ESTRO 37 Abstract book

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ESTRO 37

inter-OAR dependency on the prediction accuracy of KB models will also propagate to the performance of KB automated planning. To improve KB planning, more sophisticated KB models are needed.

virtual CT. The scans of which the quality of the warp wasn’t good enough was due to soft tissue mismatch. For example decrease of patient contour, inadequate representation of the tongue that had moved or misrepresentation of GTV shrinkage. (fig1) Conclusion The success-rate of CBCT-to-planning CT DIR is sufficiently high for clinical application of dose- recalculation. Visual verification remains necessary since in 6% the DIR-quality was insufficient. The proposed scoring methodology, demonstrated on three elements, was fast and easy, and will be applied in a DIR inspection protocol. OC-0307 Workflow optimization of image-guided adaptive radiotherapy in lung cancer patients D. Hattu 1 , J. Mannens 1 , W. Van Elmpt 1 , D. De Ruysscher 1 , M. Ollers 1 1 Maastricht Radiation Oncology MAASTRO clinic, Radiotherapy, Maastricht, The Netherlands Purpose or Objective IGRT using CBCT allows for precise patient set-up procedures. CBCT also detects anatomical changes that need corrections and may lead to an increased clinical workload. Development of decision criteria is therefore necessary. In our clinic a 'traffic light' protocol is part of the online matching procedure. For lung cancer patients this protocol also serves as a decision support tool to guide RTTs if a follow-up order is needed. These orders are due to anatomical changes caused by e.g. changes in lung density, changes in tumor volume or tumor shifts and are reviewed off-line by an RTT. The radiation oncologist determines if further action is necessary. In this study we investigated the online matching protocol to see if criteria used were adequate and clinically necessary for selecting patients that require plan adaptation. Subsequently we investigated if further optimization of the protocol allowed for a reduction in workload. Material and Methods For radical lung cancer treatments, all orders between June 2016 and June 2017 were analyzed. These orders were categorized based on the criteria: mediastinal structures present outside PRV, shift of visible gross tumor (outside CTV or outside PTV), changed anatomy of the lung (e.g. atelectasis, pleural effusion), change of tumor volume, second opinion on the match or other. For these criteria, frequency and follow up action (i.e. no action required, dose recalculation on the CBCT, new CT, new plan) was scored. For patients requiring re-planning, DVH metrics were used to quantify the dosimetric gain of a plan adaptation. Results Our protocol resulted in 585 follow-up orders, belonging to 207 patients. In 178 cases a dose recalculation on the CBCT was performed, 37 new CT scans were made and 33 new treatment plans were created. Frequency of criteria are shown in Figure 1. Out of the 585 orders: the majority (88%) did not require any further action after investigation; in 5% of the cases an adjustment of the matching method was sufficient; a new CT and/or plan adaptation was required in 7% of the cases. Adaptation of the treatment plan was most frequently seen in categories: changed lung density, change of tumor volume and shift of tumor outside the PTV (Figure 2). Follow-up orders regarding shift of the mediastinum and shift of the GTV outside the CTV were reported frequently, but hardly required further action. Omitting these latter two criteria in an optimized protocol, together with removing follow-up orders that were only received once during treatment (13%), did not increase the false negative rate.

Proffered Papers: RTT 3: Imaging and protocols for treatment verification

OC-0306 Dose-recalculation for Head and Neck patients; a RTT's perspective S. Ali 1 , S. Beek van 1 , S.R. Kranen van 1 , J. Sonke 1 , P. Remeijer 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Head and neck cancer (H&N) patients frequently show anatomical changes during the course of radiotherapy. Typically, these changes are detected in CBCT scans by visual inspection. The dosimetric consequences, however, are challenging to quantify as CBCT scans lack robust Hounsfield unit calibration. Therefore we developed a dose recalculation workflow, in which deformable image registration (DIR) is used to propagate the HU and contours of the planning CT to the geometry of the CBCT. This generates a virtual CT with corresponding structure set for dose-recalculation and evaluation in the treatment planning system (TPS). Since DIR accuracy may vary between patients, a visual inspection of the DIR by the RTT has been built into the workflow. In this study we assess the feasibility of visually inspecting the quality of the DIR in order to develop a DIR inspection protocol. Material and Methods 147 CBCT scans for 25 H&N cancer patients were retrospectively selected in consecutive order. The majority of the patients were treated in 35 fractions of 2 Gy with VMAT. CBCT-to-planning CT DIRs were performed with in-house developed software based on B-splines deformations and correlation ratio. The quality of the DIR was scored on three selected patient elements: 1) patients outer contour, 2) bony anatomy and 3) soft tissue, accumulating to a percentage of DIR-correctness: A (=not OK for dose-recalculation) and B (=OK for dose- recalculation, for this all the elements should be scored as correct). Figure1:

Results The DIR and generation of a virtual CT with corresponding structures took on average 10 minutes. The automatic dose recalculation in the TPS required another 15 minutes, after which it is available for evaluation. The scoring methodology was fast and easy to perform and may readily be expanded with further evaluation criteria. Of 147 DIRs, 94% were categorized as suitable for dose- recalculation, while 6% were not suitable to use for dose- recalculation or were needed density override of the