title: Dose-volume differences for computed tomography and magnetic resonance imaging segmentation and planning for proton prostate cancer therapy.
abstract: OBJECTIVE
  To determine the influence of magnetic-resonance-imaging (MRI)-vs. computed-tomography (CT)-based prostate and normal structure delineation on the dose to the target and organs at risk during proton therapy.
  
  
  METHODS
  Fourteen patients were simulated in the supine position using both CT and T2 MRI. The prostate, rectum, and bladder were delineated on both imaging modalities. The planning target volume (PTV) was generated from the delineated prostates with a 5-mm axial and 8-mm superior and inferior margin. Two plans were generated and analyzed for each patient: an MRI plan based on the MRI-delineated PTV, and a CT plan based on the CT-delineated PTV. Doses of 78 Gy equivalents (GE) were prescribed to the PTV.
  
  
  RESULTS
  Doses to normal structures were lower when MRI was used to delineate the rectum and bladder compared with CT: bladder V50 was 15.3% lower (p = 0.04), and rectum V50 was 23.9% lower (p = 0.003). Poor agreement on the definition of the prostate apex was seen between CT and MRI (p = 0.007). The CT-defined prostate apex was within 2 mm of the apex on MRI only 35.7% of the time. Coverage of the MRI-delineated PTV was significantly decreased with the CT-based plan: the minimum dose to the PTV was reduced by 43% (p < 0.001), and the PTV V99% was reduced by 11% (p < 0.001).
  
  
  CONCLUSIONS
  Using MRI to delineate the prostate results in more accurate target definition and a smaller target volume compared with CT, allowing for improved target coverage and decreased doses to critical normal structures.
authors.affiliation: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, FL, USA.
authors.forename: Anamaria R
authors.initials: AR
authors.lastname: Yeung
authors.forename: Carlos E
authors.initials: CE
authors.lastname: Vargas
authors.forename: Aaron
authors.initials: A
authors.lastname: Falchook
authors.forename: Debbie
authors.initials: D
authors.lastname: Louis
authors.forename: Kenneth
authors.initials: K
authors.lastname: Olivier
authors.forename: Sameer
authors.initials: S
authors.lastname: Keole
authors.forename: Daniel
authors.initials: D
authors.lastname: Yeung
authors.forename: Nancy P
authors.initials: NP
authors.lastname: Mendenhall
authors.forename: Zuofeng
authors.initials: Z
authors.lastname: Li