title: Intrafractional prostate motion during online image guided intensity-modulated radiotherapy for prostate cancer.
abstract: BACKGROUND
  Intrafractional motion consists of two components: (1) the movement between the on-line repositioning procedure and the treatment start and (2) the movement during the treatment delivery. The goal of this study is to estimate this intrafractional movement of the prostate during prostate cancer radiotherapy.
  
  
  METHODS
  Twenty-seven patients with prostate cancer and implanted fiducials underwent a marker match procedure before a five-field IMRT treatment. For all fields, in-treatment images were obtained and then processed to enable automatic marker detection. Combining the subsequent projection images, five positions of each marker were determined using the shortest path approach. The residual set-up error (RSE) after kV-MV based prostate localization, the prostate position as a function of time during a radiotherapy session and the required margins to account for intrafractional motion were determined.
  
  
  RESULTS
  The mean RSE and standard deviation in the antero-posterior, cranio-caudal and left-right direction were 2.3±1.5 mm, 0.2±1.1 mm and -0.1±1.1 mm, respectively. Almost all motions occurred in the posterior direction before the first treatment beam as the percentage of excursions>5 mm was reduced significantly when the RSE was not accounted for. The required margins for intrafractional motion increased with prolongation of the treatment. Application of a repositioning protocol after every beam could decrease the 1cm margin from CTV to PTV by 2 mm.
  
  
  CONCLUSIONS
  The RSE is the main contributor to intrafractional motion. This RSE after on-line prostate localization and patient repositioning in the posterior direction emphasizes the need to speed up the marker match procedure. Also, a prostate IMRT treatment should be administered as fast as possible, to ensure that the pre-treatment repositioning efforts are not erased by intrafractional prostate motion. This warrants an optimized workflow with the use of faster treatment techniques.
authors.affiliation: Department of Radiation Oncology, Leuven Cancer Institute, Belgium. tom.budiharto@uz.kuleuven.ac.be
authors.forename: Tom
authors.initials: T
authors.lastname: Budiharto
authors.forename: Pieter
authors.initials: P
authors.lastname: Slagmolen
authors.forename: Karin
authors.initials: K
authors.lastname: Haustermans
authors.forename: Frederik
authors.initials: F
authors.lastname: Maes
authors.forename: Sara
authors.initials: S
authors.lastname: Junius
authors.forename: Jan
authors.initials: J
authors.lastname: Verstraete
authors.forename: Raymond
authors.initials: R
authors.lastname: Oyen
authors.forename: Jeroen
authors.initials: J
authors.lastname: Hermans
authors.forename: Frank
authors.initials: F
authors.lastname: Van den Heuvel
mesh_terms: D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D009038:Motion; D056888:Patient Positioning; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*