ESTRO 2020 Abstract Book

S124 ESTRO 2020

PH-0240 An independent platform for dose calculation and log file evaluation in proton therapy G. Fonseca 1 , I. Almeida 2 , T. Wagenberg 1 , C. Wolfs 1 , G. Vilches Freixas 1 , I. Rinaldi 1 , J. Martens 1 , G. Bosmans 1 , F. Verhaegen 1 1 Maastricht University Medical Centre+, Department of Radiation Oncology MAASTRO, Maastricht, The Netherlands ; 2 Technical University of Lisbon, Center of Nuclear Sciences and Technologies, Lisboa, Portuga l Purpose or Objective Proton therapy is rapidly growing worldwide due to the potential to deliver a highly conformal dose to the tumor while sparing healthy tissues. Our clinic recently acquired a MEVION S250i™ with HYPERSCAN™ pencil beam scanning (Figure 1a) that uses a range shifter to modulate the beam energy and has an Adaptive Aperture TM (Multileaf Collimator) that generates a sharper lateral penumbra. An independent dose calculation method is desired due to the treatment complexities, which also requires treatment delivery verification (e.g. log file evaluation). This work describes an independent platform for Monte Carlo (MC) dose calculation and log files evaluation. A neuro case (15 fractions) with 3 beam directions using robust optimization on the CTV is shown as an example. Material and Methods The treatment plan and log files were imported into an in- house developed software, A Medical Image-Based Graphical platfOrm (AMIGO, Figure 1b), for visualization and analysis. AMIGO has TPS-like features working as an interface for TOPAS (TOol for PArticle Simulation). All the relevant machine components (e.g. range shifter and adaptative aperture) were modeled using TOPAS 4D geometrical features. First, the log file information (e.g. range shifter position, spot position, monitor units (MU) per pulse, etc.) is compared to the treatment plan. Secondly, TOPAS dose calculations obtained using the treatment plan and log file parameters are compared. Results Figure 1c shows the difference between log files (15 fractions) and the treatment plan for the position of each leaf of the dynamic aperture system. Deviations are below ± 0.7 mm. Spot positions have mean deviations of 0.0 ± 0.5 (1STD) and 0.1 ± 0.5 (1STD) for the inline and crossline directions, respectively. The monitor unit (MU) mean deviation is 0.00 ± 0.03 (1STD) for pulses, and 0.00 ± 0.01 (1STD) for spot positions (one spot can have multiple pulses). Total MU deviations range from 6.5 MU (≈0.3%) up to 17.5MU (≈0.8%). Figure 1d shows DVHs calculated using log files information for different fractions. Clinical metrics (e.g. D 90 and V 100 ) deviations are less than 1% for the CTV.

Conclusion A continuous and consistent analysis of the log files provides valuable information about the machine condition (e.g. an increasing number of interlocks or systematic shifts in the measurements might indicate hardware failure in an early stage). Pre-treatment QA based on log files can reduce the workload in the clinic reducing the number of measurements. In addition, dose recalculations using an independent calculation method based on log files verifies the machine parameters during dose delivery and TPS calculations. The workflow can be automated so log file based verification won’t have a negative impact in the workflow. PH-0241 A simulator of proton pencil beam scanning delivery P.R. Poulsen 1,2 , H. Nyström 3 , P.S. Skyt 1 , M.F. Jensen 1 1 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark ; 2 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark ; 3 The Scandion Clinic, The Scandion Clinic, Uppsala, Sweden Purpose or Objective Proton therapy with pencil beam scanning (PBS) in the thorax and abdomen is vulnerable to dose distortions caused by respiratory motion. The motion effects may be reduced by rescanning or treatment in breath-hold. Breath-sampling rescanning, where the rescanning of each layer is designed to cover the entire breathing cycle, provides the most efficient interplay effect mitigation, but it relies on accurate predictions of the delivery time of each PBS spot. Such predictions are also useful when evaluating the suitability of PBS plans for breath-hold, and they will be crucial for plan optimization in future proton FLASH treatment planning. Here, we develop a simulator for accurate prediction of proton PBS delivery times. Material and Methods A software tool for simulation of proton PBS fields was created and tested at a proton treatment gantry (ProBeam, Varian). The simulator modeled the timing of proton spots as a waiting time that depended on energy and distance from the preceding spot plus a delivery time equal to the number of monitor units (MU) divided by an energy layer-specific MU-rate. The MU-rate was estimated from the cyclotron beam current, which was simulated by taking interdependencies of all energy layers, cyclotron current restrictions and the minimum allowed spot duration (3 ms) into account. The parameters for the simulator were first fitted to delivery log files from a training dataset with 18 PBS plans that covered the clinical energy range (80-220 MeV in 3MeV steps) and spanned wide ranges of spot MUs (1-80 MU) and spot distances (5-180 mm). The spots in the training plans were delivered in a spiral pattern with increasing spot distances (Figure 1.A). Next, the simulator was tested by predicting the delivery time for each individual spot in 83 clinical treatment fields with a total of 1694 energy layers. The simulated delivery time of each layer was compared with the actual delivery times as given by the log files.