ESTRO 36 Abstract Book

S868 ESTRO 36 2017 _______________________________________________________________________________________________

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.

transmitted to the linac gating interface via fibre-optic cables. Results Kinect v2 was able to acquire all breath holds from each patient successfully. Extracted traces from each patient depth file were sent to the motion platform. In the gating experiments (see Fig. 2), a Kinect v2 was able to track the phantom motion with a root mean square error of between 0.6 mm and 1.3 mm. The latency of our in-house gating software was found to range between 30 and 100 ms. In all cases, the gated radiation delivery dose agreed with the baseline dose measurement without gating to better than 0.4%.

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

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.