title: Changes in the pelvic anatomy after an IMRT treatment fraction of prostate cancer.
abstract: OBJECTIVE
  To quantify the three-dimensional variations of pelvic anatomy after a single treatment fraction.
  
  
  METHODS
  Forty-six prostate cancer patients underwent computed tomography (CT) scanning with an in-room CT-on-rail system, before and immediately after one intensity-modulated radiotherapy (IMRT) session. To study the soft-tissue anatomy changes, the pre- and post-treatment CT images were registered using the bony structure with an in-house image registration software system. The center of volume for both the prostate and seminal vesicles was used to assess the relative displacement of the same structure after the treatment fraction.
  
  
  RESULTS
  During one treatment fraction (21 +/- 4 min), both the prostate and seminal vesicles showed statistically significant systematic trends in the superior and anterior directions of the patient's anatomy. The net increase in bladder volume was huge (127 +/- 79 cm(3)), yet this change did not translate into large target displacements. Although the population mean displacements in either direction were 1.3 +/- 2.9 mm for the prostate and 1.2 +/- 4.1 mm for the seminal vesicles in the anterior direction, a few patients had displacements as large as 8.4 mm and 15.6 mm, respectively. These large displacements correlated strongly (p < 0.001) with large rectal volume increases caused by gaseous build-up in the rectum.
  
  
  CONCLUSIONS
  The observed intrafraction variations in anatomy during prostate IMRT sessions suggest that, for any given fraction, the organ motion and volume changes can potentially lead to compromised target coverage in about 15% of patients in whom the prostate position shifted >4 mm.
authors.affiliation: Department of Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, TX 77030, USA.
authors.forename: Renaud
authors.initials: R
authors.lastname: de Crevoisier
authors.forename: Adam D
authors.initials: AD
authors.lastname: Melancon
authors.forename: Deborah A
authors.initials: DA
authors.lastname: Kuban
authors.forename: Andrew K
authors.initials: AK
authors.lastname: Lee
authors.forename: Rex M
authors.initials: RM
authors.lastname: Cheung
authors.forename: Susan L
authors.initials: SL
authors.lastname: Tucker
authors.forename: Rajat J
authors.initials: RJ
authors.lastname: Kudchadker
authors.forename: Wayne D
authors.initials: WD
authors.lastname: Newhauser
authors.forename: Lifei
authors.initials: L
authors.lastname: Zhang
authors.forename: Radhe
authors.initials: R
authors.lastname: Mohan
authors.forename: Lei
authors.initials: L
authors.lastname: Dong
mesh_terms: D000704:Analysis of Variance; D005740:Gases; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D009068:Movement; D015588:Observer Variation; D010388:Pelvis / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D011467:Prostate / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated; D012007:Rectum / Q000033:anatomy & histology / Q000000981:diagnostic imaging* / Q000502:physiology; D012669:Seminal Vesicles / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D012984:Software; D014057:Tomography, X-Ray Computed; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging*
chemical_list: D005740:Gases