title: Detection of fiducial gold markers for automatic on-line megavoltage position verification using a marker extraction kernel (MEK).
abstract: OBJECTIVE
  In this study automatic detection of implanted gold markers in megavoltage portal images for on-line position verification was investigated.
  
  
  METHODS
  A detection method for fiducial gold markers, consisting of a marker extraction kernel (MEK), was developed. The detection success rate was determined for different markers using this MEK. The localization accuracy was investigated by measuring distances between markers, which were fixed on a perspex template. In order to generate images comparable to images of patients with implanted markers, this template was placed on the skin of patients before the start of the treatment. Portal images were taken of lateral prostate fields at 18 MV within 1-2 monitor units (MU).
  
  
  RESULTS
  The detection success rates for markers of 5 mm length and 1.2 and 1.4 mm diameter were 0.95 and 0.99 respectively when placed at the beam entry and 0.39 and 0.86 when placed at the beam exit. The localization accuracy appears to be better than 0.6 mm for all markers.
  
  
  CONCLUSIONS
  Automatic marker detection with an acceptable accuracy at the start of a radiotherapy fraction is feasible. Further minimization of marker diameters may be achieved with the help of an a-Si flat panel imager and may increase the clinical acceptance of this technique.
authors.affiliation: Department of Radiation Oncology, University Medical Center Utrecht, The Netherlands. nederveen@radth.ruu.nl
authors.forename: A
authors.initials: A
authors.lastname: Nederveen
authors.forename: J
authors.initials: J
authors.lastname: Lagendijk
authors.forename: P
authors.initials: P
authors.lastname: Hofman
mesh_terms: D000465:Algorithms; D006046:Gold; D006801:Humans; D020007:Implants, Experimental; D055585:Physical Phenomena; D010825:Physics; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods
chemical_list: D006046:Gold