title: What CTV-to-PTV margins should be applied for prostate irradiation? Four-dimensional quantitative assessment using model-based deformable image registration techniques.
abstract: OBJECTIVE
  To quantify adequate anisotropic clinical target volume (CTV)-to-planning target volume (PTV) margins for three different setup strategies used during prostate irradiation: (1) no setup corrections, (2) on-line corrections determined from bony anatomy, and (3) on-line corrections determined from gold markers.
  
  
  METHODS
  Three radiation oncologists independently delineated the CTV on computed tomography images of 30 prostate cancer patients. Eight repeat scans were acquired to allow simulation of the delivered dose distributions in changing geometry. Different registration approaches were taken to mimic the different setup strategies. A surface model-based deformable image registration system was used to warp the delivered dose distributions back to the dose in the planning computed tomography scan. On the basis of the geometric extent of the underdosed areas, a set of anisotropic margins was derived to ensure a minimal dose to the CTV of 95% for 90% of the patients.
  
  
  RESULTS
  Without setup correction, margins of approximately 11 mm for the corpus of the prostate and 15 mm for the seminal vesicles were required. These margins could be reduced to 8 and 13 mm when aligning the patient to the bony anatomy and to 3 and 8 mm aligning the patient to implanted gold markers. A larger margin at the apex was required to account for the significant observer variability and steep dose gradients at this location (11 mm using skin marker registration, 9 mm using bony anatomy registration, and 6 mm using gold marker registration).
  
  
  CONCLUSIONS
  Novel voxel tracking techniques have enabled us to calculate accumulated dose distributions and design accurate three-dimensional CTV-to-PTV margins for prostate irradiation.
authors.affiliation: Department of Radiotherapy, Catharina Hospital, Eindhoven, The Netherlands.
authors.forename: Gert J
authors.initials: GJ
authors.lastname: Meijer
authors.forename: Jeroen
authors.initials: J
authors.lastname: de Klerk
authors.forename: Karl
authors.initials: K
authors.lastname: Bzdusek
authors.forename: Hetty A
authors.initials: HA
authors.lastname: van den Berg
authors.forename: Rogier
authors.initials: R
authors.lastname: Janssen
authors.forename: Michael R
authors.initials: MR
authors.lastname: Kaus
authors.forename: Patrick
authors.initials: P
authors.lastname: Rodrigus
authors.forename: Peter-Paul
authors.initials: PP
authors.lastname: van der Toorn
mesh_terms: D002985:Clinical Protocols; D006801:Humans; D008297:Male; D011467:Prostate / Q000033:anatomy & histology / Q000473:pathology; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D011877:Radionuclide Imaging; D011880:Radiotherapy Planning, Computer-Assisted; D015203:Reproducibility of Results; D012669:Seminal Vesicles / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D012867:Skin / Q000473:pathology / Q000528:radiation effects; D014057:Tomography, X-Ray Computed