title: Dosimetric characterization of MRI-only treatment planning for brain tumors in atlas-based pseudo-CT images generated from standard T1-weighted MR images.
abstract: OBJECTIVE
  Magnetic resonance imaging (MRI)-only radiotherapy treatment planning requires accurate pseudo-CT (pCT) images for precise dose calculation. The current work introduced an atlas-based method combined with MR intensity information. pCT analyses and Monte Carlo dose calculations for intracranial stereotactic treatments were performed.
  
  
  METHODS
  Twenty-two patients, representing 35 tumor targets, were scanned using a 3D T1-weighted MRI sequence according to the clinical protocol. The MR atlas image was registered to the MR patient image using a deformable algorithm, and the deformation was then applied to the atlas CT. Two methods were applied. The first method (MRdef) was based on deformations only, while the second (MRint) also used the actual MR intensities. pCT analysis was performed using the mean (absolute) error, as well as an in-house tool based on a gamma index. Dose differences between pCT and true CT were analyzed using dose-volume histogram (DVH) parameters, statistical tests, the gamma index, and probability density functions. An unusual case, where the patient underwent an operation (part of the skull bone was removed), was studied in detail.
  
  
  RESULTS
  Soft tissues presented a mean error inferior to 50 HUs, while low-density tissues and bones presented discrepancies up to 600 HUs for hard bone. The MRdef method led to significant dose differences compared with the true CT (p-value < 0.05; Wilcoxon-signed-rank test). The MRint method performed better. The DVH parameter differences compared with CT were between -2.9% and 3.1%, except for two cases where the tumors were located within the sphenoid bone. For these cases, the dose errors were up to 6.6% and 5.4% (D98and D95). Furthermore, for 85% of the tested patients, the mean dose to the planning target volume agreed within 2% with the calculation using the actual CT. Fictitious bone was generated in the unusual case using atlas-based methods.
  
  
  CONCLUSIONS
  Generally, the atlas-based method led to acceptable dose distributions. The use of common T1 sequences allows the implementation of this method in clinical routine. However, unusual patient anatomy may produce large dose calculation errors. The detection of large anatomic discrepancies using MR image subtraction can be realized, but an alternative way to produce synthetic CT numbers in these regions is still required.
authors.affiliation: Department of Radiotherapy, Centre Oscar Lambret, Lille 59000, France; Department of Research and Development, AQUILAB SAS, Loos Les Lille 59120, France; and Department of Research, IEMN, UMR CNRS 8520, Villeneuve d'Ascq 59650, France.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Demol
authors.affiliation: Department of Radiotherapy, Centre Oscar Lambret, Lille 59000, France.
authors.forename: Christine
authors.initials: C
authors.lastname: Boydev
authors.affiliation: Department of Radiation Oncology, Comprehensive Cancer Center, Helsinki University Central Hospital, Helsinki FI-00029, Finland and Department of Radiology, Helsinki University Central Hospital, Helsinki FI-00029, Finland.
authors.forename: Juha
authors.initials: J
authors.lastname: Korhonen
authors.affiliation: Department of Radiotherapy, Centre Oscar Lambret, Lille 59000, France.
authors.forename: Nick
authors.initials: N
authors.lastname: Reynaert
mesh_terms: D000465:Algorithms; D001932:Brain Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D006801:Humans; D008279:Magnetic Resonance Imaging; D009010:Monte Carlo Method; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014057:Tomography, X-Ray Computed