title: (Non)-migration of radiopaque markers used for on-line localization of the prostate with an electronic portal imaging device.
abstract: OBJECTIVE
  Radiopaque gold markers can be implanted in the prostate to visualize its position on portal images during radiation therapy. This procedure assumes that the markers do not move within the prostate. In this work we test this assumptiom.
  
  
  METHODS
  Three markers were implanted transrectally in the prostate of patients undergoing external radiation therapy. An orthogonal pair of portal images was acquired periodically throughout the course of radiation therapy with an a-Si electronic portal imaging device (EPID). The marker coordinates were determined, and the distances between the implanted markers were recorded. The distance time trend is used to evaluate the magnitude of marker migration.
  
  
  RESULTS
  The average standard deviation (SD) of the distances between markers was 1.3 mm (range 0.44 to 3.04 mm). Three of the 11 patients show a SD larger than 2 mm. For these patients, all three distances show a simultaneous reduction with time, compatible with a shrinking of the prostate. All had been treated with neoadjuvant hormone therapy. For 1 of the 3 patients, this reduction in volume was confirmed with a repeat computed tomographic scan.
  
  
  CONCLUSIONS
  None of the 33 markers studied migrated significantly. The implantation of three radiopaque gold markers enables accurate and precise on-line verification of the prostate position during external beam radiation therapy. The use of three markers provides a tool to monitor prostate position and volume changes that can occur over time due to hormone or radiation therapy.
authors.affiliation: Department of Radiation Oncology, University of California San Francisco, Comprehensive Cancer Center, San Francisco, CA, USA. pouliot@radonc17.ucsf.edu
authors.forename: Jean
authors.initials: J
authors.lastname: Pouliot
authors.forename: Michele
authors.initials: M
authors.lastname: Aubin
authors.forename: Katja M
authors.initials: KM
authors.lastname: Langen
authors.forename: Yu-Ming
authors.initials: YM
authors.lastname: Liu
authors.forename: Barby
authors.initials: B
authors.lastname: Pickett
authors.forename: Katsuto
authors.initials: K
authors.lastname: Shinohara
authors.forename: Mack
authors.initials: M
authors.lastname: Roach
mesh_terms: D006046:Gold; D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D013997:Time Factors; D014057:Tomography, X-Ray Computed
chemical_list: D006046:Gold