title: A methodology for TLD postal dosimetry audit of high-energy radiotherapy photon beams in non-reference conditions.
abstract: OBJECTIVE
  A strategy for national TLD audit programmes has been developed by the International Atomic Energy Agency (IAEA). It involves progression through three sequential dosimetry audit steps. The first step audits are for the beam output in reference conditions for high-energy photon beams. The second step audits are for the dose in reference and non-reference conditions on the beam axis for photon and electron beams. The third step audits involve measurements of the dose in reference, and non-reference conditions off-axis for open and wedged symmetric and asymmetric fields for photon beams. Through a co-ordinated research project the IAEA developed the methodology to extend the scope of national TLD auditing activities to more complex audit measurements for regular fields.
  
  
  METHODS
  Based on the IAEA standard TLD holder for high-energy photon beams, a TLD holder was developed with horizontal arm to enable measurements 5cm off the central axis. Basic correction factors were determined for the holder in the energy range between Co-60 and 25MV photon beams.
  
  
  RESULTS
  New procedures were developed for the TLD irradiation in hospitals. The off-axis measurement methodology for photon beams was tested in a multi-national pilot study. The statistical distribution of dosimetric parameters (off-axis ratios for open and wedge beam profiles, output factors, wedge transmission factors) checked in 146 measurements was 0.999+/-0.012.
  
  
  CONCLUSIONS
  The methodology of TLD audits in non-reference conditions with a modified IAEA TLD holder has been shown to be feasible.
authors.affiliation: Dosimetry and Medical Radiation Physics Section, International Atomic Energy Agency, Vienna, Austria. j.izewska@iaea.org
authors.forename: Joanna
authors.initials: J
authors.lastname: Izewska
authors.forename: Dietmar
authors.initials: D
authors.lastname: Georg
authors.forename: Pranabes
authors.initials: P
authors.lastname: Bera
authors.forename: David
authors.initials: D
authors.lastname: Thwaites
authors.forename: Mehenna
authors.initials: M
authors.lastname: Arib
authors.forename: Margarita
authors.initials: M
authors.lastname: Saravi
authors.forename: Katia
authors.initials: K
authors.lastname: Sergieva
authors.forename: Kaibao
authors.initials: K
authors.lastname: Li
authors.forename: Fernando Garcia
authors.initials: FG
authors.lastname: Yip
authors.forename: Ashok Kumar
authors.initials: AK
authors.lastname: Mahant
authors.forename: Wojciech
authors.initials: W
authors.lastname: Bulski
mesh_terms: D005240:Feasibility Studies; D006801:Humans; D007390:International Agencies; D008485:Medical Audit; D017785:Photons; D010865:Pilot Projects; D011786:Quality Control; D011874:Radiometry; D011879:Radiotherapy Dosage / Q000592:standards; D011882:Radiotherapy, High-Energy / Q000592:standards*; D012015:Reference Standards; D015203:Reproducibility of Results; D013819:Thermoluminescent Dosimetry / Q000592:standards*