title: Intrafraction motion of the prostate during external-beam radiation therapy: analysis of 427 patients with implanted fiducial markers.
abstract: OBJECTIVE
  To analyze the intrafraction motion of the prostate during external-beam radiation therapy of patients with prostate cancer.
  
  
  METHODS
  Between August 2001-December 2005, 427 patients with Stage T3Nx/0Mx/0 prostate carcinoma received intensity-modulated radiation therapy treatment combined with position verification with fiducial gold markers. For a total of 11,426 treatment fractions (average, 27 per patient), portal images were taken of the first segment of all five beams. The irradiation time of the technique varied between 5-7 min. From these data, the location of gold markers could be established within every treatment beam under the assumption of minimal marker movement.
  
  
  RESULTS
  In 66% of treatment fractions, a motion outside a range of 2 mm was observed, with 28% outside a range of 3 mm. The intrafraction marker movements showed that motion directions were often reversed. However, the effect was small. Even with perfect online position-correction at the start of irradiation, intrafraction motion caused position uncertainty, but systematic errors (Sigma) were limited to <0.6 mm, and random errors (sigma) to <0.9 mm. This would result in a lower limit of 2 mm for margins, in the absence of any other uncertainties.
  
  
  CONCLUSIONS
  Intrafraction motion of the prostate occurs frequently during external-beam irradiation on a time scale of 5-7 min. Margins of 2 mm account for these intrafraction motions. However, larger margins are required in practice to accommodate other uncertainties in the treatment.
authors.affiliation: Department of Radiotherapy, University Medical Center, Utrecht, The Netherlands. A.N.T.J.Kotte@umcutrecht.nl
authors.forename: Alexis N T J
authors.initials: AN
authors.lastname: Kotte
authors.forename: Pieter
authors.initials: P
authors.lastname: Hofman
authors.forename: Jan J W
authors.initials: JJ
authors.lastname: Lagendijk
authors.forename: Marco
authors.initials: M
authors.lastname: van Vulpen
authors.forename: Uulke A
authors.initials: UA
authors.lastname: van der Heide
mesh_terms: D006046:Gold; D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D019736:Prostheses and Implants; D050397:Radiotherapy, Intensity-Modulated
chemical_list: D006046:Gold