Premium
Measurement of absorbed dose‐to‐water for an HDR 192 Ir source with ionization chambers in a sandwich setup
Author(s) -
Araki Fujio,
Kouno Tomohiro,
Ohno Takeshi,
Kakei Kiyotaka,
Yoshiyama Fumiaki,
Kawamura Shinji
Publication year - 2013
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4816673
Subject(s) - ionization chamber , absorbed dose , dosimetry , brachytherapy , materials science , imaging phantom , calibration , monte carlo method , percentage depth dose curve , ionization , reproducibility , nuclear medicine , optics , physics , ion , chemistry , radiation therapy , mathematics , medicine , statistics , chromatography , quantum mechanics
Purpose: 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, k Q , 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 k Q 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.