ESTRO 36 Abstract Book

S177 ESTRO 36 2017 _______________________________________________________________________________________________

planned to confirm and consolidate correction factors and determine the overall uncertainty on absorbed dose-to- water obtained using each system. The next experimental step is to perform the same experimental comparison for a real clinical situation: a dose cube of 10 x 10 x 10 cm³, created by a superposition of mono-energetic layers. OC-0340 Validation of HU to mass density conversion curve: Proton range measurements in animal tissues J. Góra 1 , G. Kragl 1 , S. Vatnitsky 1 , T. Böhlen 1 , M. Teichmeister 1 , M. Stock 1 1 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria Purpose or Objective Proton dose calculation in the treatment planning system (TPS) is based on HU information taken from the CT scans and its relation to the relative stopping powers (RSP). However, tissue equivalent substitutes commonly used in the process of conversion curve definition may not reflect precisely the properties of real, human tissues. Therefore, various animal tissues were used for validation of the CT number to mass density (MD) conversion curves implemented in the TPS (RayStation v5.0.2). Material and Methods 10 animal tissue samples (pig) were used in this study (muscle, brain, bone, blood, liver, spleen, lung, fat, kidney and heart). Each sample was prepared and wrapped separately. 3-4 tissues were placed in dedicated phantoms (head and pelvis) at a time and CT scans were taken in the clinically accepted planning protocols. Specially designed PMMA phantoms where composed of two parts: a) an internal box, which could fit the animal tissues inside, b) the outer PMMA cover, designed to simulate pelvis (see fig.1c) and head during CT scan. The design of the phantoms not only helped to reduce imaging artefacts but also allowed to apply a slight pressure on the tissues in order to remove unwanted air. Subsequently, the tissue phantom was attached to the front of the water phantom, where with the use of 2 Bragg peak chambers, range measurements were performed. All measurements were performed within 24h after the animal was slaughtered with the use of one, central, 160.3 MeV pencil beam. For each sample, multiple irradiation positions were chosen in a very precise matter, as it was extremely important to choose the most homogeneous path through which the proton beam would pass. Acquired CT data was used to read out the HU, correlate them with the measured RSP and validate against implemented CT number to MD conversion curves. Results Figure 1, shows the comparison between measured RSP and HU for real tissue samples and implemented conversion curve in the TPS a), CT scan of the adult, abdomen protocol b), and measurement set-up c). The measured data for all soft tissues were found to be within 1% agreement with the calculated data. Only for lung tissue the deviations were up to 3.5%. For bone, both the difficulty in assessing the actual thickness of the part where the beam was passing through, as well as the inhomogeneous nature of this tissue, prevented us from the accurate RSP assessment. However, for 2 measurements out of 3, the measured RSP where within 3.5% uncertainty. Conclusion The experimental validation of the conversion curve resulted in good agreement between measured and calculated data, therefore we can use it in the clinical set- up with confidence. There is a number of uncertainty sources related to these measurements, starting from HU to RSP model, real tissue heterogeneities or uncertainties related to acquisition of the CT data due to beam hardening. The last one, we tried to minimize by using especially dedicated phantom.

OC-0341 Monte Carlo dose calculations using different dual energy CT scanners for proton range verification I.P. Almeida 1 1 Maastricht Radiation Oncology MAASTRO clinic, Physics Research, Maastricht, The Netherlands Purpose or Objective To simulate the dose profile for proton range verification by means of Monte Carlo calculations and to quantify the difference in dose using extracted values of relative electron densities ( ρ e ) and effective atomic numbers ( Z eff ) for three commercial dual-energy computed tomography (DECT) scanners from the same vendor: a novel single- source split-filter (i.e. twin-beam), a novel single-source dual-spiral and a dual source device. This study aims also to provide a comparison between the use of different DECT modalities and the conventional single-energy CT (SECT) technique in terms of dose distributions and proton range. Material and Methods Measurements were made with three third generation DECT scanners: a novel dual spiral at 80/140 kVp, a novel twin-beam at 120 kVp with gold and tin filters, and a dual- source scanner at 90/150kVp with tin filtration in the high energy tube. Images were acquired with equivalent CTDI vol of approximately 20 mGy and reconstructed with equivalent iterative reconstruction algorithms. Two phantoms with tissue mimicking inserts were used for calibration and validation. Monte Carlo proton dose calculations were performed with GEANT4, in which the materials and densities were assigned using the DECT extracted values of ρ e and Z eff for both phantoms. Simulations were done with monoenergetic proton beams impinging under directions to the cylindrical phantoms, covering different tissue-equivalent inserts. Dose calculations were also performed on images from a third generation SECT scanner at 120 kVp. Simulations based on DECT and SECT images were compared to a reference phantom. Results Range shifts on the 80% distal dose fall-off (R80) were quantified and compared for the different beam directions and media involved to a reference phantom. Maximum R80 range shifts from the reference values for the calibration phantoms based on DECT images were 3.5 mm for the twin-beam, 2.1 mm for the dual-spiral and for the dual- source. For the same phantom, simulations based on SECT images had a maximum range shift of 4.9 mm. 2D stopping power maps were computed and compared for the different techniques.