Abstract Book

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observed in and around sinuses, which were not modelled. 3D gamma index comparison for the entire geometry resulted in a passing rate of 97.98% which was also reflected in DVH comparison. The mean dose for the smallest target was found to deviate by 1.4%, while for all remaining targets discrepancies did not exceed 0.6%.

corrected” WEPL. A new energy is calculated from the range, and a new plan is created. Results Several scenarios have been generated to test the proposed methodology. The figure shows the DVHs for the dose reconstruction when a setup error of 1 cm is generated from the original CT. The plot compares the reconstructed (MCDose_Density, MCDose_Energy) and expected dose (MCDose_NewCT). The planned dose on the original CT image is also reported for comparison. The discrepancy observed are caused by the partial coverage of the treated field: the density/energy correction is only partially applied, since only 9 over 17 layers have been imaged with the prompt gamma camera. Since the target is relatively homogeneous we can make the reasonable assumption of a stable range shift. We correct for the missing layers propagating the range shift from the last measured layer back to the shallow layers. Currently the same correction factor is applied for all the spots of the missing layer. This solution is clearly not optimal but a reasonable improvement can be seen already (MCDose_DensityAllLayers). A spot-wise correction is under development.

Figure: (a) Profile on an axial slice through two targets. (b) Isolines comparison on a coronal slice including two targets. (c) Indicative DVHs for one metastasis. Conclusion An anatomical replica of the patient was constructed using commercially available 3D printer. Dosimetric evaluation revealed that the phantom can be regarded as patient equivalent (except for the skin and sinuses areas), enabling truly patient-specific plan verification if coupled with a comprehensive dosimetric system. OC-0081 From range measurements to dose: in-vivo dosimetry using the prompt gamma camera S. Toscano 1 , K. Souris 2 , E. Sterpin 1 , G. Janssens 3 , J. Petzoldt 3 , F. Vander Stappen 3 , J. Smeets 3 , X. Geets 4 , K. Teo 1 KU Leuven, Laboratory of Experimental Radiotherapy- Department of Oncology-, Leuven, Belgium 2 Université catholique de Louvain, Institute of Experimental and Clinical Research IREC, Louvain-la- Neuve, Belgium 3 IBA - Ion Beam Applications, R&D, Louvain-la-Neuve, Belgium 4 Clinique Universitaire Saint Luc- UCL, Radiation oncology, Brussels, Belgium 5 University of Pennsylvania School of Medicine, Radiation oncology, Philadelphia, USA Purpose or Objective In this contribution we present a new methodology to reconstruct the delivered dose during a pencil beam scanning proton therapy treatment, using prompt gamma imaging for range verification of the proton beam. Material and Methods A prototype prompt gamma camera was used to record the signal emitted along the proton tracks during the delivery of proton therapy for a brain cancer patient. By comparing the recorded prompt gamma depth profiles with simulation, a verification of the proton range is possible. Using the range shift calculation we developed a new method based on Monte Carlo calculations to reconstruct the delivered dose. Our Monte Carlo simulation tool, MCsquare, is a fast and accurate method of simulating heavy charged particles inside voxelized geometry, such that a full treatment can be simulated in few minutes. In our approach the shift measured from the prompt gamma imaging can be simulated either as a density scaling of the voxels along the proton trajectory or a spot-specific energy correction. Two methodologies have been implemented and compared: 1. Density correction: A ray-tracing inside the CT image geometry is performed to calculate the water equivalent path length (WEPL) for every proton beamlet, and a density correction factor is calculated as the ratio between the 'shift-corrected” WEPL and the expected one. In MCsquare calculations the stopping powers are scaled by this spot-specific scaling factor and the dose is recalculated accordingly. 2. Energy correction: For every beamlet the range in water is calculated from the 'shift-

Conclusion The first dose reconstruction using prompt gamma imaging data has been developed and tested on real patient data. First results show that the implemented methods have the potential to verify the delivered dose and could be used as a tool to assure the quality of the treatment plan delivery. OC-0082 Using prompt gamma emission profiles to monitor day-to-day dosimetric changes in proton therapy E. Lens 1 , T. Jagt 2 , M. Hoogeman 2 , M. Staring 3 , D. Schaart 1 1 Delft University of Technology, Radiation Science and Technology, Delft, The Netherlands 2 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands 3 Leiden University Medical Center, Radiology, Leiden, The Netherlands Purpose or Objective Prompt gamma (PG) emission profiles can be used to determine the proton range in patients, but studies on the correlation between PG measurements and relevant dosimetric parameters are mostly lacking. The aim of this study was to investigate the feasibility of using PG emission profiles to monitor dosimetric changes in pencil beam scanning (PBS) proton therapy as a result of day-to- day variation in patient anatomy. Material and Methods We included 11 prostate patients with a planning CT scan and 7–9 repeat CT scans (99 CT scans in total), illustrating daily variation in patient anatomy. For each patient, we had a PBS treatment plan with two lateral fields. We determined the real-time PG emission profiles on a cylindrical surface around the patient by simulating each plan on the planning CT and on the repeat CT scans of each patient using the Geant4-based TOPAS Monte Carlo code. The scored (i.e. detected) PGs were discriminated