title: Measurement of intrafractional prostate motion using magnetic resonance imaging.
abstract: OBJECTIVE
  To quantify the three-dimensional intrafractional prostate motion over typical treatment time intervals with cine-magnetic resonance imaging (cine MRI) studies.
  
  
  METHODS
  Forty-two patients with prostate cancer were scanned supine in an alpha cradle cast using cine MRI. Twenty sequential slices were acquired in the sagittal and axial planes through the center of the prostate. Each scan took approximately 9 min. The posterior, lateral, and superior edges of the prostate were tracked on each frame relative to the initial prostate position, and the size and duration of each displacement was recorded.
  
  
  RESULTS
  The prostate displacements were (mean +/- SD): 0.2 +/- 2.9 mm, 0.0 +/- 3.4 mm, and 0.0 +/- 1.5 mm in the anterior-posterior, superior-inferior, and medial-lateral dimensions respectively. The prostate motion appeared to have been driven by peristalsis in the rectum. Large displacements of the prostate (up to 1.2 cm) moved the prostate both anteriorly and superiorly and in some cases compressed the organ. For such motions, the prostate did not stay displaced, but moved back to its original position. To account for the dosimetric consequences of the motion, we also calculated the time-averaged displacement to be approximately 1 mm.
  
  
  CONCLUSIONS
  Cine MRI can be used to measure intrafractional prostate motion. Although intrafractional prostate motions occur, their effects are negligible compared to interfractional motion and setup error. No adjustment in margin is necessary for three-dimensional conformal or intensity-modulated radiation therapy.
authors.affiliation: Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA, USA. dm2066@columbia.edu
authors.forename: Dennis
authors.initials: D
authors.lastname: Mah
authors.forename: Gary
authors.initials: G
authors.lastname: Freedman
authors.forename: Bart
authors.initials: B
authors.lastname: Milestone
authors.forename: Alexandra
authors.initials: A
authors.lastname: Hanlon
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Palacio
authors.forename: Theresa
authors.initials: T
authors.lastname: Richardson
authors.forename: Benjamin
authors.initials: B
authors.lastname: Movsas
authors.forename: Raj
authors.initials: R
authors.lastname: Mitra
authors.forename: Eric
authors.initials: E
authors.lastname: Horwitz
authors.forename: Gerald E
authors.initials: GE
authors.lastname: Hanks
mesh_terms: D006801:Humans; D019028:Magnetic Resonance Imaging, Cine; D008297:Male; D009068:Movement; D015588:Observer Variation; D011467:Prostate; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D013997:Time Factors