title: A new fiducial marker for Image-guided radiotherapy of prostate cancer: clinical experience.
abstract: BACKGROUND
  A new fiducial marker for image guided radiotherapy (IGRT) based on a removable prostate stent made of Ni Ti has been developed during two previous clinical feasibility studies. The marker is currently being evaluated for IGRT treatment in a third clinical study.
  
  
  METHODS
  The new marker is used to co-register MR and planning CT scans with high accuracy in the region around the prostate. The co-registered MR-CT volumes are used for delineation of GTV before planning. In each treatment session the IGRT system is used to position the patient before treatment. The IGRT system use a stereo pair of kV images matched to corresponding Digital Reconstructed Radiograms (DRR) from the planning CT scan. The match is done using mutual gray scale information. The pair of DRR's for positioning is created in the IGRT system with a threshold in the Look Up Table (LUT). The resulting match provides the necessary shift in couch coordinates to position the stent with an accuracy of 1-2 mm within the planned position.
  
  
  RESULTS
  At the present time 39 patients have received the new marker. Of the 39 one has migrated to the bladder. Deviations of more than 5 mm between CTV outlined on CT and MR are seen in several cases and in anterior-posterior (AP), left-right (LR) and cranial-caudal (CC) directions. Intra-fraction translation movements up to +/- 3 mm are seen as well. As the stent is also clearly visible on images taken with high voltage x-rays using electronic portal images devices (EPID), the positioning has been verified independently of the IGRT system.
  
  
  CONCLUSIONS
  The preliminary result of an on going clinical study of a Ni Ti prostate stent, potentially a new fiducial marker for image guided radiotherapy, looks promising. The risk of migration appears to be much lower compared to previous designs.
authors.affiliation: Department of Medical Physics, Oncology, Aalborg Hospital, Denmark. jhc@rn.dk
authors.forename: Jesper
authors.initials: J
authors.lastname: Carl
authors.forename: Jane
authors.initials: J
authors.lastname: Nielsen
authors.forename: Mats
authors.initials: M
authors.lastname: Holmberg
authors.forename: Erik
authors.initials: E
authors.lastname: Højkjaer Larsen
authors.forename: Knud
authors.initials: K
authors.lastname: Fabrin
authors.forename: Rune V
authors.initials: RV
authors.lastname: Fisker
mesh_terms: D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D011471:Prostatic Neoplasms / Q000175:diagnosis / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D015607:Stents; D014057:Tomography, X-Ray Computed