ESTRO 2020 Abstract book

S766 ESTRO 2020

The tolerance limits determined based on measurement data for IMRT and VMAT plans expressed in GPR are higher than the action level set analogously, It confirms the stability of the verification process (Tab. 1). Both tolerance and action limits for the VMAT reach higher values than for the IMRT (Fig. 1).

results: PB works well in homogenous district while in low density regions CCC must be used.

Conclusion DC is a promising in vivo tool, and since it is possible to use a CCC algorithm for patients in which low density regions are involved, this system can improve quality assurance, as it provides the added value of the in vivo dosimetry PO‐1354 Are we on the safe side? Action and tolerance limits for verification of IMRT and VMAT plans A. Borda-Ziemacka 1 , K. Zelechowska-Matysiak 2 , M. Gabor 3 , A. Rygielska 1 , D. Pruska-Pich 3 , A. WALEWSKA 4 1 Military Institute od Medicine, Radiotherapy Department- Laboratory of Medical Physics, Warsaw, Poland ; 2 University of Warsaw- Faculty of Physics, Department of Biomedical Physics, Warsaw, Poland ; 3 Military Institute od Medicine, Radiotherapy Department- Laboratory of Medical Physics, Warsaw, Poland ; 4 Military Institute od Medicine, Radiotherapy Department, Warsaw, Poland Purpose or Objective There are two ways to define action limits for pre- treatment verification: accept commonly defined values or determined own values based on collected statistics. The optimal solution is to use action levels adapted to the conditions of treatment. The work aimed to determine the action and tolerance limits based on the AAPM TG 218 recommendations [1] for the results of dynamic plan verification (IMRT and VMAT). Action limit has been described as gamma passing rate (GPR) limits, exceeding of which may be associated with an increased risk of negative effects of patient treatment on a given dynamic plan. Tolerance limits determine the scope of gamma passing rate (GPR), in which the results of the quality control process of dynamic plans are considered to be unchanged. affected only by random errors. Material and Methods The results of verification measurements of 42 IMRT and 127 VMAT plans for 6 MV, collected during 6 months of 2018 were analyzed The pre-treatment measurements were performed with the Octavius 1500 (PTW) matrix placed in the Octavius 4D (PTW) modular phantom system. For each plan, measurements from individual fields/arcs were added and after reconstruction, the cumulative (composite) dose distribution in the phantom volume was compared (volume analysis option) in Verisoft v.7.1 (PTW) with the TPS Monaco v.5.1 (Elekta) calculations. Gamma analysis was performed for 3mm /3% parameters (max. and local dose) with 5%.threshold. Based on the results of the gamma passing rate (GPR) and AAPM TG 218 guidelines, tolerance and action limits for the pre-treatment verification of IMRT and VMAT performed in our department were determined. Results

Conclusion The determined values of action and tolerance limits will allow better monitoring of the verification process. In the event of a change in the system/verification method or gamma analysis parameters or cut-off threshold, tolerance and action limits should be re-determined based on the new data collected after the changed method/parameters have been implemented. [1] Miften et al.: Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218 , Med.Phys. 45(4) April 2018 http://dx.doi.org/10.1002/mp.12810 PO‐1355 Analyzing Tumor Treating Fields (TTFields) delivery by Water‐based electrical properties tomography Z. Bomzon 1 , A. Kinzel 2 , C. Tempel-Brami 1 , H.S. Hershkovich 1 , M. Giladi 1 , C. Wenger 3 1 Novocure Ltd., Research and Development, Haifa, Israel ; 2 Novocure GmbH, Medical, Munich, Germany ; 3 Novocure GmbH, Research and Development, Root D4, Switzerland Purpose or Objective Tumor Treating Fields (TTFields) are low intensity, intermediate frequency alternating electric fields and a TTFields frequency of 200 kHz is currently used to treat glioblastoma multiforme. Previous data have shown that the efficacy of TTFields not only depends on a specific frequency, but also strongly on intensity. Therefore, a maximal TTFields dose at the site of the tumor is important. The distribution of TTFields in the brain is dependent on the tissue electric properties (EP) which are heterogeneous throughout the brain. Water content based EP tomography (wEPT) is a technique that uses the ratio of two T1w images with different relaxation times (TRs). Here we report on wEPT adaptation in order to map EPs in An empirical model was generated using 23 tissue samples from 3 juvenile bovine brains and 1 porcine CSF sample. This model linked water content (WC), T1 images and EPs in the range of 100-1000 kHz. For our analysis, we used T1w MRIs with TRs {700, 4000} ms. In order to estimate the water content, the dry and wet mass of the samples were measured and the differences between the two measurements were calculated. Curve fitting was then the 100-1000 kHz range. Material and Methods