title: Quantification of intra-fraction changes during radiotherapy of cervical cancer assessed with pre- and post-fraction Cone Beam CT scans.
abstract: OBJECTIVE
  With the introduction of Intensity Modulated Radiotherapy (IMRT) and image-guided plan-of-the-day strategies, the treatment of cervical cancer has become more sensitive to intra-fraction uncertainties. In this study we quantified intra-fraction changes in cervix-uterus shape, bladder and rectum filling, and patient setup using pre- and post-fraction CBCT scans.
  
  
  METHODS
  A total of 632 CBCT scans were analyzed for 16 patients with large tip-of-uterus displacement (>2.5 cm) measured in an empty and full bladder CT scan. In all scans, the bladder, cervix-uterus, and rectum were delineated. For rectum and bladder, intra-fraction volume changes were assessed. Systematic cervix-uterus intra-fraction displacements were obtained by non-rigidly aligning the pre-fraction cervix-uterus to that in the post-fraction CBCT. Intra-fraction patient setup changes were obtained by rigidly aligning pre- and post-CBCTs using the bony anatomy.
  
  
  RESULTS
  The mean time between pre- and post-fraction CBCT scan was 20.8 min. The group-mean intra-fraction displacements averaged over the cervix-uterus were 0.1±1.4/1.8±1.5/-2.8±1.8 (LR/CC/AP) mm. The group-mean 5th and 95th percentile intra-fraction displacements were -2.3,2.1/-0.8,4.9/-5.8,0.5 (LR/CC/AP) mm. There was a significant correlation between bladder inflow rate and cervix-uterus motion (r=0.6 and p<0.01). Intra-fraction changes in patient setup were 1.3/0.4/0.6 and 1.4/1.0/1.1 mm (LR/CC/AP), for systematic and random changes, respectively.
  
  
  CONCLUSIONS
  Intra-fraction cervix-uterus motion can be considerable and should be taken into account using appropriate PTV margins.
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Cancer Institute, Rotterdam, The Netherlands. Electronic address: s.heijkoop@erasmusmc.nl.
authors.forename: Sabrina T
authors.initials: ST
authors.lastname: Heijkoop
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Cancer Institute, Rotterdam, The Netherlands.
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Langerak
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Cancer Institute, Rotterdam, The Netherlands.
authors.forename: Sandra
authors.initials: S
authors.lastname: Quint
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Cancer Institute, Rotterdam, The Netherlands.
authors.forename: Jan Willem M
authors.initials: JW
authors.lastname: Mens
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Cancer Institute, Rotterdam, The Netherlands.
authors.forename: Andras G
authors.initials: AG
authors.lastname: Zolnay
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Cancer Institute, Rotterdam, The Netherlands.
authors.forename: Ben J M
authors.initials: BJ
authors.lastname: Heijmen
authors.affiliation: Department of Radiation Oncology, Erasmus MC - Cancer Institute, Rotterdam, The Netherlands.
authors.forename: Mischa S
authors.initials: MS
authors.lastname: Hoogeman
mesh_terms: D054893:Cone-Beam Computed Tomography / Q000379:methods*; D005260:Female; D006801:Humans; D061089:Radiotherapy, Image-Guided; D012007:Rectum / Q000000981:diagnostic imaging; D001743:Urinary Bladder / Q000000981:diagnostic imaging; D002583:Uterine Cervical Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*
keywords: IMRT; Intra-fraction motion; Locally advanced cervical cancer; Online adaptive radiotherapy; Plan-of-the-day; VMAT