title: An atlas-based electron density mapping method for magnetic resonance imaging (MRI)-alone treatment planning and adaptive MRI-based prostate radiation therapy.
abstract: OBJECTIVE
  Prostate radiation therapy dose planning directly on magnetic resonance imaging (MRI) scans would reduce costs and uncertainties due to multimodality image registration. Adaptive planning using a combined MRI-linear accelerator approach will also require dose calculations to be performed using MRI data. The aim of this work was to develop an atlas-based method to map realistic electron densities to MRI scans for dose calculations and digitally reconstructed radiograph (DRR) generation.
  
  
  METHODS
  Whole-pelvis MRI and CT scan data were collected from 39 prostate patients. Scans from 2 patients showed significantly different anatomy from that of the remaining patient population, and these patients were excluded. A whole-pelvis MRI atlas was generated based on the manually delineated MRI scans. In addition, a conjugate electron-density atlas was generated from the coregistered computed tomography (CT)-MRI scans. Pseudo-CT scans for each patient were automatically generated by global and nonrigid registration of the MRI atlas to the patient MRI scan, followed by application of the same transformations to the electron-density atlas. Comparisons were made between organ segmentations by using the Dice similarity coefficient (DSC) and point dose calculations for 26 patients on planning CT and pseudo-CT scans.
  
  
  RESULTS
  The agreement between pseudo-CT and planning CT was quantified by differences in the point dose at isocenter and distance to agreement in corresponding voxels. Dose differences were found to be less than 2%. Chi-squared values indicated that the planning CT and pseudo-CT dose distributions were equivalent. No significant differences (p > 0.9) were found between CT and pseudo-CT Hounsfield units for organs of interest. Mean ± standard deviation DSC scores for the atlas-based segmentation of the pelvic bones were 0.79 ± 0.12, 0.70 ± 0.14 for the prostate, 0.64 ± 0.16 for the bladder, and 0.63 ± 0.16 for the rectum.
  
  
  CONCLUSIONS
  The electron-density atlas method provides the ability to automatically define organs and map realistic electron densities to MRI scans for radiotherapy dose planning and DRR generation. This method provides the necessary tools for MRI-alone treatment planning and adaptive MRI-based prostate radiation therapy.
authors.affiliation: Australian e-Health Research Center, CSIRO ICT Commonwealth Scientific and Industrial Research Organisation Information and Communication Technologies Centre, Queensland, Australia. jason.dowling@csiro.au
authors.forename: Jason A
authors.initials: JA
authors.lastname: Dowling
authors.forename: Jonathan
authors.initials: J
authors.lastname: Lambert
authors.forename: Joel
authors.initials: J
authors.lastname: Parker
authors.forename: Olivier
authors.initials: O
authors.lastname: Salvado
authors.forename: Jurgen
authors.initials: J
authors.lastname: Fripp
authors.forename: Anne
authors.initials: A
authors.lastname: Capp
authors.forename: Chris
authors.initials: C
authors.lastname: Wratten
authors.forename: James W
authors.initials: JW
authors.lastname: Denham
authors.forename: Peter B
authors.initials: PB
authors.lastname: Greer
mesh_terms: D000368:Aged; D016009:Chi-Square Distribution; D004583:Electrons; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008488:Medical Illustration; D008875:Middle Aged; D058958:Organs at Risk / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D010315:Particle Accelerators; D010384:Pelvic Bones / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D011467:Prostate / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D011856:Radiographic Image Enhancement / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods; D012007:Rectum / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed / Q000379:methods; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging