title: Set-up errors and planning margins in planar and CBCT image-guided radiotherapy using three different imaging systems: A clinical study for prostate and head-and-neck cancer.
abstract: OBJECTIVE
  The purpose of this work is to compare the positioning accuracy achieved by three different imaging techniques and planar vs. CBCT imaging for two common IGRT indications.
  
  
  METHODS
  A collective of prostate cancer and head-and-neck cancer patients treated at our institution during the year 2013 was retrospectively analyzed. For all treatment fractions (3078 in total), the kind of acquired set-up image and the performed couch shift before treatment were assessed. The distribution of couch corrections was compared for three different imaging systems available at our institution: the treatment beam line operating at 6 MV, a dedicated imaging beam line of nominally 1 MV, and the kVision system at 70-121 kV. Shifts were analyzed for planar and cone-beam CT images. Based on the set-up corrections, CTV to PTV expansion margins were calculated.
  
  
  RESULTS
  The difference in set-up corrections performed for the three energies and both techniques (planar vs. CBCT) was not significant for head-and-neck cancer patients. For prostate cancer all shifts had equal variance. Averages ranged from -0.7 to +0.7 mm. The set-up margins calculated on the basis of the observed shifts are 4.0 mm (AP) and 3.8 mm (SI, LR) for the head-and-neck PTV and 6.6 mm (SI), 6.7 mm (AP) and 7.9 mm (LR) for the prostate cancer patients.
  
  
  CONCLUSIONS
  For three different linac-based imaging energies and planar/CBCT imaging, no relevant differences in set-up shifts were observed. The suggested set-up margins for these indications are of the order of 4 mm for head-and-neck and 6-8 mm for prostate treatment.
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Saarland University Medical Center, Homburg, Saar, Germany. Electronic address: yvonne.dzierma@uks.eu.
authors.forename: Yvonne
authors.initials: Y
authors.lastname: Dzierma
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Saarland University Medical Center, Homburg, Saar, Germany.
authors.forename: Monica
authors.initials: M
authors.lastname: Beyhs
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Saarland University Medical Center, Homburg, Saar, Germany.
authors.forename: Jan
authors.initials: J
authors.lastname: Palm
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Saarland University Medical Center, Homburg, Saar, Germany.
authors.forename: Marcus
authors.initials: M
authors.lastname: Niewald
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Saarland University Medical Center, Homburg, Saar, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Bell
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Saarland University Medical Center, Homburg, Saar, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Nuesken
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Saarland University Medical Center, Homburg, Saar, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Licht
authors.affiliation: Department of Radiotherapy and Radiation Oncology, Saarland University Medical Center, Homburg, Saar, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Rübe
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D006801:Humans; D008297:Male; D019047:Phantoms, Imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011877:Radionuclide Imaging; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061106:Radiotherapy Setup Errors; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D012189:Retrospective Studies
keywords: Image-guided radiotherapy with planar and cone-beam CT; Positioning accuracy; Prostate cancer and head-and-neck cancer treatment margins; kV and MV imaging