TU‐A‐BRB‐03: Realization of the Unit of Absorbed Dose to Water for I‐125 Interstitial Brachytherapy Sources

Purpose: To establish a primary standard for I‐125 interstitial brachytherapy sources in terms of absorbed dose to water and thereby to improve the accuracy and effectiveness of the dosimetry for brachytherapy. Methods: The primary standard is a large, air‐filled, parallel‐plate extrapolation chamber in a phantom of water‐equivalent material (RW1). The thickness of the entrance plate defines the measurement depth within the water phantom and the inner side of the front plate the reference plane of the chamber. The method of evaluation was derived analytically based on radiation transport theory. A conversion factor has to be applied to the difference of ionization charges measured at two plate separations. This factor is composed of quotients of kerma values calculated for different plate separations in the chamber. In a recent publication it has been shown that this conversion factor is robust against uncertainties of database and spectra in comparison to conversion factors needed to convert the Reference Air Kerma Rate (RAKR) into Dw. With this measure the relative uncertainties of the conversion into Dw could be reduced from 10% to 1 %. Results: Dw and RAKR of four ‘BEBIG Symmetra I25.S16’‐ seeds were determined by means of primary standards. With this an experimentally determined dose rate constant was obtained, which could be compared to literature data. The uncertainty of the experimentally determined dose‐rate constant results to 1.6% as the uncertainties of the determination of Dw is 1.3% and of the RAKR is 0.9%. The mean value of the determined dose rate constants amounts to 1.021×10−4 m‐2 and is in good agreement with the TG‐43 consensus value of 1.012×10−4 m‐2. Conclusions: A primary standard was developed which is able to determine the absorbed dose to water for I‐125 interstitial brachytherapy sources with an uncertainty of 1.3%. The research within this EURAMET joint research project leading to these results has received funding from the European Community's Seventh Framework Programme, ERA‐NET Plus, under Grant Agreement No. 217257.