title: Impact of selection of post-implant technique on dosimetry parameters for permanent prostate implants.
abstract: OBJECTIVE
  To investigate the variability of prostate implant quality indices between three different methods of calculating the post-implant dose distribution.
  
  
  METHODS
  In a study of 9 permanent prostate implant patients, post-implant dosimetry was carried out using three methods of identifying seed positions within the prostate volume: (1) prostate volumes defined by transrectal ultrasound (TRUS) immediately following implant were registered with shift-film defined seed positions, (2) seeds were identified directly from the post-implant TRUS images, and (3) CT was used to define seed positions and prostate volumes from images acquired at 41-65 days post-implant. For each method, the volume of prostate receiving 90%, 100%, and 150% of the prescribed dose (V90, V100, V150) and the dose delivered to 90% of the prostate volume (D90) were calculated.
  
  
  RESULTS
  Post-implant TRUS volumes were within 15% of the preimplant TRUS volumes in 8 of the 9 patients investigated. The post-implant CT volume was within 15% of the preimplant (TRUS) volume in only 3 of the 9 cases. The value of the dosimetry parameters was dependent on the method used and varied by 5-25% for V90, 5-30% for V100, 42-134% for V150, and 9-60% for D90. No simple relationship was found between change in volume and the resultant change in dosimetry parameter. Differences in dosimetry parameters due to source localization uncertainties was found to be small (< or = 10% for V100) when comparing methods (1) and (2).
  
  
  CONCLUSIONS
  There are many uncertainties in the calculation of parameters that are commonly used to describe the quality of a permanent prostate implant. Differences in the parameters calculated were most likely a result of a combination of factors including uncertainties in delineating the prostate with different imaging modalities, differences in source identification techniques, and intraobserver variability.
authors.affiliation: Department of Radiation Oncology, Sir Charles Gairdner Hospital, Perth, WA, Australia. ahaworth@cyllene.uwa.edu.au
authors.forename: Annette
authors.initials: A
authors.lastname: Haworth
authors.forename: Martin
authors.initials: M
authors.lastname: Ebert
authors.forename: Shaun
authors.initials: S
authors.lastname: St Clair
authors.forename: Brendan M
authors.initials: BM
authors.lastname: Carey
authors.forename: Anthony
authors.initials: A
authors.lastname: Flynn
authors.forename: David M
authors.initials: DM
authors.lastname: Bottomley
authors.forename: Gillian M
authors.initials: GM
authors.lastname: Duchesne
authors.forename: David
authors.initials: D
authors.lastname: Joseph
authors.forename: Daniel
authors.initials: D
authors.lastname: Ash
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation*; D004307:Dose-Response Relationship, Radiation; D019160:Endosonography; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D019736:Prostheses and Implants; D011829:Radiation Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012007:Rectum; D014057:Tomography, X-Ray Computed; D016896:Treatment Outcome