title: Intra- and inter-radiation therapist reproducibility of daily isocenter verification using prostatic fiducial markers.
abstract: BACKGROUND
  We sought to determine the intra- and inter-radiation therapist reproducibility of a previously established matching technique for daily verification and correction of isocenter position relative to intraprostatic fiducial markers (FM).
  
  
  METHODS
  With the patient in the treatment position, anterior-posterior and left lateral electronic images are acquired on an amorphous silicon flat panel electronic portal imaging device. After each portal image is acquired, the therapist manually translates and aligns the fiducial markers in the image to the marker contours on the digitally reconstructed radiograph. The distances between the planned and actual isocenter location is displayed. In order to determine the reproducibility of this technique, four therapists repeated and recorded this operation two separate times on 20 previously acquired portal image datasets from two patients. The data were analyzed to obtain the mean variability in the distances measured between and within observers.
  
  
  RESULTS
  The mean and median intra-observer variability ranged from 0.4 to 0.7 mm and 0.3 to 0.6 mm respectively with a standard deviation of 0.4 to 1.0 mm. Inter-observer results were similar with a mean variability of 0.9 mm, a median of 0.6 mm, and a standard deviation of 0.7 mm. When using a 5 mm threshold, only 0.5% of treatments will undergo a table shift due to intra or inter-observer error, increasing to an error rate of 2.4% if this threshold were reduced to 3 mm.
  
  
  CONCLUSIONS
  We have found high reproducibility with a previously established method for daily verification and correction of isocenter position relative to prostatic fiducial markers using electronic portal imaging.
authors.affiliation: Radiation Oncology Branch, Center for Cancer Research, National Cancer Institute, NIH, DHHS, Bldg 10, CRC Rm B2(SW) 3500, 9000 Rockville Pike, Bethesda, MD, 20892, USA. ullmank@mail.nih.gov
authors.forename: Karen L
authors.initials: KL
authors.lastname: Ullman
authors.forename: Holly
authors.initials: H
authors.lastname: Ning
authors.forename: Robert C
authors.initials: RC
authors.lastname: Susil
authors.forename: Asna
authors.initials: A
authors.lastname: Ayele
authors.forename: Lucresse
authors.initials: L
authors.lastname: Jocelyn
authors.forename: Jan
authors.initials: J
authors.lastname: Havelos
authors.forename: Peter
authors.initials: P
authors.lastname: Guion
authors.forename: Huchen
authors.initials: H
authors.lastname: Xie
authors.forename: Guang
authors.initials: G
authors.lastname: Li
authors.forename: Barbara C
authors.initials: BC
authors.lastname: Arora
authors.forename: Angela
authors.initials: A
authors.lastname: Cannon
authors.forename: Robert W
authors.initials: RW
authors.lastname: Miller
authors.forename: C Norman
authors.initials: CN
authors.lastname: Coleman
authors.forename: Kevin
authors.initials: K
authors.lastname: Camphausen
authors.forename: Cynthia
authors.initials: C
authors.lastname: Ménard
mesh_terms: D004867:Equipment Design; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D019047:Phantoms, Imaging; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011829:Radiation Dosage; D011859:Radiography; D011878:Radiotherapy / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D015203:Reproducibility of Results