title: Measurement of absorbed dose-to-water for an HDR (192)Ir source with ionization chambers in a sandwich setup.
abstract: OBJECTIVE
  In this study, a dedicated device for ion chamber measurements of absorbed dose-to-water for a Nucletron microSelectron-v2 HDR (192)Ir brachytherapy source is presented. The device uses two ionization chambers in a so-called sandwich assembly. Using this setup and by taking the average reading of the two chambers, any dose error due to difficulties in absolute positioning (centering) of the source in between the chambers is cancelled to first order. The method's accuracy was examined by comparing measurements with absorbed dose-to-water determination based on the AAPM TG-43 protocol.
  
  
  METHODS
  The optimal source-to-chamber distance (SCD) for (192)Ir dosimetry was determined from ion chamber measurements in a water phantom. The (192)Ir source was sandwiched between two Exradin A1SL chambers (0.057 cm(3)) at the optimal SCD separation. The measured ionization was converted to the absorbed dose-to-water using a (60)Co calibration factor and a Monte Carlo-calculated beam quality conversion factor, kQ, for (60)Co to (192)Ir. An uncertainty estimate of the proposed method was determined based on reproducibility of measurements at different institutions for the same type of source.
  
  
  RESULTS
  The optimal distance for the A1SL chamber measurements was determined to be 5 cm from the (192)Ir source center, considering the depth dependency of kQ for (60)Co to (192)Ir and the chamber positioning. The absorbed dose to water measured at (5 cm, 90°) on the transverse axis was 1.3% lower than TG-43 values and its reproducibility and overall uncertainty were 0.8% and 1.7%, respectively. The measurement doses at anisotropic points agreed within 1.5% with TG-43 values.
  
  
  CONCLUSIONS
  The ion chamber measurement of absorbed dose-to-water with a sandwich method for the (192)Ir source provides a more accurate, direct, and reference dose compared to the dose-to-water determination based on air-kerma strength in the TG-43 protocol. Due to the simple but accurate assembly, the sandwich measurement method is useful for daily dose management of (192)Ir sources.
authors.affiliation: Department of Health Sciences, Faculty of Life Sciences, Kumamoto University, 4-24-1 Kuhonji, Kumamoto 862-0976, Japan. f_araki@kumamoto-u.ac.jp
authors.forename: Fujio
authors.initials: F
authors.lastname: Araki
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Kouno
authors.forename: Takeshi
authors.initials: T
authors.lastname: Ohno
authors.forename: Kiyotaka
authors.initials: K
authors.lastname: Kakei
authors.forename: Fumiaki
authors.initials: F
authors.lastname: Yoshiyama
authors.forename: Shinji
authors.initials: S
authors.lastname: Kawamura
mesh_terms: D001918:Brachytherapy; D007496:Iridium Radioisotopes / Q000627:therapeutic use*; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation*; D011880:Radiotherapy Planning, Computer-Assisted; D035501:Uncertainty; D014867:Water
chemical_list: D007496:Iridium Radioisotopes; D014867:Water