title: An evaluation of four CT-MRI co-registration techniques for radiotherapy treatment planning of prone rectal cancer patients.
abstract: OBJECTIVE
  MRI is the preferred staging modality for rectal carcinoma patients. This work assesses the CT-MRI co-registration accuracy of four commercial rigid-body techniques for external beam radiotherapy treatment planning for patients treated in the prone position without fiducial markers.
  
  
  METHODS
  17 patients with biopsy-proven rectal carcinoma were scanned with CT and MRI in the prone position without the use of fiducial markers. A reference co-registration was performed by consensus of a radiologist and two physicists. This was compared with two automated and two manual techniques on two separate treatment planning systems. Accuracy and reproducibility were analysed using a measure of target registration error (TRE) that was based on the average distance of the mis-registration between vertices of the clinically relevant gross tumour volume as delineated on the CT image.
  
  
  RESULTS
  An automated technique achieved the greatest accuracy, with a TRE of 2.3 mm. Both automated techniques demonstrated perfect reproducibility and were significantly faster than their manual counterparts. There was a significant difference in TRE between registrations performed on the two planning systems, but there were no significant differences between the manual and automated techniques.
  
  
  CONCLUSIONS
  For patients with rectal cancer, MRI acquired in the prone treatment position without fiducial markers can be accurately registered with planning CT. An automated registration technique offered a fast and accurate solution with associated uncertainties within acceptable treatment planning limits.
authors.affiliation: Department of Medical Physics, St James's Institute of Oncology, Leeds, UK. chris.dean@bartsandthelondon.nhs.uk
authors.forename: C J
authors.initials: CJ
authors.lastname: Dean
authors.forename: J R
authors.initials: JR
authors.lastname: Sykes
authors.forename: R A
authors.initials: RA
authors.lastname: Cooper
authors.forename: P
authors.initials: P
authors.lastname: Hatfield
authors.forename: B
authors.initials: B
authors.lastname: Carey
authors.forename: S
authors.initials: S
authors.lastname: Swift
authors.forename: S E
authors.initials: SE
authors.lastname: Bacon
authors.forename: D
authors.initials: D
authors.lastname: Thwaites
authors.forename: D
authors.initials: D
authors.lastname: Sebag-Montefiore
authors.forename: A M
authors.initials: AM
authors.lastname: Morgan
mesh_terms: D002277:Carcinoma / Q000175:diagnosis* / Q000532:radiotherapy*; D006801:Humans; D008279:Magnetic Resonance Imaging; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012004:Rectal Neoplasms / Q000175:diagnosis* / Q000532:radiotherapy*; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed