ESTRO 37 Abstract book

(Superior-inferior) and Z (Anterior-Posterior) were calculated. For each patient, the average shift in every direction was obtained. The final result reported are the median value between those determinated by the two radiographers. Results

patients and the subset was 20 patients containing 40 CBCT images. The results of the study were analyzed statistically per patient-specifically (n = 20) and couch shifts-specifically (n = 40). Results Auto-matching with the region of interest (ROI) and gold fiducial markers produced the best positioning results. In regard to couch shifts, the manual positioning who use orthogonal X-ray images differed significantly statistically (p < 0.05) from the positioning done by auto-matching in the vertical direction (-0.7 mm ± 1.3 mm). Admittedly, less than one millimeter differences are virtually meaningless in radiation therapy. Differences in longitude and lateral were 0.1 mm ± 1.7 mm and -0.2 mm ± 1.0 mm. In the same way, other positioning methods included auto-matching with ROI, and auto- matching with ROI, gold markers and couch rotation. Results also indicated that couch rotation does not improve positioning accuracy. Conclusion The Department of Radiotherapy in the Oulu University Hospital (OUH) has shifted towards the use of Cone Beam Computed Tomography (CBTC) in the positioning of prostate cancer patients in 2016. Daily online review with the use of CBCT enables the safe reduction of the treatment margins without jeopardizing the tumor control probability (TCP) and also makes possible to switch to the hypofractionated treatment, which leads to multiple benefits, i.e. the optimization of resources, lowered costs, and the patients' improved quality of life. EP-2348 MRI acquisition in radiotherapy treatment position: sequence optimization for head and neck cancer A. Largent 1 , L. Marage 1 , I. Gicquiau 1 , G. Gambarota 1 , H. Chajon 2 , O. Acosta 1 , J.C. Nunes 1 , R. De Crevoisier 2 , H. Saint-Jalmes 1 1 University of Rennes 1- INSERM- UMR 1099, Laboratoire du traitement du signal et de l'image LTSI, Rennes, France 2 Centre Eugène Marquis, Radiotherapy department, Rennes, France Purpose or Objective The use of MRI in radiotherapy treatment planning (RTP) for head and neck cancers requires acquisition in RT position with a RTP system (flat table, thermoplastic mask, and head board). Nowadays, this system does not fit with the standard MRI receiving coils. Specific receiving coils for RTP were designed by manufacturers, but at expense of image quality. In this context, this study aims to describe our sequence optimization method designed to obtain fast and reliable MR images for RTP. Material and Methods A water test-object and three volunteers, in treatment position with thermoplastic masks, were scanned. MR imaging was performed at 1.5 T (Optima 450W, GEM, Milwaukee, USA) using the GEM RT open head & neck Suite coil system. Three sets of parameters for isotropic 3D T1 gradient echo sequence and four sets of parameters for isotropic 3D T2 fast spin echo sequence were tested (Table 1). Each image was acquired twice with five repetitions (Fig 1). This double acquisition allows for reliable SNR computation even with parallel imaging acceleration. Except for the v4 T2, all the acquisitions were performed in sagittal direction and then reconstructed in axial to minimize the acquisition time and avoid aliasing. The v4 T2 MRI were acquired directly in axial. To quantify the image quality and find Electronic Poster: RTT track: Imaging acquisition and registration, OAR and target definition

Table 1

The median shift (± Standard Deviation, SD) along the X was 3,20± 2,06 mm, along the axis Y was 3,61± 2,99 mm and along the axis Z was of 4,25± 2,93 mm. The results obtained are summarized in the table 1. The diagram shows clearly that the most relevant shifts occur in Z direction, where actually the value of the third quartile is of 5,9 mm. Conclusion The use of PBBP, which is usually recommended to displace the bowel bag during pelvic 3DCRT treatments, would require the use of daily IGRT techniques to take into account potential systematic and random setup errors that are inevitably linked to it. Where this is not feasible, it is advisable to follow Center-dependent procedures in order to minimize such uncertainties, such as running daily EPID in the first 5 days( for the systematic errors) and then weekly (for the random ones), in addition to adopting an appropriate Center- specific CTV-PTV margin, calculated through the statistical measurement of systematic and random errors. EP-2347 Positioning of prostate cancer patients by the CBCT: Guide for the staff of Radiotherapy Department A.L. Jussila 1 , A. Määttä 2 , M. Kurttila 2 , K. Marttila-Tornio 2 , K. Paalimäki-Paakki 1 1 Oulu University of Applied Sciences, Special Fields of Health Care, Oulu, Finland 2 Oulu University Hospital, Radiotherapy, Oulu, Finland Purpose or Objective The purpose of the study was to determine how the manual positioning of the prostate cancer patient done by radiation therapists, who use orthogonal X-ray images, differs from the auto-matching, which was done in the offline review and based in the CBCT images taken from the patient during treatment sessions in the Oulu University Hospital (OUH). Material and Methods Firstly a guide was created for the positioning of prostate cancer patients by the CBCT. The positioning guide was pre-tested in the Radiotherapy Department, and completely finalized according to user feedback. Secondly, the data collected in the offline review was analyzed statistically to determine how the auto- matching positioning differs from the manual positioning done by radiation therapists. The research data consisted of prostate cancer patients who were treated with Simultaneous Integrated Boost (SIB) treatment and who also had images taken by the CBCT during treatment sessions. The basic set of the study consisted of 51