title: Prostate movement during simulation resulting from retrograde urethrogram compared with "natural" prostate movement.
abstract: OBJECTIVE
  Retrograde urethrography (UG) is commonly used at the time of simulation to assist in defining the prostate apex. Some investigators have reported that performing the UG introduces error by causing prostate displacement. We investigate the movement of the prostate caused by the retrograde UG.
  
  
  METHODS
  Twenty-four patients treated with three-dimensional conformal radiotherapy for prostate cancer who had gold marker seeds placed into their prostates were studied. Marker seed locations at the time of simulation and on the portal images acquired just before the treatment were compared with the locations on digitally reconstructed radiographs (DRR). Movement in the superior-inferior and anteroposterior directions as seen on lateral images was measured from 402 portal images by offline customized imaging software and evaluated using analysis of variance methods for continuous variables and chi-square statistics for categoric variables.
  
  
  RESULTS
  "Natural" nonrandom movement of the prostate around an "origin" as defined by markers on DRR was observed. This movement tends to be in a superior and anterior direction, with the average shift being 1 mm and 0.82 mm, respectively. The magnitude of movement in the superior direction averaged 2.88 mm compared with 1.64 mm in the inferior direction (p = 0.04). There was slightly greater movement after the UG compared with mean "natural" movement but the difference was less than 3 mm in either direction on average (difference: superior-inferior = 2.64 mm, p = 0.004; anteroposterior = 2.24, p = 0.035).
  
  
  CONCLUSIONS
  Use of the UG induces a small but clinically insignificant displacement of the prostate when "natural" movement is taken into account.
authors.affiliation: Department of Radiation Oncology, University of California San Francisco, San Francisco, CA, USA.
authors.forename: Yu-Ming
authors.initials: YM
authors.lastname: Liu
authors.forename: Stella
authors.initials: S
authors.lastname: Ling
authors.forename: K M
authors.initials: KM
authors.lastname: Langen
authors.forename: Katsuto
authors.initials: K
authors.lastname: Shinohara
authors.forename: Vivian
authors.initials: V
authors.lastname: Weinberg
authors.forename: Jean
authors.initials: J
authors.lastname: Pouliot
authors.forename: Mack
authors.initials: M
authors.lastname: Roach
mesh_terms: D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D011859:Radiography; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal / Q000295:instrumentation; D014521:Urethra / Q000000981:diagnostic imaging*