title: Comparison of megavoltage position verification for prostate irradiation based on bony anatomy and implanted fiducials.
abstract: OBJECTIVE
  The patient position during radiotherapy treatment of prostate cancer can be verified with the help of portal images acquired during treatment. In this study we quantify the clinical consequences of the use of image-based verification based on the bony anatomy and the prostate target itself.
  
  
  METHODS
  We analysed 2025 portal images and 23 computed tomography (CT) scans from 23 patients with prostate cancer. In all patients gold markers were implanted prior to CT scanning. Statistical data for both random and systematic errors were calculated for displacements of bones and markers and we investigated the effectiveness of an off-line correction protocol.
  
  
  RESULTS
  Standard deviations for systematic marker displacement are 2.4 mm in the lateral (LR) direction, 4.4 mm in the anterior-posterior (AP) direction and 3.7 mm in the caudal-cranial direction (CC). Application of off-line position verification based on the marker positions results in a shrinkage of the systematic error to well below 1 mm. Position verification based on the bony anatomy reduces the systematic target uncertainty to 50% in the AP direction and in the LR direction. No reduction was observed in the CC direction. For six out of 23 patients we found an increase of the systematic error after application of bony anatomy-based position verification.
  
  
  CONCLUSIONS
  We show that even if correction based on the bony anatomy is applied, considerable margins have to be set to account for organ motion. Our study highlights that for individual patients the systematic error can increase after application of bony anatomy-based position verification, whereas the population standard deviation will decrease. Off-line target-based position verification effectively reduces the systematic error to well below 1 mm, thus enabling significant margin reduction.
authors.affiliation: Department of Radiation Oncology, University Medical Center Utrecht, MS Q00.118, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
authors.forename: Aart J
authors.initials: AJ
authors.lastname: Nederveen
authors.forename: Homan
authors.initials: H
authors.lastname: Dehnad
authors.forename: Uulke A
authors.initials: UA
authors.lastname: van der Heide
authors.forename: R Jeroen A
authors.initials: RJ
authors.lastname: van Moorselaar
authors.forename: Pieter
authors.initials: P
authors.lastname: Hofman
authors.forename: Jan J W
authors.initials: JJ
authors.lastname: Lagendijk
mesh_terms: D016000:Cluster Analysis; D006046:Gold; D006801:Humans; D008297:Male; D009068:Movement; D010347:Patient Care Planning; D010384:Pelvic Bones / Q000000981:diagnostic imaging*; D011187:Posture; D011467:Prostate / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*
chemical_list: D006046:Gold