title: A 2D ion chamber array audit of wedged and asymmetric fields in an inhomogeneous lung phantom.
abstract: OBJECTIVE
  The Australian Clinical Dosimetry Service (ACDS) has implemented a new method of a nonreference condition Level II type dosimetric audit of radiotherapy services to increase measurement accuracy and patient safety within Australia. The aim of this work is to describe the methodology, tolerances, and outcomes from the new audit.
  
  
  METHODS
  The ACDS Level II audit measures the dose delivered in 2D planes using an ionization chamber based array positioned at multiple depths. Measurements are made in rectilinear homogeneous and inhomogeneous phantoms composed of slabs of solid water and lung. Computer generated computed tomography data sets of the rectilinear phantoms are supplied to the facility prior to audit for planning of a range of cases including reference fields, asymmetric fields, and wedged fields. The audit assesses 3D planning with 6 MV photons with a static (zero degree) gantry. Scoring is performed using local dose differences between the planned and measured dose within 80% of the field width. The overall audit result is determined by the maximum dose difference over all scoring points, cases, and planes. Pass (Optimal Level) is defined as maximum dose difference ≤3.3%, Pass (Action Level) is ≤5.0%, and Fail (Out of Tolerance) is >5.0%.
  
  
  RESULTS
  At close of 2013, the ACDS had performed 24 Level II audits. 63% of the audits passed, 33% failed, and the remaining audit was not assessable. Of the 15 audits that passed, 3 were at Pass (Action Level). The high fail rate is largely due to a systemic issue with modeling asymmetric 60° wedges which caused a delivered overdose of 5%-8%.
  
  
  CONCLUSIONS
  The ACDS has implemented a nonreference condition Level II type audit, based on ion chamber 2D array measurements in an inhomogeneous slab phantom. The powerful diagnostic ability of this audit has allowed the ACDS to rigorously test the treatment planning systems implemented in Australian radiotherapy facilities. Recommendations from audits have led to facilities modifying clinical practice and changing planning protocols.
authors.affiliation: Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia.
authors.forename: Jessica
authors.initials: J
authors.lastname: Lye
authors.affiliation: Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia and Radiation Oncology Queensland, Toowoomba, Queensland 4350, Australia.
authors.forename: John
authors.initials: J
authors.lastname: Kenny
authors.affiliation: Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia and School of Applied Science, RMIT University, Melbourne 3000, Australia.
authors.forename: Joerg
authors.initials: J
authors.lastname: Lehmann
authors.affiliation: Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia.
authors.forename: Leon
authors.initials: L
authors.lastname: Dunn
authors.affiliation: School of Applied Science, RMIT University, Melbourne 3000, Australia and Peter MacCallum Cancer Centre, Melbourne 3008, Australia.
authors.forename: Tomas
authors.initials: T
authors.lastname: Kron
authors.affiliation: Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia.
authors.forename: Andrew
authors.initials: A
authors.lastname: Alves
authors.affiliation: Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia and Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Victoria 3085, Australia.
authors.forename: Andrew
authors.initials: A
authors.lastname: Cole
authors.affiliation: Australian Clinical Dosimetry Service, Yallambie, Victoria 3085, Australia and School of Applied Science, RMIT University, Melbourne 3000, Australia.
authors.forename: Ivan
authors.initials: I
authors.lastname: Williams
mesh_terms: D001315:Australia; D006283:Health Physics; D006801:Humans; D008168:Lung / Q000528:radiation effects*; D008485:Medical Audit; D019047:Phantoms, Imaging; D017785:Photons; D011874:Radiometry / Q000295:instrumentation / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D014057:Tomography, X-Ray Computed; D014867:Water
chemical_list: D014867:Water