title: Prostate delineation using CT and MRI for radiotherapy patients with bilateral hip prostheses.
abstract: OBJECTIVE
  Prostate cancer patients with bilateral hip prostheses (b-HP) are rare, but pose unique challenges for high-dose radiotherapy. The purpose of this study was to determine the effect of b-HP on prostate delineation using CT and MR.
  
  
  METHODS
  The planning CT and MR of 7 patients with b-HP were co-registered. Four observers delineated the prostate on both data sets, blinded to the volumes delineated by their peers and those on the alternate image set. Inter-modality differences and inter-observer variability were quantified by calculating 3D volume overlaps. The effect of prostheses physical extent on these variables was evaluated. MR geometric distortion was explored using the differences in vector distances between three intra-prostatic markers (IPMs) on CT and MR.
  
  
  RESULTS
  Prostate volumes on CT were consistently larger than MR volumes in all patients when averaged over the 4 observers (+10% to +46%, p<0.001). The mean inter-modality volume overlap was 1.59, which varied significantly between patients (1.35 to 1.82, p=0.04). There was a trend toward smaller inter-observer variability in the MR volumes (1.95 vs. 1.71, p=0.08). No differences could be identified between the IPM vectors on CT and MR.
  
  
  CONCLUSIONS
  Although limited by a small sample size, this study found that MR prostate volumes were significantly smaller than those from CT for b-HP patients. The largest differences between CT and MR prostate volumes and the greatest inter-observer variability on CT tended to be seen in patients with the largest prostheses. b-HP do not appear to cause measurable MR geometric distortions in the sequences used.
authors.affiliation: Princess Margaret Hospital, Toronto, Canada. Tara.Rosewall@rmp.uhn.on.ca
authors.forename: Tara
authors.initials: T
authors.lastname: Rosewall
authors.forename: Vickie
authors.initials: V
authors.lastname: Kong
authors.forename: Danny
authors.initials: D
authors.lastname: Vesprini
authors.forename: Charles
authors.initials: C
authors.lastname: Catton
authors.forename: Peter
authors.initials: P
authors.lastname: Chung
authors.forename: Cynthia
authors.initials: C
authors.lastname: Ménard
authors.forename: Andrew
authors.initials: A
authors.lastname: Bayley
mesh_terms: D006622:Hip Prosthesis; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D015588:Observer Variation; D009929:Organ Size; D011467:Prostate / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D014057:Tomography, X-Ray Computed