ESTRO 36 Abstract Book

S880 ESTRO 36 _______________________________________________________________________________________________

Figure 2: Top: A patient breathing trace programmed into the motion platform, monitored for gating by a Kinect v2 . Bottom: corresponding beam state transmitted to linac. Conclusion The Kinect v2 provides a cost-effective method of monitoring patients during VBH, and gating the delivery of radiation to only the peak inhale phase. This is a markerless, convenient alternative to manual monitoring. EP-1625 Comprehensive prospective evaluation tool for treatments of thoracic tumours with scanned protons C. Ribeiro 1 , A. Meijers 1 , G. Janssens 2 , J. Widder 1 , J. Langendijk 1 , E. Korevaar 1 , A. Knopf 1 1 University Medical Center Groningen UMCG, Department of Radiation Oncology, Groningen, The Netherlands 2 Ion Beam Applications IBA, Advanced Technology Group, Louvain-la-Neuve, Belgium Purpose or Objective Due to the high sensitivity of Pencil Beam Scanning (PBS) to water equivalent thickness (WET) variations, differences between the planned and delivered dose to the CTV (robustness) are of great concern, especially for the treatment of moving targets located in the thorax. Effects that influence the robustness of plans created for patients with moving targets are: machine uncertainties, setup and range errors and the interplay effect, which occurs due to the interference of the time structure of treatment delivery and target motion. The aim of this study is the development and application of a tool that realistically evaluates PBS deliveries to patients with moving targets prior to the actual treatment. Material and Methods A robustly optimized plan with a nominal dose of 60 Gy to the CTV was created using our treatment planning system for an exemplary lung cancer patient (non-small cell lung cancer (NSCLC) stage III). We considered the delivery of this nominal plan over 8 fractions, which has been shown representative for the clinical delivery over 30 fractions. Our tool simulates (1) machine uncertainties (spot position, dose, and energy errors), (2) setup and range errors (by shifting the patient and 3% scaling the CT intensity values in order to create 14 scenarios representing 14 possible treatment courses), (3) breathing motion (by performing 4D dose accumulation in the planning 4DCT and in repeated 4DCTs), (4) interplay effect (incorporating the time structure of delivery by splitting the nominal plan in 10 different sub-plans with the help of the scanning control system ScanAlgo), and (5) a combination of all previously mentioned effects (1) - (4) . To evaluate robustness, the V95 of the CTV was analysed. In case of presence of multiple scenarios ( (2) and (5) ) the V95 of the voxel-wise minimum dose distribution of the CTV (minimum dose obtained from all the scenarios in each voxel of this structure) was determined. Results V95 values for the simulation scenarios (1) - (5) are present in Table 1. The V95 for the CTV dropped from 100% (nominal case) to 90.11% when all effects were considered in combination (simulation (5) ). Figure 1 shows dose distributions of the nominal plan and the voxel-wise minimum obtained for simulation (5) . Furthermore, DVH curves of the nominal plan, all treatment scenarios resulting from the realistic combination of effects and the corresponding voxel-wise minimum dose are shown.

Conclusion We developed a realistic and comprehensive tool for a prospective robustness analysis of PBS treatment plans for patients with moving targets. The power of this tool was demonstrated in one exemplary lung cancer patient, showing the significant impact of the combination of PBS delivery effects for target coverage. In clinical practice this tool will help to make decisions concerning the necessity to employ further motion mitigation techniques. EP-1626 Predicting motion of normal tissue using incomplete real-time data during lung radiotherapy. L.S.H. Bendall 1 , M. Partridge 1 , M.A. Hawkins 1 , J. Fenwick 2 1 CRUK MRC Oxford Institute for Radiation Oncology, Department of Oncology- University of Oxford, Oxford, United Kingdom 2 University of Liverpool, Institute of Translational Medicine, Liverpool, United Kingdom Purpose or Objective Imaging during radiotherapy treatment has the potential to increase the accuracy and precision of radiotherapy. MR-linacs can produce high quality images during treatment delivery. However, for image acquisition and analysis to be performed in real-time, fast registration techniques based on incomplete data is required. Target motion in lung radiotherapy has been extensively investigated but motion of surrounding organs at risk (OARs) remain relatively uncharacterised. Consequences of irradiating nearby OARs to high doses can be fatal. We have investigated the bronchial tree with the aim of characterising the motion of the whole structure