title: Treatment planning of intracranial targets on MRI derived substitute CT data.
abstract: BACKGROUND
  The use of magnetic resonance imaging (MRI) as a complement to computed tomography (CT) in the target definition procedure for radiotherapy is increasing. To eliminate systematic uncertainties due to image registration, a workflow based entirely on MRI may be preferable. In the present pilot study, we investigate dose calculation accuracy for automatically generated substitute CT (s-CT) images of the head based on MRI. We also produce digitally reconstructed radiographs (DRRs) from s-CT data to evaluate the feasibility of patient positioning based on MR images.
  
  
  METHODS
  Five patients were included in the study. The dose calculation was performed on CT, s-CT, s-CT data without inhomogeneity correction and bulk density assigned MRI images. Evaluation of the results was performed using point dose and dose volume histogram (DVH) comparisons, and gamma index evaluation.
  
  
  RESULTS
  The results demonstrate that the s-CT images improve the dose calculation accuracy compared to the method of non-inhomogeneity corrected dose calculations (mean improvement 2.0% points) and that it performs almost identically to the method of bulk density assignment. The s-CT based DRRs appear to be adequate for patient positioning of intra-cranial targets, although further investigation is needed on this subject.
  
  
  CONCLUSIONS
  The s-CT method is very fast and yields data that can be used for treatment planning without sacrificing accuracy.
authors.affiliation: Radiation Physics, Department of Radiation Sciences, Umeå University, Sweden. joakim.jonsson@radfys.umu.se
authors.forename: Joakim H
authors.initials: JH
authors.lastname: Jonsson
authors.forename: Adam
authors.initials: A
authors.lastname: Johansson
authors.forename: Karin
authors.initials: K
authors.lastname: Söderström
authors.forename: Thomas
authors.initials: T
authors.lastname: Asklund
authors.forename: Tufve
authors.initials: T
authors.lastname: Nyholm
mesh_terms: D000368:Aged; D001921:Brain / Q000528:radiation effects*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000379:methods*
keywords: Dose calculation; Magnetic resonance; Radiotherapy; Substitute CT; Treatment planning