ESTRO 2020 Abstract book

S912 ESTRO 2020

neck cancer. No atlas patients were included in this cohort; deep learning training was performed on patients from another institute. Mean dose was extracted from all contralateral parotid gland clinical and auto-contours. Patient-rated baseline xerostomia scores and 12 months following treatment were prospectively collected as part as the MDASI follow-up program. Mean dose differences were tested with paired Samples Wilcoxon Test. Logistic regression models with the contralateral parotid gland mean dose and baseline xerostomia scores were fitted for doses for clinical, AB and DL auto-contours. In addition, external validation of a published Xer 12m model, developed in a different institute, was performed with mean parotid dose from the different contours and baseline xerostomia scores (Table 1). Results Fifty-one (30%) patients developed moderate-to-severe Xer 12m (MDASI>4). Mean dose values of the clinical contours (16.2 ±6.8 Gy) were significantly different with for DL contours (17.3 ±7.8 Gy; p<0.001), but not for AB contours (16.4 ±7.3 Gy; p=0.46). Coefficients of the Xer 12m models fitted to the dose data from different contours were nearly identical with slight changes in the intercept (see Table 1). Performance of the both auto-contoured based models was slightly better than the one based on the clinically used contours (e.g. lower BIC, higher AUC and explained variance R 2 ). In addition, validation of Xer 12m model that was previously published in literature showed similar performance and coefficients. Conclusion Coefficients and prediction performance of Xer 12m with both AB and DL auto-contouring were comparable to clinical contours, as well as the previously published models from a different institute. This work shows the potential utility of auto-segmentation methods to contour OARs for large number of patients in developing and validating toxicity prediction models.

Poster: Physics track: Intra-fraction motion management

PO-1587 Novel biocompatible artifact-robust and highly visible fiducial marker for image-guided radiotherapy T. Wang 1 , N. Ikeo 2 , K. Okumura 3 , H. Akasaka 1 , R. Yada 1 , K. Yoshida 1 , D. Miyawaki 1 , T. Ishihara 1 , N. Mukumoto 1 , Y. Shimizu 1 , T. Mukai 2 , A. Nakaoka 1 , R. Sasaki 1 1 Kobe University Graduate School of Medicine, Radiation Oncology, Kobe, Japan ; 2 Kobe University, Department of Mechanical Engineering, Kobe, Japan ; 3 Kobe University Hospital, Centre for Radiology and Radiation Oncology, Kobe, Japan Purpose or Objective Develop a novel biocompatible, artifact-robust and highly visible solid fiducial marker for kilo-voltage X-ray image- guided radiation therapy (IGRT). Material and Methods The fiducial marker is made of one of the physiologically essential metal elements in the human body. An in-house phantom was designed to systematically evaluate its image artifacts (by the metal artifact index) and image visibility (by the contrast-to-noise ratio) under different conditions. Image artifacts were assessed on kilovoltage computed tomography (CT) and cone-beam CT (CBCT). The marker visibility was evaluated on two types of kilo-voltage 2D planar X-ray images from the linear accelerator. In-vivio biocompatibility characteristics, including histological and biochemical blood testing, were evaluated using the animal model of rats over 12 weeks. Results Results were compared to those from the conventional gold fiducial marker. Imaging artifacts were found dramatically suppressed by our proposed biocompatible marker compared to the gold marker (as the artifact index decreased by up to 68% in CT and by up 81% and 65% in CBCT, respectively. The visibility of the proposed marker exhibited satisfied CNRs above 2.0 for 23/32 exposures under various phantom settings, being comparable to the results of gold marker (24/32 exposures with CNRs >2.0). Further, minimal local fibrosis at the implantation site and absence of adverse effects in nearby organs over the observation period, suggest excellent biocompatibility of the proposed marker. Conclusion We developed a novel biocompatible artifact-robust and highly visible solid fiducial marker. Although further evaluation is needed in clinical settings, our findings suggest its feasibility and benefits for implementation in kilo-voltage X-ray IGRT. PO-1588 Effect of breathing motion on robustness of locoregional proton plans for left-sided breast cancer A. Petoukhova 1,2 , L. Klaassen 2,3 , S. Habraken 2,4 , J. Jacobs 2 , M. Sattler 2,4 , K. Verhoeven 5 , Y. Klaver 2 1 Haaglanden Medical Centre, Department of Medical Physics, Leidschendam, The Netherlands ; 2 HollandPTC, Department of Radiation Oncology, Delft, The Netherlands ; 3 Delft University of Technology, Faculty of Mechanical- Maritime and Materials Engineering, Delft, The Netherlands ; 4 Erasmus MC Cancer Institute, Department of Radiation Oncology, Rotterdam, The Netherlands ; 5 GROW School for Oncology and Developmental Biology- Maastricht University Medical Centre, Department of Radiation Oncology MAASTRO, Maastricht, The Netherlands Purpose or Objective To investigate dosimetric impact of breathing motion on robustly optimised proton therapy treatment plans for