title: Prostate position late in the course of external beam therapy: patterns and predictors.
abstract: OBJECTIVE
  To examine prostate and seminal vesicles position late in the course of radiation therapy and to determine the effect and predictive value of the bladder and rectum on prostate and seminal vesicles positioning.
  
  
  METHODS
  Twenty-four patients with localized prostate cancer underwent a computerized tomography scan (CT1) before the start of radiation therapy. After 4-5 weeks of radiation therapy, a second CT scan (CT2) was obtained. All patients were scanned in the supine treatment position with instructions to maintain a full bladder. The prostate, seminal vesicles, bladder, and rectum were contoured. CT2 was aligned via fixed bony anatomy to CT1. The geometrical center and volume of each structure were obtained and directly compared.
  
  
  RESULTS
  The prostate shifted along a diagonal axis extending from an anterior-superior position to a posterior-inferior position. The dominant shift was to a more posterior-inferior position. On average, bladder and rectal volumes decreased to 51% (+/-29%) and 82% (+/-45%) of their pretreatment values, respectively. Multiple regression analysis (MRA) revealed that bladder movement and volume change and upper rectum movement were independently associated with prostate motion (p = 0.016, p = 0. 003, and p = 0.052 respectively).
  
  
  CONCLUSIONS
  Patients are often instructed to maintain a full bladder during a course of external beam radiation therapy, in the hopes of decreasing bladder and small bowel toxicity. However, our study shows that large bladder volumes late in therapy are strongly associated with posterior prostate displacement. This prostate displacement may result in marginal miss.
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor 20007, USA.
authors.forename: R C
authors.initials: RC
authors.lastname: Zellars
authors.forename: P L
authors.initials: PL
authors.lastname: Roberson
authors.forename: M
authors.initials: M
authors.lastname: Strawderman
authors.forename: D
authors.initials: D
authors.lastname: Zhang
authors.forename: H M
authors.initials: HM
authors.lastname: Sandler
authors.forename: R K
authors.initials: RK
authors.lastname: Ten Haken
authors.forename: D
authors.initials: D
authors.lastname: Osher
authors.forename: P W
authors.initials: PW
authors.lastname: McLaughlin
mesh_terms: D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D020266:Radiotherapy, Conformal; D012007:Rectum / Q000000981:diagnostic imaging; D012044:Regression Analysis; D012669:Seminal Vesicles / Q000000981:diagnostic imaging*; D014057:Tomography, X-Ray Computed; D001743:Urinary Bladder / Q000000981:diagnostic imaging