ESTRO 36 Abstract Book

S78 ESTRO 36 2017 _______________________________________________________________________________________________

Analysis in the direction orthogonal to the applied deformation showed minimal errors, as expected ( = 0.32 mm, SD = 0.43 mm). Across 14 clinical MR-CT registration datasets, mean magnitude registration errors within the GTV varied from 0.4 to 5.4 mm (population mean = 1.8 mm), indicating that MM-DIR errors can be significant for RT planning.

propagate urethra delineation to the test patients. The n- most similar individuals were selected and final segmentation was obtained by a weighted vote. Leave-one out cross validation of the atlas for urethra segmentation was first performed on the training data set. Mean Centerline Dispersion (MCD) and Hausdorff Distance (HD) were used for accuracy assessment. The method was then applied to a second set of 95 patients having received 78 Gy by IMRT for prostate cancer. Target volume and organs at risks (bladder, prostate) were delineated on computed tomography (CT) slices according to the French GETUG group recommendations. Then, the urethra was segmented using the proposed approach and dose was measured inside the resulting segmentation and compared to the dose to the prostate. Results From the training data set, the number of most similar atlases was optimized to 10 in the leave one out scheme. Average MCD of 2.3 mm and HD of 3.5 mm were thereby obtained. In the testing data base dose received by the segmented urethra were significantly higher than the whole prostate in a range of dose from 74 Gy to 79 Gy (Wilcoxon test p<0.01). Conclusion An accurate atlas based segmentation method was proposed allowing assessment of dose within prostatic urethra. Dose in this organ was significantly higher than the whole prostate, mainly in the highest dose range. Results open the way to further NTCP studies relating urinary toxicity such as obstructive symptoms to the urethra dose.

Conclusion Reference free localised registration quality assessment offers clinicians a tool to judge registration reliability, which could increase confidence in and clinical usage of MM-DIR in radiotherapy. A software tool was developed and validated to achieve this. A strong correlation was found between detected and applied registration errors. Mean GTV error is a potential indicator for clinical acceptability of registrations. OC-0157 Atlas-based segmentation of prostatic urethra in the planning CT of prostate cancer O. Acosta 1 , M. Le Dain 1 , C. Voisin 1 , R. Bastien 1 , C. Lafond 2 , K. Gnep 2 , R. De Crevoisier 2 1 LTSI-INSERM UMR 1099, Université de Rennes 1, Rennes, 2 Centre Eugene Marquis, Radiotherapy, Rennes, France Purpose or Objective to the dose delivered mainly to the bladder) and likely also to the urethra (obstructive symptoms). Identification of urethra for dose assessment from planning CT scans is however challenging as the organ lies inside the prostate and is not visible. Moreover, the dose received by the urethra may not be superposed to the dose received by the whole prostate. In case of prostate IMRT, the goals of this work were therefore: i) to propose an automatic method for urethra segmentation from the planning CT and ii) to quantify the dose received by the urethra. Material and Methods An original weighted multi atlas-based segmentation method was devised standing on a global characterization of the urethra wrt the surrounding organs. For building the atlas a first set of CT scans (512×512 0.63×0.63 mm axial pixels and 3 mm slices) from 80 patients treated for localized prostate cancer with Iodine 125 brachytherapy was used. All the patients had an urinary probe allowing an ease manual urethra segmentation. Prostate, bladder and urethra were delineated by a radiation oncologist. An average patient, in terms of prostate volume, was selected as common reference system where all the patients were rigidly aligned. Each segmented urethra was characterized by its central line, the relative bladder position and prostate characteristics (height, excentricity and volume). An in-house demons based registration using prostate contours and Laplacian maps was performed to

OC-0158 a priori scatter correction of cone-beam CT projections in photon vs. proton therapy gantries A.G. Andersen 1 , Y. Park 2 , O. Casares-Magaz 1 , U. Elstrøm 1 , J. Petersen 1 , B. Winey 2 , L. Dong 3 , L. Muren 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus V, Denmark 2 Massachusetts General Hospital, Department of Radiation Oncology, Boston- Massachusetts, USA 3 Scripps Proton Therapy Center, Department of Medical Physics, San Diego- California, USA Purpose or Objective Cone-beam (CB) CT is becoming available on proton therapy gantries, to allow image/dose-guidance and adaptation for protons. To use these techniques clinically, the challenges related to image quality and Hounsfield Unit accuracy need to be solved. Algorithms for scatter correction have been developed, and have been explored for CBCT systems on photon therapy gantries but so far not