title: Converting from CT- to MRI-only-based target definition in radiotherapy of localized prostate cancer: A comparison between two modalities.
abstract: OBJECTIVE
  To investigate the conversion of prostate cancer radiotherapy (RT) target definition from CT-based planning into an MRI-only-based planning procedure.
  
  
  METHODS
  Using the CT- and MRI-only-based RT planning protocols, 30 prostate cancer patients were imaged in the RT fixation position. Two physicians delineated the prostate in both CT and T2-weighted MRI images. The CT and MRI images were coregistered based on gold seeds and anatomic borders of the prostate. The uncertainty of the coregistration, as well as differences in target volumes and uncertainty of contour delineation were investigated. Conversion of margins and dose constraints from CT- to MRI-only-based treatment planning was assessed.
  
  
  RESULTS
  On average, the uncertainty of image coregistration was 0.4 ± 0.5 mm (one standard deviation, SD), 0.9 ± 0.8 mm and 0.9 ± 0.9 mm in the lateral, anterior-posterior and base-apex direction, respectively. The average ratio of the prostate volume between CT and MRI was 1.20 ± 0.15 (one SD). Compared to the CT-based contours, the MRI-based contours were on average 2-7 mm smaller in the apex, 0-1 mm smaller in the rectal direction and 1-4 mm smaller elsewhere.
  
  
  CONCLUSIONS
  When converting from a CT-based planning procedure to an MRI-based one, the overall planning target volumes (PTV) are prominently reduced only in the apex. The prostate margins and dose constraints can be retained by this conversion.
authors.affiliation: Comprehensive Cancer Center, University of Helsinki and Helsinki University Hospital, POB 180, 00029, Helsinki, Finland. tiina.seppala@hus.fi.
authors.forename: Tiina
authors.initials: T
authors.lastname: Seppälä
authors.affiliation: Comprehensive Cancer Center, University of Helsinki and Helsinki University Hospital, POB 180, 00029, Helsinki, Finland.
authors.forename: Harri
authors.initials: H
authors.lastname: Visapää
authors.affiliation: Comprehensive Cancer Center, University of Helsinki and Helsinki University Hospital, POB 180, 00029, Helsinki, Finland.
authors.forename: Juhani
authors.initials: J
authors.lastname: Collan
authors.affiliation: Comprehensive Cancer Center, University of Helsinki and Helsinki University Hospital, POB 180, 00029, Helsinki, Finland.
authors.forename: Mika
authors.initials: M
authors.lastname: Kapanen
authors.affiliation: HUS Medical Imaging Center, Radiology, University of Helsinki and Helsinki University Hospital, POB 180, 00029, Helsinki, Finland.
authors.forename: Annette
authors.initials: A
authors.lastname: Beule
authors.affiliation: Comprehensive Cancer Center, University of Helsinki and Helsinki University Hospital, POB 180, 00029, Helsinki, Finland.
authors.forename: Mauri
authors.initials: M
authors.lastname: Kouri
authors.affiliation: Comprehensive Cancer Center, University of Helsinki and Helsinki University Hospital, POB 180, 00029, Helsinki, Finland.
authors.forename: Mikko
authors.initials: M
authors.lastname: Tenhunen
authors.affiliation: Comprehensive Cancer Center, University of Helsinki and Helsinki University Hospital, POB 180, 00029, Helsinki, Finland.
authors.forename: Kauko
authors.initials: K
authors.lastname: Saarilahti
mesh_terms: D000230:Adenocarcinoma / Q000175:diagnosis / Q000532:radiotherapy*; D000368:Aged; D000369:Aged, 80 and over; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D058958:Organs at Risk / Q000528:radiation effects; D011471:Prostatic Neoplasms / Q000175:diagnosis / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D020266:Radiotherapy, Conformal / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*; D016896:Treatment Outcome
keywords: Bladder; Imaging; Intensity-modulated radiotherapy; Organs at risk; Rectum