title: Image-guided localization accuracy of stereoscopic planar and volumetric imaging methods for stereotactic radiation surgery and stereotactic body radiation therapy: a phantom study.
abstract: OBJECTIVE
  To evaluate the positioning accuracies of two image-guided localization systems, ExacTrac and On-Board Imager (OBI), in a stereotactic treatment unit.
  
  
  METHODS
  An anthropomorphic pelvis phantom with eight internal metal markers (BBs) was used. The center of one BB was set as plan isocenter. The phantom was set up on a treatment table with various initial setup errors. Then, the errors were corrected using each of the investigated systems. The residual errors were measured with respect to the radiation isocenter using orthogonal portal images with field size 3 × 3 cm(2). The angular localization discrepancies of the two systems and the correction accuracy of the robotic couch were also studied. A pair of pre- and post-cone beam computed tomography (CBCT) images was acquired for each angular correction. Then, the correction errors were estimated by using the internal BBs through fiducial marker-based registrations.
  
  
  RESULTS
  The isocenter localization errors (μ ±σ) in the left/right, posterior/anterior, and superior/inferior directions were, respectively, -0.2 ± 0.2 mm, -0.8 ± 0.2 mm, and -0.8 ± 0.4 mm for ExacTrac, and 0.5 ± 0.7 mm, 0.6 ± 0.5 mm, and 0.0 ± 0.5 mm for OBI CBCT. The registration angular discrepancy was 0.1 ± 0.2° between the two systems, and the maximum angle correction error of the robotic couch was 0.2° about all axes.
  
  
  CONCLUSIONS
  Both the ExacTrac and the OBI CBCT systems showed approximately 1 mm isocenter localization accuracies. The angular discrepancy of two systems was minimal, and the robotic couch angle correction was accurate. These positioning uncertainties should be taken as a lower bound because the results were based on a rigid dosimetry phantom.
authors.affiliation: Department of Radiation Oncology, Henry Ford Health System, Detroit, MI, USA. jkim3@hfhs.org
authors.forename: Jinkoo
authors.initials: J
authors.lastname: Kim
authors.forename: Jian-Yue
authors.initials: JY
authors.lastname: Jin
authors.forename: Nicole
authors.initials: N
authors.lastname: Walls
authors.forename: Teamour
authors.initials: T
authors.lastname: Nurushev
authors.forename: Benjamin
authors.initials: B
authors.lastname: Movsas
authors.forename: Indrin J
authors.initials: IJ
authors.lastname: Chetty
authors.forename: Samuel
authors.initials: S
authors.lastname: Ryu
mesh_terms: D000465:Algorithms; D054893:Cone-Beam Computed Tomography / Q000379:methods; D004867:Equipment Design; D021621:Imaging, Three-Dimensional / Q000295:instrumentation / Q000379:methods / Q000592:standards; D008953:Models, Anatomic; D010315:Particle Accelerators; D010388:Pelvis; D019047:Phantoms, Imaging; D055585:Physical Phenomena; D015641:Radiography, Interventional / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D012015:Reference Standards; D012371:Robotics / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D012399:Rotation; D013679:Technology, Radiologic / Q000295:instrumentation / Q000379:methods / Q000592:standards