.32)%, Q was 0.705 ± 0.001. For PIQT the SNR reached values of 71.38 ± 1.25 (TE=30ms) and 53.99 ± 0.92 (TE=100ms), specification SNR > 59 or 44, respectively. The slice profile measurements led to values of FWTM =

Conclusion The presented QA data for the Linac and the MR part as well as the tests for the hybrid MR-Linac system showed a high level of stability and robustness. QA tests established on a regular weekly, monthly or quarterly basis seem to be adequate for routine machine QA of a clinical MR-Linac system. PO‐1370 GATE/Geant4 as a Monte Carlo simulation toolkit for light ion beam dosimetry M. Bolsa-Ferruz 1 , H. Palmans 1,2 , A. Carlino 1 , M. Stock 1 , E. Traneus 3 , L. Grevillot 1 1 EBG MedAustron GmbH, Medical Physics Department, Wiener Neustadt, Austria ; 2 National Physical Laboratory, Chemical- Medical and Environmental Science Department, Teddington, United Kingdom ; 3 RaySearch Laboratories AB, Sales Particle Therapy, Stockholm, Sweden Purpose or Objective The growing interest in Light Ion Beam Therapy (LIBT) has fostered the efforts to improve the accuracy of ion beam dosimetry. Solid-state detectors are a good alternative to ionization chambers. To derive dose to water from the detector signal, the water-to-medium stopping power ratio (SPR) and the relative effectiveness (RE) of the dosimeter must be taken into account. Monte Carlo (MC) codes are essential tools for their determination since the complete particle spectra are required. Our aim is to compute these quantities in clinically relevant conditions using a fully commissioned clinical beam model and full clinical delivery sequences. This work intends to support end-to-end testing activities using alanine detectors,