title: Time-resolved dosimetry using a pinpoint ionization chamber as quality assurance for IMRT and VMAT.
abstract: OBJECTIVE
  To develop a method to verify the dose delivery in relation to the individual control points of intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) using an ionization chamber. In addition to more effective problem solving during patient-specific quality assurance (QA), the aim is to eventually map out the limitations in the treatment chain and enable a targeted improvement of the treatment technique in an efficient way.
  
  
  METHODS
  Pretreatment verification was carried out for 255 treatment plans that included a broad range of treatment indications in two departments using the equipment of different vendors. In-house developed software was used to enable calculation of the dose delivery for the individual beamlets in the treatment planning system (TPS), for data acquisition, and for analysis of the data. The observed deviations were related to various delivery and measurement parameters such as gantry angle, field size, and the position of the detector with respect to the field edge to distinguish between error sources.
  
  
  RESULTS
  The average deviation of the integral fraction dose during pretreatment verification of the planning target volume dose was -2.1% ± 2.2% (1 SD), -1.7% ± 1.7% (1 SD), and 0.0% ± 1.3% (1 SD) for IMRT at the Radboud University Medical Center (RUMC), VMAT (RUMC), and VMAT at the Wellington Blood and Cancer Centre, respectively. Verification of the dose to organs at risk gave very similar results but was generally subject to a larger measurement uncertainty due to the position of the detector at a high dose gradient. The observed deviations could be related to limitations of the TPS beam models, attenuation of the treatment couch, as well as measurement errors. The apparent systematic error of about -2% in the average deviation of the integral fraction dose in the RUMC results could be explained by the limitations of the TPS beam model in the calculation of the beam penumbra.
  
  
  CONCLUSIONS
  This study showed that time-resolved dosimetry using an ionization chamber is feasible and can be largely automated which limits the required additional time compared to integrated dose measurements. It provides a unique QA method which enables identification and quantification of the contribution of various error sources during IMRT and VMAT delivery.
authors.affiliation: Department of Radiation Oncology, Wellington Blood and Cancer Centre, Wellington Hospital, Wellington 6242, New Zealand.
authors.forename: Robert J W
authors.initials: RJ
authors.lastname: Louwe
authors.affiliation: Department of Radiation Oncology, Radboud University Medical Center, Nijmegen 6500 HB, The Netherlands.
authors.forename: Markus
authors.initials: M
authors.lastname: Wendling
authors.affiliation: Department of Radiation Oncology, Radboud University Medical Center, Nijmegen 6500 HB, The Netherlands.
authors.forename: René
authors.initials: R
authors.lastname: Monshouwer
authors.affiliation: Department of Radiation Oncology, Wellington Blood and Cancer Centre, Wellington Hospital, Wellington 6242, New Zealand.
authors.forename: Thomas
authors.initials: T
authors.lastname: Satherley
authors.affiliation: Department of Radiation Oncology, Wellington Blood and Cancer Centre, Wellington Hospital, Wellington 6242, New Zealand.
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Day
authors.affiliation: Department of Radiation Oncology, Wellington Blood and Cancer Centre, Wellington Hospital, Wellington 6242, New Zealand.
authors.forename: Lynne
authors.initials: L
authors.lastname: Greig
mesh_terms: D005240:Feasibility Studies; D006801:Humans; D009369:Neoplasms / Q000532:radiotherapy; D010315:Particle Accelerators; D010363:Pattern Recognition, Automated / Q000379:methods; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use; D011137:Polystyrenes; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D012984:Software; D013997:Time Factors
chemical_list: D011137:Polystyrenes