title: Analysis of interfraction prostate motion using megavoltage cone beam computed tomography.
abstract: OBJECTIVE
  Determine the degree of interfraction prostate motion and its components measured by using daily megavoltage (MV) cone beam computed tomography (CBCT) imaging.
  
  
  METHODS
  A total of 984 daily MV CBCT images from 24 patients undergoing definitive intensity-modulated radiotherapy for localized prostate cancer were analyzed retrospectively. Pretreatment couch shifts, based on physician registration of MV CBCT to planning CT data sets, were used as a measure of daily interfraction motion. Off-line bony registration was performed to separate bony misalignment from internal organ motion. Interobserver and intraobserver variation studies were performed on 20 MV CBCT images.
  
  
  RESULTS
  Mean interfraction prostate motion was 6.7 mm, with the greatest single-axis deviation in the anterior-posterior (AP) direction. The largest positional inaccuracy was accounted for by systematic deviations in bony misalignment, whereas random deviations occurred from bony misalignment and internal prostate motion. In the aggregate, AP motion did not correlate with days elapsed since beginning therapy or on average with rectal size at treatment planning. Interobserver variation was greatest in the AP direction, decreased in experienced observers, and further decreased in intraobserver studies. Mean interfraction motion during the first 6 days of therapy, when used as a subsequent offset, reduced acceptable AP planning target volume margins by 50%.
  
  
  CONCLUSIONS
  The MV CBCT is a practical direct method of daily localization that shows significant interfraction motion with respect to conventional three-dimensional conformal and intensity-modulated radiotherapy margins, similar to that measured in other modalities.
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, IA, USA.
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Bylund
authors.forename: John E
authors.initials: JE
authors.lastname: Bayouth
authors.forename: Mark C
authors.initials: MC
authors.lastname: Smith
authors.forename: A Curtis
authors.initials: AC
authors.lastname: Hass
authors.forename: Sudershan K
authors.initials: SK
authors.lastname: Bhatia
authors.forename: John M
authors.initials: JM
authors.lastname: Buatti
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D011467:Prostate / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011630:Pubic Bone / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012007:Rectum / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging