title: Repeat CT assessed CTV variation and PTV margins for short- and long-course pre-operative RT of rectal cancer.
abstract: OBJECTIVE
  To quantify the inter-fraction shape variation of the CTV in rectal-cancer patients treated with 5 × 5 (SCRT) and 25 × 2 Gy (LCRT) and derive PTV margins.
  
  
  METHODS
  Thirty-three SCRT with daily repeat CT scans and 30 LCRT patients with daily scans during the first week followed by weekly scans were included. The CTV was delineated on all scans and local shape variation was calculated with respect to the planning CT. Margin estimation was done using the local shape variation to assure 95% minimum dose for at least 90% of patients.
  
  
  RESULTS
  Using 482 CT scans, systematic and random CTV shape variation was heterogeneous, ranging from 0.2 cm close to bony structures up to 1.0 cm SD at the upper-anterior CTV region. A significant reduction in rectal volume during LCRT resulted in an average 0.5 cm posterior shift of the upper-anterior CTV. Required margins ranged from 0.7 cm close to bony structures up to 3.1 and 2.3 cm in the upper-anterior region for SCRT and LCRT, respectively.
  
  
  CONCLUSIONS
  Heterogeneous shape variation demands anisotropic PTV margins. Required margins were substantially larger in the anterior direction compared to current clinical margins. These larger margins were, however, based on strict delineated CTVs, resulting in smaller PTVs compared to current practice.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Jasper
authors.initials: J
authors.lastname: Nijkamp
authors.forename: Maurits
authors.initials: M
authors.lastname: Swellengrebel
authors.forename: Birgit
authors.initials: B
authors.lastname: Hollmann
authors.forename: Rianne
authors.initials: R
authors.lastname: de Jong
authors.forename: Corrie
authors.initials: C
authors.lastname: Marijnen
authors.forename: Corine
authors.initials: C
authors.lastname: van Vliet-Vroegindeweij
authors.forename: Baukelien
authors.initials: B
authors.lastname: van Triest
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
mesh_terms: D005260:Female; D006801:Humans; D008297:Male; D011880:Radiotherapy Planning, Computer-Assisted; D012004:Rectal Neoplasms / Q000473:pathology / Q000532:radiotherapy* / Q000601:surgery; D014057:Tomography, X-Ray Computed; D047368:Tumor Burden