title: Adaptive radiotherapy with an average anatomy model: evaluation and quantification of residual deformations in head and neck cancer patients.
abstract: OBJECTIVE
  To develop and validate an adaptive intervention strategy for radiotherapy of head-and-neck cancer that accounts for systematic deformations by modifying the planning-CT (pCT) to the average misalignments in daily cone beam CT (CBCT) measured with deformable registration (DR).
  
  
  METHODS
  Daily CBCT scans (808 scans) for 25 patients were retrospectively registered to the pCT with B-spline DR. The average deformation vector field (<DVF>) was used to deform the pCT for adaptive intervention. Two strategies were simulated: single intervention after 10 fractions and weekly intervention with an <DVF> from the previous week. The model was geometrically validated with the residual misalignment of anatomical landmarks both on bony-anatomy (BA; automatically generated) and soft-tissue (ST; manually identified).
  
  
  RESULTS
  Systematic deformations were 2.5/3.4mm vector length (BA/ST). Single intervention reduced deformations to 1.5/2.7 mm (BA/ST). Weekly intervention resulted in 1.0/2.2mm (BA/ST) and accounted better for progressive changes. 15 patients had average systematic deformations >2mm (BA): reductions were 1.1/1.9 mm (single/weekly BA). ST improvements were underestimated due to observer and registration variability.
  
  
  CONCLUSIONS
  Adaptive intervention with a pCT modified to the average anatomy during treatment successfully reduces systematic deformations. The improved accuracy could possibly be exploited in margin reduction and/or dose escalation.
authors.affiliation: Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
authors.forename: Simon
authors.initials: S
authors.lastname: van Kranen
authors.forename: Angelo
authors.initials: A
authors.lastname: Mencarelli
authors.forename: Suzanne
authors.initials: S
authors.lastname: van Beek
authors.forename: Coen
authors.initials: C
authors.lastname: Rasch
authors.forename: Marcel
authors.initials: M
authors.lastname: van Herk
authors.forename: Jan-Jakob
authors.initials: JJ
authors.lastname: Sonke
mesh_terms: D000465:Algorithms; D054893:Cone-Beam Computed Tomography / Q000379:methods; D057918:Fiducial Markers; D006258:Head and Neck Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D008953:Models, Anatomic; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D012189:Retrospective Studies
keywords: Adaptive radiotherapy; Head and neck cancer; Non-rigid registration; Systematic deformations