ESTRO 37 Abstract book

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ESTRO 37

Figure 2: Lateral dose distribution comparison of FLUKA and FROG plan recalculation. Conclusion A comprehensive validation of the newly introduced GPU- based plan recalculation platform, FROG, is currently underway. Here, a preliminary evaluation of the platform demonstrates acceptable agreement with the gold standard Monte Carlo-calculated dose distributions for the four ions at HIT. References: [1] Tessonnier T et al. Phase Space Generation for Proton and Carbon Ion Beams for External Users’ Applications at the Heidelberg Ion Therapy Center. Front Oncol. 2016. [2] Russo G et al. A novel algorithm for the calculation of physical and biological irradiation quantities in scanned ion beam therapy: the beamlet superposition approach. Phys Med Biol. 2016. EP-1839 Evaluation of a radiotherapy-dedicated Monte Carlo environment in clinical VMAT plans. G. Reggiori 1 , A. Stravato 1 , L. Paganini 1 , M. Di Filippo 2 , A. Fogliata 1 , V. Palumbo 1 , P. Mancosu 1 , S. Tomatis 1 , M. Scorsetti 3,4 1 Istituto Clinico Humanitas, Medical Physics Service of the Department of Radiation Oncology, Rozzano Milan, Italy 2 ETH Zurich, Department of Nuclear Engineering, Zurich, Switzerland 3 Istituto Clinico Humanitas, Department of Radiation Oncology, Rozzano Milan, Italy 4 Humanitas University, Department of Biomedical Sciences, Rozzano-Milan, Italy Purpose or Objective PRIMO is a graphical environment for MonteCarlo (MC) simulations. The DPM is a fast MC algorithm for the simulation of the deposited dose under radiotherapy conditions. The objective of this work was to validate the DPM-calculated beams against our LINAC and compare the performances of a clinical algorithm (Acuros, Varian) and of PRIMO DPM MC in two different clinical scenarios. Material and Methods The TB phsp files supplied by Varian were used to simulate the 10 MV FFF beam of out LINAC. These phsp files were validated against experimental measurements in terms of profiles, PDDs and Output Factors (OF). The Dosimetric Leaf Gap (DLG) and transmitted dose of the Millennium 120 HD MLC were evaluated too. A set of 20 patients was selected for this study: 10 patients with brain metastases and 10 with lung metastases. For all patients VMAT plans with 10 MV FFF beam were optimized in Eclipse and calculated with Acuros. The generated DICOM files (plan, structure and images) were imported in the PRIMO environment. The Dose Planning Method (DPM) simulation engine was then used to calculate the dose distribution in the patients CTs. The different plans were compared in terms of DVHs (dose volume histograms) through Homogeneity (HI) and Gradient (GI) Indexes. Results The agreement between simulated and real beam was good with differences ≤1% for all parameters considered. Some differences were observed in the MLC modeling: PRIMO resulted in a higher end-leaf transmission compared to Acuros with differences up to 10%. PRIMO showed a larger DLG too with values significantly higher than both Acuros and the related experimental measurements. The MC dose distributions of the VMAT plans had mean uncertainties within the PTV <1% and <1.5% for the brain and lung patients respectively. Comparing the VMAT plans an overall good agreement was observed though homogeneity was higher and dose fall-off was steeper in the plans calculated with Acuros, both for lung and brain patients.

Finally dose profiles along tissue inhomogeneities were evaluated for some patients showing some slight differences in correspondence of bone-cartilage interface, probably due to the different way of defining thecomposition of the different materials (Figure 1). Conclusion PRIMO is an interesting tool for the clinical environment useful to verify and support the commercially available TPS, being a reliable tool even in highly inhomogeneous tissues. The agreement with Acuros is good though a detailed analysis on the role of material composition in dose-to-matter calculations should be performed. It is an affordable Monte Carlo environment even in hospital departments where low IT resources are available. EP-1840 Dosimetric verification of IMRT head and neck plans using a 3D printed heterogeneous phantom J. Olasolo Alonso 1 , P. Collado Chamorro 1 , C.J. Sanz Freire 1 , V. Diaz Pascual 1 , A. Vazquez Galiñanes 1 1 Centro de Investigación Biomédica de La Rioja, Medical Physics, Logroño, Spain Purpose or Objective In external beam radiotherapy, phantoms are used to perform dosimetric checks. Basic phantoms are homogeneous, but patient anatomy may be more realistically mimic through heterogeneous phantoms. Our objective is to develop a home-made heterogeneous phantom, with realistic geometry of a patient. We study the suitability of the materials used (PLA and polymerizable silicone), to perform dosimetric calculations in an external beam radiotherapy planning system. Material and Methods A Prusa i3 Hephestos 3D printer, based on a fused plastic extrusion technique, was used for constructing the phantom. The material employed for soft tissue was PLA plastic (polylactic acid). To simulate the bone, a dental polymerizable silicone was used. The design of the phantom was based on a head and neck CT scan of a real patient. Soft tissue segmentation was performed using ImageJ 1.48v, excluding both bone and air. The volume was sliced, and each slice surface extracted and exported in stl format. The gcode for the 3D printer was generated by Cura 15.04.6 software. The holes corresponding to bone were manually filled with the polymerizable silicone. A CT scan of the phantom was performed using the usual protocol for radiotherapy patients in our institution. The CT scan was imported in Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA). Ten head and neck IMRT plans were optimized with the DVO 13.6 algorithm using 7 coplanar fields and calculated with AAA 13.6 algorithm. Dose was calculated at the chamber reference point, in a RW3 insert. The treatment plans were delivered to the phantom with a Clinac 2100 C/D treatment unit. A PTW 31010 semiflex ionization chamber (0.125cm 3 ) was used for dose measurements. Results Values around 100HU were obtained for the PLA, and 780HU for the silicone from the phantom CT scan. The differences between the dose calculated with the TPS and measured with ionization chamber were analyzed. Plan doses shown differences below 1.5% in all cases. Analysing the fields individually, the biggest difference was 3.6 % and the average difference was 0.68±1.54%. Conclusion We can conclude that it is feasible to design and construct a 3D printed heterogeneous phantom representing patient anatomy. This phantom can be routinely used to perform dosimetric checks of advance external beam treatment modalities.