ESTRO 2020 Abstract book

S767 ESTRO 2020

used to obtain empirical models that link Ir, WC, and EPs. Such empirical curves were then used in order to map EP and WC in tumor-bearing rat brains which were also imaged by T1w MRIs. In addition to that, 6 excised samples of each imaged brain were used to measure EPs and WC. Finally, the WC and EPs of the wEPT map were compared to measured values. Results The wEPT maps of rat brains allowed to identify anatomical structures and the tumor. The data also showed that the estimated WC determined by wEPT was in line with measurements done on excised sample. In addition, the results indicate that EPs that are estimated with the wEPT method are connected with experimentally measured values. However, wEPT- derived EP values and measurements differed in some of the samples and this was particularly found for the conductivity of samples taken from the rat tumors. Conclusion Our results show that wEPT can be applied to map WC in brain tissues between 100-1000 kHz. On the contrary, wEPT did not yield reliable estimates of conductivity within tumors and therefore further examination is required to clarify the relationship between WC and EP between 100-1000 kHz. This method might then be used non-invasively to measure electrical properties within brain tissue and to further understand the distribution of TTFields. PO‐1356 Ion Stopping Powers and Dual Energy CT Numbers of Animal Tissues for Monte Carlo Dose Calculations L. Zeng 1 , F.M. Michael 1 1 Shanghai Proton and Heavy Ion Center, Department of Medical Physics, Shanghai, China Purpose or Objective For ion beam treatment, most treatment planning systems(TPSs) use a pencil beam algorithm that calculates dose distributions using depth dose data measured in water and an algorithm that converts the X-ray computed tomography number of a given material to its linear stopping power relative to water(RLSP). Recently some TPSs have started using Monte Carlo type dose calculations. These calculations typically need the physical density and elemental composition of the tissues to determine penetration and calculate dose. Material and Methods Samples of 10 different animal tissues were obtained and packed in regularity-shaped containers including brain. heart. fat. cartilage. hard bone. muscle. liver. kidney and lung. The physical density of each sample was measured. The samples were then scanned with a dual energy CT scanner. Additionally, The samples were placed between the exit of a beaming and a Peak Finder to measure single- spot integrated depth dose distributions. Beams of protons accelerated to energies of 118.0,167.3,and222.1Mev and carbon ions accelerated to 216.7, 322.8 and 430 MeV/u were used.

Figure1: PTW Peak Finder and measurement setup Results For each tissue a dual-energy CT index was derived. This index may be used for determining the elemental composition classification for each tissue used in the Monte Carlo calculations. RLSPs were derived for each tissue for verification of the correct conversion function. In addition. two pairs of data. the Bragg peak width and the distal gradient. relative electron density and effective atomic number for each tissue as the specific material indicators were obtained respectively.

Figure 2:Measured single spot integrate depth dose distribution for 167.3 MeV proton for each sample. empty box. air and water placed between the radiation head and the Peak Finder. Conclusion Data for converting dual energy CT number for Monte Carlo dose calculation were obtained as well as data for verifying the correct conversion. A software workflow for the use of Siemens DEXCT images for Monte Carlo in the SPHIC in the future was suggested PO‐1357 Creating individually computed head models to simulate TTFields distribution Z. Bomzon 1 , A. Kinzel 2 , N. Urman 1 , S. Levi 1 , A. Naveh 1 , D. Manzur 1 , H.S. Hershkovich 1 1 Novocure Ltd., Research and Development, Haifa, Israel ; 2 Novocure GmbH, Medical, Munich, Germany Purpose or Objective Tumor Treating Fields (TTFields) are locally applied alternating electric fields of intermediate frequency used to treat glioblastoma multiforme (200 kHz). Currently, TTFields are also investigated in clinical phase III trials in other solid cancers. TTFields distribution in the tissue is affected by the position of the transducer arrays delivering the therapy, but also by the patient’s anatomy and the electric properties of tissue and tumor. For investigating the influence of TTFields distribution on patient outcome, we aimed to design realistic, patient-specific