ESTRO 36 Abstract Book

S924 ESTRO 36 2017 _______________________________________________________________________________________________

Radiooncology, Dresden, Germany 5 OncoRay, National Center for Radiation Research in Oncology, Dresden, Germany 6 Nanovi Radiotherapy A/S, Development, Kgs. Lyngby, Denmark Purpose or Objective Use of solid fiducial markers in proton radiation therapy has been approached with care as their presence may cause significant local dose perturbations. Recently, a liquid carbohydrate based fiducial marker (BioXmark®) has been introduced with minimal dose perturbation (relative stopping power = 1.164) and visibility properties suitable for use in image-guided proton therapy (IGPT). The purpose of this work was to investigate the chemical stability of the marker for use in both normofractionated and single fraction proton treatment regimes. Material and Methods Ten identical custom-made cylindrical polymethylmethacrylate (PMMA) inserts (V = 0.95 mL, d outer = 10.0 mm, d inner = 5.0 mm, l = 48 mm) were prepared. BioXmark® markers (150±30 mg) were added to the bottom of the inserts and water (700 µL) was added on top of the markers. The inserts were sealed with a rubber stopper. A QA dosimetry phantom was modified to accommodate four PMMA inserts simultaneously by inserting these sideway into the proton irradiation field (10 × 10cm) (Figure 1). Four markers (Group A) were irradiated during daily QA for a period of 51 days with 43 fractions ranging from 1.44-1.86 Gy resulting in an accumulated dose of 67.4 Gy. Four other markers (Group B) were irradiated with a single dose of 155.4 Gy and two non-irradiated Control markers were kept on site for the duration of the experiments. Possible chemical alterations caused by proton irradiation were evaluated by high-performance liquid chromatography (HPLC), electrospray ionization mass spectrometry (ESI-MS),thin-layer chromatography (TLC) and visual inspection of the markers and the aqueous phase above the markers.

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·The manual and automatic contours were compared after every 5 pts added to the atlas at a time up to a total of 30. Results Comparing the mean scores between initial manual and the MBS contours (Table1), the manual approach performed better for spinal cord and parotids with averaged scores of 8.4 (manual) vs 7.2 (MBS). Standard deviation showed comparable intra-organs variability. DSC scores were: 1.0 mandible, 0.71 spinal cord, 0.73 right parotid, 0.72 left parotid, 0.80 brainstem. All 30 pts were then used to build a customized atlas. Contours analysis, tested on 4 new pts, is shown in Figure1. After training, the performance of the ABAS increased for all the OARs automatically contoured. Best outcomes resulted for mandible, spinal cord and brainstem for which the score and DSC are respectively: 10, 1.0; 10, 1.0; 10, 0.95. Parotids showed lower results: 7.5, 0.70 for right parotid; 7.4, 0.71 for left parotid.

Conclusion The default MBS tool showed a difference in structure delineation that does not exceed the inter-clinician variability. The customized atlas developed reached performances comparable with the clinical gold standard for mandible, spinal cord and brainstem. To increase outcomes, several atlases trained on specific sub- populations could be created reducing the intra-patient variability and making results closer to optimal segmentation. As a next step, the influence of image quality on automatic segmentation will be analyzed. EP-1710 Chemical stability of BioXmark® following normofractionated and single-fraction proton beam therapy E. Troost 1,2,3,4,5 , S. Menkel 1 , W. Enghardt 1,4,5 , J. Hytry 5 , D. Kunath 1 , S. Makocki 1 , A. Hoffmann 1,4,5 , R. Jølck 6 1 TU Dresden- Med. Faculty Carl Gustav Carus, Radiotherapy and Radiation Oncology, Dresden, Germany 2 German Cancer Consortium DKTK, Partner site Dresden, Dresden, Germany 3 National Center for Tumor Diseases, Partner site Dresden, Dresden, Germany 4 Helmholtz-Zentrum Dresden-Rossendorf, Institute of

Figure 1. QA dosimetry phantom setup for proton irradiation of BioXmark® inserts. Results No visual alterations between markers from Group A+B and the Control markers were observed. HPLC and TLC analysis of the markers and the aqueous phase above the markers from all three groups did not indicate chemical degradation of the marker materials (Figure 2). This observation was further supported by ESI-MS analysis, which showed identical m/z fragments for all three groups (Figure 2).