title: Adaptive radiotherapy for long course neo-adjuvant treatment of rectal cancer.
abstract: OBJECTIVE
  To quantify the potential margin reduction with adaptive radiotherapy (ART) during neo-adjuvant treatment of locally-advanced rectal cancer.
  
  
  METHODS
  Repeat CT scans were acquired for 28 patients treated with 25×2 Gy, daily during the first week, and followed by weekly scans. The CTV was delineated on all scans, and shape variation was estimated. Five ART strategies were tested, consisting of an average CTV over the planning CT and one to five repeat CTs. Required PTV margins were calculated for adapted and non-adapted treatment. The strategy with the least PTV volume over the whole treatment was selected and bowel area dose reduction was estimated.
  
  
  RESULTS
  Substantial systematic and random shape variation demanded for a PTV margin up to 2.4 cm at the upper-anterior part of the CTV. Plan adaptation after fraction 4 resulted in a maximum 0.7 cm margin reduction and a significant PTV reduction from 1185 to 1023 cc (p<0.0001). The bowel area volume receiving 15, 45, and 50 Gy was reduced from 436 to 402 cc, 111 to 81 cc, and 49 to 29 cc, respectively (p<0.0001).
  
  
  CONCLUSIONS
  With adaptive radiotherapy, maximum required PTV margins can be reduced from 2.4 to 1.7 cm, resulting in significantly less dose to the bowel area.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Jasper
authors.initials: J
authors.lastname: Nijkamp
authors.forename: Corrie
authors.initials: C
authors.lastname: Marijnen
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: Baukelien
authors.initials: B
authors.lastname: van Triest
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
mesh_terms: D006801:Humans; D007422:Intestines / Q000528:radiation effects; D020360:Neoadjuvant Therapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012004:Rectal Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy*; D012007:Rectum / Q000528:radiation effects; D014057:Tomography, X-Ray Computed; D001743:Urinary Bladder / Q000528:radiation effects