Measurement of absorbed dose-to-water for an HDR 192Ir source with ionization chambers in a sandwich setup

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Abstract

Purpose:

In this study, a dedicated device for ion chamber measurements of absorbed dose-to-water for a Nucletron microSelectron-v2 HDR192Ir 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) for192Ir dosimetry was determined from ion chamber measurements in a water phantom. The 192Ir source was sandwiched between two Exradin A1SL chambers (0.057 cm3) at the optimal SCD separation. The measured ionization was converted to the absorbed dose-to-water using a 60Co calibration factor and a Monte Carlo-calculated beam quality conversion factor, kQ, for 60Co to 192Ir. 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 the192Ir source center, considering the depth dependency of kQ for 60Co to 192Ir 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 the192Ir 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 192Ir sources.

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