Abstract Book

S161

ESTRO 37

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.

Conclusion Our protocol was adequate for selecting patients needing adaptive re-planning, but also led to many false positives. Criteria related to changes in lung density, tumor volume and shift outside the PTV were appropriate and clinically relevant indicators for possible adaptation. The optimized protocol leads to a reduction of the workload of 28% (in 47% of patients) while remaining adequate for detecting treatments requiring adaptation. OC-0308 Error in node level positioning after 3D CBCT match in loco-regional breast cancer radiotherapy V. Althof 1 , E. Hoogeboom 1 , A. Moes 1 , P. Jeene 1 1 Radiotherapiegroep, Deventer, Deventer, The Netherlands Purpose or Objective Measurement of set up differences between the matches that focus on node levels I to IV and humeral head, and the clinical Cone Beam CT (CBCT) match respectively. Material and Methods Delineation of lymph node levels I - IV in 3D CBCT (XVI Elekta) was performed, based on the ESTRO Consensus Guideline on Target Volume Delineation for Breast Cancer 2015, making use of surrogate structures. Between June 2016 and April 2017, 34 patients received post-operative loco-regional radiotherapy in our institution. In 20 patients (59%) the CBCT image quality was sufficient to discern the surrogate structures. During treatment 6 to 7 CBCT’s were made. Per CBCT 6 matches were performed, that is: the clinical match, using bony structures and lung- and outer contours and 5 non-clinical matches that focused on humeral head and the four node levels. To calculate the PTV margin necessary to cope with the set up variations of the clinical match, combined with the residual set up variations of the node levels, both systematic and random variations of both sources of variation are added quadratically and the result is used in the PTV margin recipe M=2∑+0.7σ .