title: Confidence limit variation for a single IMRT system following the TG119 protocol.
abstract: OBJECTIVE
  To evaluate the robustness of TG119-based quality assurance metrics for an IMRT system.
  
  
  METHODS
  Four planners constructed treatment plans for the five IMRT test cases described in TG119. All plans were delivered to a 30 cm x 30 cm x 15 cm solid water phantom in one treatment session in order to minimize session-dependent variation from phantom setup, film quality, machine performance, etc. Composite measurements utilized film and an ionization chamber. Per-field measurements were collected using a diode array device at an effective depth of 5 cm. All data collected were analyzed using the TG119 specifications to determine the confidence limit values for each planner separately and then compared.
  
  
  RESULTS
  The mean variance of ion chamber measurements for each planner was within 1.7% of the planned dose. The resulting confidence limits were 3.13%, 1.98%, 3.65%, and 4.39%. Confidence limit values determined by composite film analysis were 8.06%, 13.4%, 9.30%, and 16.5%. Confidence limits from per-field measurements were 1.55%, 0.00%, 0.00%, and 2.89%.
  
  
  CONCLUSIONS
  For a single IMRT system, the accuracy assessment provided by TG119-based quality assurance metrics showed significant variations in the confidence limits between planners across all composite and per-field evaluations. This observed variation is likely due to the different levels of modulation between each planner's set of plans. Performing the TG119 evaluation using plans produced by a single planner may not provide an adequate estimation of IMRT system accuracy.
authors.affiliation: Department of Radiation Oncology, The University of Arizona, Tucson, Arizona 85724, USA. jdgordon@email.arizona.edu
authors.forename: J D
authors.initials: JD
authors.lastname: Gordon
authors.forename: S P
authors.initials: SP
authors.lastname: Krafft
authors.forename: S
authors.initials: S
authors.lastname: Jang
authors.forename: L
authors.initials: L
authors.lastname: Smith-Raymond
authors.forename: M Y
authors.initials: MY
authors.lastname: Stevie
authors.forename: R J
authors.initials: RJ
authors.lastname: Hamilton
mesh_terms: D011874:Radiometry; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*