title: The frequency of re-planning and its variability dependent on the modification of the re-planning criteria and IGRT correction strategy in head and neck IMRT.
abstract: BACKGROUND
  To analyse the frequency of re-planning and its variability dependent on the IGRT correction strategy and on the modification of the dosimetric criteria for re-planning for the spinal cord in head and neck IG-IMRT.
  
  
  METHODS
  Daily kV-control-CTs of six head and neck patients (=175 CTs) were analysed. All volumes of interest were re-contoured using deformable image registration. Three IGRT correction strategies were simulated and the resulting dose distributions were computed for all fractions. Different sets of criteria with varying dose thresholds for re-planning were investigated. All sets of criteria ensure equivalent target coverage of both CTVs, but vary in the tolerance threshold of the spinal cord.
  
  
  RESULTS
  The variations of the D95 and D2 in respect to the planned values ranged from -7% to +3% for both CTVs, and -2% to +6% for the spinal cord. Despite different correction vectors of the three IGRT strategies, the dosimetric differences were small. The number of fractions not requiring re-planning varied between 0% and 11% dependent on the applied IGRT correction strategy. In contrast, this number ranged between 32% and 70% dependent on the dosimetric thresholds, even though these thresholds were only gently modified.
  
  
  CONCLUSIONS
  The more precise the planned dose needs to be maintained over the treatment course, the more frequently re-planning is required. The influence of different IGRT correction strategies, even though geometrically notable, was found to be of only limited relevance for the re-planning frequency. In contrast, the definition and modification of thresholds for re-planning have a major impact on the re-planning frequency.
authors.affiliation: Department of Medical Physics in Radiation Oncology, DKFZ, INF 280, 69120 Heidelberg, Germany. m.stoll@dkfz.de.
authors.forename: Markus
authors.initials: M
authors.lastname: Stoll
authors.forename: Kristina
authors.initials: K
authors.lastname: Giske
authors.forename: Jürgen
authors.initials: J
authors.lastname: Debus
authors.forename: Rolf
authors.initials: R
authors.lastname: Bendl
authors.forename: Eva Maria
authors.initials: EM
authors.lastname: Stoiber
mesh_terms: D003198:Computer Simulation; D003657:Decision Making; D006258:Head and Neck Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D007090:Image Interpretation, Computer-Assisted; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D061089:Radiotherapy, Image-Guided; D050397:Radiotherapy, Intensity-Modulated; D014057:Tomography, X-Ray Computed; D047368:Tumor Burden
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