title: Quantifying appropriate PTV setup margins: analysis of patient setup fidelity and intrafraction motion using post-treatment megavoltage computed tomography scans.
abstract: OBJECTIVE
  To present a technique that can be implemented in-house to evaluate the efficacy of immobilization and image-guided setup of patients with different treatment sites on helical tomotherapy. This technique uses an analysis of alignment shifts between kilovoltage computed tomography and post-treatment megavoltage computed tomography images. The determination of the shifts calculated by the helical tomotherapy software for a given site can then be used to define appropriate planning target volume internal margins.
  
  
  METHODS
  Twelve patients underwent post-treatment megavoltage computed tomography scans on a helical tomotherapy machine to assess patient setup fidelity and net intrafraction motion. Shifts were studied for the prostate, head and neck, and glioblastoma multiforme. Analysis of these data was performed using automatic and manual registration of the kilovoltage computed tomography and post-megavoltage computed tomography images.
  
  
  RESULTS
  The shifts were largest for the prostate, followed by the head and neck, with glioblastoma multiforme having the smallest shifts in general. It appears that it might be more appropriate to use asymmetric planning target volume margins. Each margin value reported is equal to two standard deviations of the average shift in the given direction.
  
  
  CONCLUSIONS
  This method could be applied using individual patient post-image scanning and combined with adaptive planning to reduce or increase the margins as appropriate.
authors.affiliation: Department of Physics, University of Alberta, Edmonton, AB, Canada.
authors.forename: Donata M
authors.initials: DM
authors.lastname: Drabik
authors.forename: Marc A
authors.initials: MA
authors.lastname: MacKenzie
authors.forename: Gino B
authors.initials: GB
authors.lastname: Fallone
mesh_terms: D001932:Brain Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D005909:Glioblastoma / Q000473:pathology / Q000532:radiotherapy*; D006258:Head and Neck Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D008297:Male; D009068:Movement; D055585:Physical Phenomena; D010825:Physics; D011471:Prostatic Neoplasms / Q000473:pathology / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D013679:Technology, Radiologic / Q000379:methods; D036542:Tomography, Spiral Computed; D047368:Tumor Burden