title: On-line use of three-dimensional marker trajectory estimation from cone-beam computed tomography projections for precise setup in radiotherapy for targets with respiratory motion.
abstract: OBJECTIVE
  To develop and evaluate accurate and objective on-line patient setup based on a novel semiautomatic technique in which three-dimensional marker trajectories were estimated from two-dimensional cone-beam computed tomography (CBCT) projections.
  
  
  METHODS
  Seven treatment courses of stereotactic body radiotherapy for liver tumors were delivered in 21 fractions in total to 6 patients by a linear accelerator. Each patient had two to three gold markers implanted close to the tumors. Before treatment, a CBCT scan with approximately 675 two-dimensional projections was acquired during a full gantry rotation. The marker positions were segmented in each projection. From this, the three-dimensional marker trajectories were estimated using a probability based method. The required couch shifts for patient setup were calculated from the mean marker positions along the trajectories. A motion phantom moving with known tumor trajectories was used to examine the accuracy of the method. Trajectory-based setup was retrospectively used off-line for the first five treatment courses (15 fractions) and on-line for the last two treatment courses (6 fractions). Automatic marker segmentation was compared with manual segmentation. The trajectory-based setup was compared with setup based on conventional CBCT guidance on the markers (first 15 fractions).
  
  
  RESULTS
  Phantom measurements showed that trajectory-based estimation of the mean marker position was accurate within 0.3 mm. The on-line trajectory-based patient setup was performed within approximately 5 minutes. The automatic marker segmentation agreed with manual segmentation within 0.36 ± 0.50 pixels (mean ± SD; pixel size, 0.26 mm in isocenter). The accuracy of conventional volumetric CBCT guidance was compromised by motion smearing (≤21 mm) that induced an absolute three-dimensional setup error of 1.6 ± 0.9 mm (maximum, 3.2) relative to trajectory-based setup.
  
  
  CONCLUSIONS
  The first on-line clinical use of trajectory estimation from CBCT projections for precise setup in stereotactic body radiotherapy was demonstrated. Uncertainty in the conventional CBCT-based setup procedure was eliminated with the new method.
authors.affiliation: Department of Oncology, Aarhus University Hospital, Aarhus, Denmark. esbeworm@rm.dk
authors.forename: Esben S
authors.initials: ES
authors.lastname: Worm
authors.forename: Morten
authors.initials: M
authors.lastname: Høyer
authors.forename: Walther
authors.initials: W
authors.lastname: Fledelius
authors.forename: Jens E
authors.initials: JE
authors.lastname: Nielsen
authors.forename: Lars P
authors.initials: LP
authors.lastname: Larsen
authors.forename: Per R
authors.initials: PR
authors.lastname: Poulsen
mesh_terms: D001650:Bile Duct Neoplasms; D001653:Bile Ducts, Intrahepatic; D001943:Breast Neoplasms; D018281:Cholangiocarcinoma / Q000000981:diagnostic imaging / Q000556:secondary / Q000601:surgery; D015179:Colorectal Neoplasms; D054893:Cone-Beam Computed Tomography; D019583:Dose Fractionation, Radiation; D005260:Female; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008113:Liver Neoplasms / Q000000981:diagnostic imaging* / Q000556:secondary / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D009068:Movement; D019047:Phantoms, Imaging; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012119:Respiration
chemical_list: D006046:Gold