title: Motion and shape change when using an endorectal balloon during prostate radiation therapy.
abstract: OBJECTIVE
  To investigate motion and shape change when using an endorectal balloon (ERB) in patients receiving radiotherapy for prostate cancer.
  
  
  METHODS
  In nine patients treated for prostate cancer using an ERB, the anterior wall of the ERB was contoured on right lateral images taken immediately before irradiation, and on left lateral images taken immediately after irradiation. Changes in the contours were used to calculate inter-fraction shape change and inter-imaging motion and shape change. Inter-imaging motion describes changes that occur after the right lateral image is taken that are seen in the left lateral image.
  
  
  RESULTS
  Eighty-six percent of all inter-imaging shifts of the anterior wall of the ERB were in the posterior direction (mean: 1.8 mm, 1 SD: 1.8 mm, maximum posterior shift: 2.8-7.2 mm). The inter-fraction shape change (1 SD) of the anterior wall was equivalent to a change in the angle of the balloon of 2.5-5.7 degrees, with a range of 8-20 degrees, depending on the patient. Inter-imaging shape changes were similar in size.
  
  
  CONCLUSIONS
  The inter-imaging motion and shape changes may be explained by the patient relaxing some time after insertion of the ERB, indicating that it could be reduced by a waiting period after insertion before irradiation. Development of image-guided localization strategies should consider intra-fraction motion and also inter- and intra-fraction shape change.
authors.affiliation: Department of Radiation Oncology, Dana-Farber/Brigham and Women's Cancer Center, Boston, MA 02115, USA. lcourt@lroc.harvard.edu
authors.forename: Laurence E
authors.initials: LE
authors.lastname: Court
authors.forename: Anthony V
authors.initials: AV
authors.lastname: D'Amico
authors.forename: Dnyanesh
authors.initials: D
authors.lastname: Kadam
authors.forename: Robert
authors.initials: R
authors.lastname: Cormack
mesh_terms: D002404:Catheterization; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D009038:Motion; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D011859:Radiography; D020266:Radiotherapy, Conformal / Q000379:methods*; D012007:Rectum