title: Feasibility of MRI-based reference images for image-guided radiotherapy of the pelvis with either cone-beam computed tomography or planar localization images.
abstract: OBJECTIVE
  This study introduces methods to conduct image-guided radiotherapy (IGRT) of the pelvis with either cone-beam computed tomography (CBCT) or planar localization images by relying solely on magnetic resonance imaging (MRI)-based reference images.
  
  
  METHODS
  Feasibility of MRI-based reference images for IGRT was evaluated against kV CBCT (50 scans, 5 prostate cancer patients) and kV & MV planar (5 & 5 image pairs and patients) localization images by comparing the achieved patient position corrections to those obtained by standard CT-based reference images. T1/T2*-weighted in-phase MRI, Hounsfield unit conversion-based heterogeneous pseudo-CT, and bulk pseudo-CT images were applied for reference against localization CBCTs, and patient position corrections were obtained by automatic image registration. IGRT with planar localization images was performed manually by 10 observers using reference digitally reconstructed radiographs (DRRs) reconstructed from the pseudo-CTs and standard CTs. Quality of pseudo-DRRs against CT-DRRs was evaluated with image similarity metrics.
  
  
  RESULTS
  The SDs of differences between CBCT-to-MRI and CBCT-to-CT automatic gray-value registrations were ≤1.0 mm & ≤0.8° and ≤2.5 mm & ≤3.6° with 10 cm diameter cubic VOI and prostate-shaped VOI, respectively. The corresponding values for reference heterogeneous pseudo-CT were ≤1.0 mm & ≤0.7° and ≤2.2 mm & ≤3.3°, respectively. Heterogeneous pseudo-CT was the only type of MRI-based reference image working reliably with automatic bone registration (SDs were ≤0.9 mm & ≤0.7°). The differences include possible residual errors from planning CT to MRI registration. The image similarity metrics were significantly (p≤0.01) better in agreement between heterogeneous pseudo-DRRs and CT-DRRs than between bulk pseudo-DRRs and CT-DRRs. The SDs of differences in manual registrations (3D) with planar kV and MV localization images were ≤1.0 mm and ≤1.7 mm, respectively, between heterogeneous pseudo-DRRs and CT-DRRs, and ≤1.4 mm and ≤2.1 mm between bulk pseudo-DRRs and CT-DRRs.
  
  
  CONCLUSIONS
  This study demonstrated that it is feasible to conduct IGRT of the pelvis with MRI-based reference images.
authors.affiliation: Clinical Research Institute Helsinki University Central Hospital Ltd , Helsinki , Finland.
authors.forename: Juha
authors.initials: J
authors.lastname: Korhonen
authors.forename: Mika
authors.initials: M
authors.lastname: Kapanen
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
authors.forename: Leonard
authors.initials: L
authors.lastname: Wee
authors.forename: Eero
authors.initials: E
authors.lastname: Salli
authors.forename: Jani
authors.initials: J
authors.lastname: Keyriläinen
authors.forename: Tiina
authors.initials: T
authors.lastname: Seppälä
authors.forename: Mikko
authors.initials: M
authors.lastname: Tenhunen
mesh_terms: D054893:Cone-Beam Computed Tomography; D005240:Feasibility Studies; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D010388:Pelvis / Q000000981:diagnostic imaging / Q000473:pathology; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided / Q000379:methods*