title: Accuracy of surface registration compared to conventional volumetric registration in patient positioning for head-and-neck radiotherapy: a simulation study using patient data.
abstract: OBJECTIVE
  3D optical surface imaging has been applied to patient positioning in radiation therapy (RT). The optical patient positioning system is advantageous over conventional method using cone-beam computed tomography (CBCT) in that it is radiation free, frameless, and is capable of real-time monitoring. While the conventional radiographic method uses volumetric registration, the optical system uses surface matching for patient alignment. The relative accuracy of these two methods has not yet been sufficiently investigated. This study aims to investigate the theoretical accuracy of the surface registration based on a simulation study using patient data.
  
  
  METHODS
  This study compares the relative accuracy of surface and volumetric registration in head-and-neck RT. The authors examined 26 patient data sets, each consisting of planning CT data acquired before treatment and patient setup CBCT data acquired at the time of treatment. As input data of surface registration, patient's skin surfaces were created by contouring patient skin from planning CT and treatment CBCT. Surface registration was performed using the iterative closest points algorithm by point-plane closest, which minimizes the normal distance between source points and target surfaces. Six degrees of freedom (three translations and three rotations) were used in both surface and volumetric registrations and the results were compared. The accuracy of each method was estimated by digital phantom tests.
  
  
  RESULTS
  Based on the results of 26 patients, the authors found that the average and maximum root-mean-square translation deviation between the surface and volumetric registrations were 2.7 and 5.2 mm, respectively. The residual error of the surface registration was calculated to have an average of 0.9 mm and a maximum of 1.7 mm.
  
  
  CONCLUSIONS
  Surface registration may lead to results different from those of the conventional volumetric registration. Only limited accuracy can be achieved for patient positioning with an approach based solely on surface information.
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305 and Center for Bionics, Korea Institute of Science and Technology, Seoul 136-791, South Korea.
authors.forename: Youngjun
authors.initials: Y
authors.lastname: Kim
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305.
authors.forename: Ruijiang
authors.initials: R
authors.lastname: Li
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305.
authors.forename: Yong Hum
authors.initials: YH
authors.lastname: Na
authors.affiliation: Department of Radiation Oncology, Ewha Womans University School of Medicine, Seoul 158-710, South Korea.
authors.forename: Rena
authors.initials: R
authors.lastname: Lee
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305.
authors.forename: Lei
authors.initials: L
authors.lastname: Xing
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D006257:Head / Q000000981:diagnostic imaging; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation / Q000379:methods*; D008954:Models, Biological; D009333:Neck / Q000000981:diagnostic imaging / Q000528:radiation effects; D056888:Patient Positioning / Q000379:methods*; D019047:Phantoms, Imaging; D061089:Radiotherapy, Image-Guided / Q000379:methods*