title: Magnetic resonance assessment of prostate localization variability in intensity-modulated radiotherapy for prostate cancer.
abstract: OBJECTIVE
  To measure prostate motion with magnetic resonance imaging (MRI) during a course of intensity-modulated radiotherapy.
  
  
  METHODS
  Seven patients with prostate carcinoma were scanned supine on a 1.5-Tesla MRI system with weekly pretreatment and on-treatment HASTE T2-weighted images in 3 orthogonal planes. The bladder and rectal volumes and position of the prostatic midpoint (PMP) and margins relative to the bony pelvis were measured.
  
  
  RESULTS
  All pretreatment positions were at the mean position as computed from the on-treatment scans in each patient. The PMP variability (given as 1 SD) in the anterior-posterior (AP), superior-inferior (SI), and right-left (RL) directions was 2.6, 2.4, and 1.0 mm, respectively. The largest variabilities occurred at the posterior (3.2 mm), superior (2.6 mm), and inferior (2.6 mm) margins. A strong correlation was found between large rectal volume (>95th percentile) and anterior PMP displacement. A weak correlation was found between bladder volume and superior PMP displacement.
  
  
  CONCLUSIONS
  All pretreatment positions were representative of the subsequent on-treatment positions. A clinical target volume (CTV) expansion of 5.3 mm in any direction was sufficient to ascertain a 95% coverage of the CTV within the planning target volume (PTV), provided that a rectal suppository is administered to avoid rectal overdistension and that the patient has a comfortably filled bladder (<300 mL).
authors.affiliation: Department of Radiology, Ghent University Hospital, Ghent, Belgium. Geert.Villeirs@ugent.be
authors.forename: Geert M
authors.initials: GM
authors.lastname: Villeirs
authors.forename: Gert O
authors.initials: GO
authors.lastname: De Meerleer
authors.forename: Koenraad L
authors.initials: KL
authors.lastname: Verstraete
authors.forename: Wilfried J
authors.initials: WJ
authors.lastname: De Neve
mesh_terms: D000368:Aged; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009068:Movement; D011446:Prospective Studies; D011467:Prostate / Q000033:anatomy & histology*; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D020266:Radiotherapy, Conformal / Q000379:methods*; D012007:Rectum / Q000033:anatomy & histology; D001743:Urinary Bladder / Q000033:anatomy & histology