title: Random variation in rectal position during radiotherapy for prostate cancer is two to three times greater than that predicted from prostate motion.
abstract: OBJECTIVE
  Radiotherapy for prostate cancer does not explicitly take into account daily variation in the position of the rectum. It is important to accurately assess accumulated dose (DA) to the rectum in order to understand the relationship between dose and toxicity. The primary objective of this work was to quantify systematic (Σ) and random (σ) variation in the position of the rectum during a course of prostate radiotherapy.
  
  
  METHODS
  The rectum was manually outlined on the kilo-voltage planning scan and 37 daily mega-voltage image guidance scans for 10 participants recruited to the VoxTox study. The femoral heads were used to produce a fixed point to which all rectal contours were referenced.
  
  
  RESULTS
  Σ [standard deviation (SD) of means] between planning and treatment was 4.2 mm in the anteroposterior (AP) direction and 1.3 mm left-right (LR). σ (root mean square of SDs) was 5.2 mm AP and 2.7 mm LR. Superior-inferior variation was less than one slice above and below the planning position.
  
  
  CONCLUSIONS
  Our results for Σ are in line with published data for prostate motion. σ, however, was approximately twice as great as that seen for prostate motion. This suggests that DA may differ from planned dose in some patients treated with radiotherapy for prostate cancer.
  
  
  CONCLUSIONS
  This work is the first to use daily imaging to quantify Σ and σ of the rectum in prostate cancer. σ was found to be greater than published data, providing strong rationale for further investigation of individual DA.
authors.affiliation: 1 Department of Oncology, Addenbrooke's Hospital, University of Cambridge, Cambridge, UK.
authors.forename: J
authors.initials: J
authors.lastname: Scaife
authors.forename: K
authors.initials: K
authors.lastname: Harrison
authors.forename: M
authors.initials: M
authors.lastname: Romanchikova
authors.forename: A
authors.initials: A
authors.lastname: Parker
authors.forename: M
authors.initials: M
authors.lastname: Sutcliffe
authors.forename: S
authors.initials: S
authors.lastname: Bond
authors.forename: S
authors.initials: S
authors.lastname: Thomas
authors.forename: S
authors.initials: S
authors.lastname: Freeman
authors.forename: R
authors.initials: R
authors.lastname: Jena
authors.forename: A
authors.initials: A
authors.lastname: Bates
authors.forename: N
authors.initials: N
authors.lastname: Burnet
mesh_terms: D000368:Aged; D006801:Humans; D008297:Male; D009068:Movement; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted; D012007:Rectum / Q000033:anatomy & histology* / Q000528:radiation effects; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed / Q000379:methods