title: Subsecond and Submillimeter Resolution Positional Verification for Stereotactic Irradiation of Spinal Lesions.
abstract: OBJECTIVE
  Spine stereotactic body radiation therapy (SBRT) requires highly accurate positioning. We report our experience with markerless template matching and triangulation of kilovoltage images routinely acquired during spine SBRT, to determine spine position.
  
  
  METHODS
  Kilovoltage images, continuously acquired at 7, 11 or 15 frames/s during volumetric modulated spine SBRT of 18 patients, consisting of 93 fluoroscopy datasets (1 dataset/arc), were analyzed off-line. Four patients were immobilized in a head/neck mask, 14 had no immobilization. Two-dimensional (2D) templates were created for each gantry angle from planning computed tomography data and registered to prefiltered kilovoltage images to determine 2D shifts between actual and planned spine position. Registrations were considered valid if the normalized cross correlation score was ≥0.15. Multiple registrations were triangulated to determine 3D position. For each spine position dataset, average positional offset and standard deviation were calculated. To verify the accuracy and precision of the technique, mean positional offset and standard deviation for twenty stationary phantom datasets with different baseline shifts were measured.
  
  
  RESULTS
  For the phantom, average standard deviations were 0.18 mm for left-right (LR), 0.17 mm for superior-inferior (SI), and 0.23 mm for the anterior-posterior (AP) direction. Maximum difference in average detected and applied shift was 0.09 mm. For the 93 clinical datasets, the percentage of valid matched frames was, on average, 90.7% (range: 49.9-96.1%) per dataset. Average standard deviations for all datasets were 0.28, 0.19, and 0.28 mm for LR, SI, and AP, respectively. Spine position offsets were, on average, -0.05 (range: -1.58 to 2.18), -0.04 (range: -3.56 to 0.82), and -0.03 mm (range: -1.16 to 1.51), respectively. Average positional deviation was <1 mm in all directions in 92% of the arcs.
  
  
  CONCLUSIONS
  Template matching and triangulation using kilovoltage images acquired during irradiation allows spine position detection with submillimeter accuracy at subsecond intervals. Although the majority of patients were not immobilized, most vertebrae were stable at the sub-mm level during spine SBRT delivery.
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands. Electronic address: c.hazelaar@vumc.nl.
authors.forename: Colien
authors.initials: C
authors.lastname: Hazelaar
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Max
authors.initials: M
authors.lastname: Dahele
authors.affiliation: Ginzton Technology Center, Varian Medical Systems, Palo Alto, California.
authors.forename: Hassan
authors.initials: H
authors.lastname: Mostafavi
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Lineke
authors.initials: L
authors.lastname: van der Weide
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Ben J
authors.initials: BJ
authors.lastname: Slotman
authors.affiliation: Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Wilko F A R
authors.initials: WF
authors.lastname: Verbakel
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D054893:Cone-Beam Computed Tomography / Q000379:methods; D005260:Female; D006801:Humans; D007103:Immobilization; D008297:Male; D008875:Middle Aged; D009068:Movement; D058958:Organs at Risk / Q000528:radiation effects; D019047:Phantoms, Imaging; D011832:Radiation Injuries / Q000517:prevention & control; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated; D013116:Spinal Cord / Q000528:radiation effects; D013125:Spinal Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D013131:Spine / Q000000981:diagnostic imaging*