title: CT- and MRI-based seed localization in postimplant evaluation after prostate brachytherapy.
abstract: OBJECTIVE
  To compare the uncertainties in CT- and MRI-based seed reconstruction in postimplant evaluation after prostate seed brachytherapy in terms of interobserver variability and quantify the impact of seed detection variability on a selection of dosimetric parameters for three postplan techniques: (1) CT, (2) MRI-T1 weighted fused with MRI-T2 weighted, and (3) CT fused with MRI-T2 weighted.
  
  
  METHODS
  Seven physicists reconstructed the seed positions on postimplant CT and MRI-T1 images of three patients. For each patient and imaging modality, the interobserver variability was calculated with respect to a reference seed set. The effect of this variability on dosimetry was calculated for CT and CT + MRI-T2 (CT-based seed reconstruction), as well as for MRI-T1 + MRI-T2 (MRI-T1-based seed reconstruction), using fixed CT and MRI-T2 prostate contours.
  
  
  RESULTS
  Averaged over three patients, the interobserver variability in CT-based seed reconstruction was 1.1 mm (1 SDref, i.e., standard deviation with respect to the reference value). The D90 (dose delivered to 90% of the target) variability was 1.5% and 1.3% (1 SDref) for CT and CT + MRI-T2, respectively. The mean interobserver variability in MRI-based seed reconstruction was 3.0 mm (1 SDref), and the impact of this variability on D90 was 6.6% for MRI-T1 + MRI-T2.
  
  
  CONCLUSIONS
  Seed reconstruction on MRI-T1-weighted images was less accurate than on CT. This difference in uncertainties should be weighted against uncertainties due to contouring and image fusion when comparing the overall reliability of postplan techniques.
authors.affiliation: Department of Radiation Oncology, University Hospital Gasthuisberg, Leuven, Belgium. Electronic address: marisol.debrabandere@uzleuven.be.
authors.forename: Marisol
authors.initials: M
authors.lastname: De Brabandere
authors.forename: Bashar
authors.initials: B
authors.lastname: Al-Qaisieh
authors.forename: Liesbeth
authors.initials: L
authors.lastname: De Wever
authors.forename: Karin
authors.initials: K
authors.lastname: Haustermans
authors.forename: Christian
authors.initials: C
authors.lastname: Kirisits
authors.forename: Marinus A
authors.initials: MA
authors.lastname: Moerland
authors.forename: Raymond
authors.initials: R
authors.lastname: Oyen
authors.forename: Alex
authors.initials: A
authors.lastname: Rijnders
authors.forename: Frank
authors.initials: F
authors.lastname: Van den Heuvel
authors.forename: Frank-André
authors.initials: FA
authors.lastname: Siebert
mesh_terms: D001918:Brachytherapy / Q000379:methods*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D015588:Observer Variation; D011467:Prostate / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000175:diagnosis / Q000532:radiotherapy*; D011874:Radiometry / Q000379:methods; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed / Q000379:methods*
keywords: Postplanning; Prostate; Seed reconstruction