title: Development of CBCT-based prostate setup correction strategies and impact of rectal distension.
abstract: BACKGROUND
  Cone-beam computed tomography (CBCT) image-guided radiotherapy (IGRT) systems are widely used tools to verify and correct the target position before each fraction, allowing to maximize treatment accuracy and precision. In this study, we evaluate automatic three-dimensional intensity-based rigid registration (RR) methods for prostate setup correction using CBCT scans and study the impact of rectal distension on registration quality.
  
  
  METHODS
  We retrospectively analyzed 115 CBCT scans of 10 prostate patients. CT-to-CBCT registration was performed using (a) global RR, (b) bony RR, or (c) bony RR refined by a local prostate RR using the CT clinical target volume (CTV) expanded with 1-to-20-mm varying margins. After propagation of the manual CT contours, automatic CBCT contours were generated. For evaluation, a radiation oncologist manually delineated the CTV on the CBCT scans. The propagated and manual CBCT contours were compared using the Dice similarity and a measure based on the bidirectional local distance (BLD). We also conducted a blind visual assessment of the quality of the propagated segmentations. Moreover, we automatically quantified rectal distension between the CT and CBCT scans without using the manual CBCT contours and we investigated its correlation with the registration failures. To improve the registration quality, the air in the rectum was replaced with soft tissue using a filter. The results with and without filtering were compared.
  
  
  RESULTS
  The statistical analysis of the Dice coefficients and the BLD values resulted in highly significant differences (p<10(-6)) for the 5-mm and 8-mm local RRs vs the global, bony and 1-mm local RRs. The 8-mm local RR provided the best compromise between accuracy and robustness (Dice median of 0.814 and 97% of success with filtering the air in the rectum). We observed that all failures were due to high rectal distension. Moreover, the visual assessment confirmed the superiority of the 8-mm local RR over the bony RR.
  
  
  CONCLUSIONS
  The most successful CT-to-CBCT RR method proved to be the 8-mm local RR. We have shown the correlation between its registration failures and rectal distension. Furthermore, we have provided a simple (easily applicable in routine) and automatic method to quantify rectal distension and to predict registration failure using only the manual CT contours.
authors.affiliation: Signal Processing Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland. christine.boydev@epfl.ch.|Laboratory of Industrial and Human Automation control, Mechanical engineering and Computer Science, University of Valenciennes and Hainaut-Cambrésis, Valenciennes, France. christine.boydev@epfl.ch.
authors.forename: Christine
authors.initials: C
authors.lastname: Boydev
authors.affiliation: Laboratory of Industrial and Human Automation control, Mechanical engineering and Computer Science, University of Valenciennes and Hainaut-Cambrésis, Valenciennes, France. taleb@univ-valenciennes.fr.
authors.forename: Abdelmalik
authors.initials: A
authors.lastname: Taleb-Ahmed
authors.affiliation: Unité de Traitements de Signaux Biomédicaux, Faculté Libre de Médecine, Lille, France. foued.derraz@icl-lille.fr.
authors.forename: Foued
authors.initials: F
authors.lastname: Derraz
authors.affiliation: Unité de Traitements de Signaux Biomédicaux, Hautes Etudes d'Ingénieur, Lille, France. Laurent.PEYRODIE@hei.fr.
authors.forename: Laurent
authors.initials: L
authors.lastname: Peyrodie
authors.affiliation: Signal Processing Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland. jean-philippe.thiran@epfl.ch.|Department of Radiology, University Hospital Center (CHUV) and University of Lausanne, Lausanne, Switzerland. jean-philippe.thiran@epfl.ch.
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Thiran
authors.affiliation: Academic Department of Radiation Oncology, Centre Oscar Lambret, Lille, France. d-pasquier@o-lambret.fr.
authors.forename: David
authors.initials: D
authors.lastname: Pasquier
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D009038:Motion; D009929:Organ Size; D058958:Organs at Risk; D010384:Pelvic Bones / Q000000981:diagnostic imaging; D011467:Prostate / Q000000981:diagnostic imaging; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors / Q000517:prevention & control*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated; D012007:Rectum / Q000000981:diagnostic imaging; D012189:Retrospective Studies; D001743:Urinary Bladder / Q000000981:diagnostic imaging