Importance of HDR Source Calibration and Measurement

摘要

The most common radionucide used in High-dose rate (HDR) brachytherapy is ~(192)Ir and has been used clinically for over 25 years. The first calibration of this source was performed at the University of Wisconsin - Madison. Other HDR sources are beginning to be introduced into the market, each of which requires a unique calibration. One of these sources is electronic brachytherapy (eBT). The calibration of these sources involves the use of different standards, namely a calibrated ion chamber traceable to a primary standards dosimetry laboratory such as NIST or a free-air chamber. The traceability of the ion chambers and the details of the free-air chamber will be described in detail. An uncertainty estimate for HDR sources (other source types are described elsewhere) will also be provided. [1] The University of Wisconsin has measured most HDR ~(192)Ir source models and found that the calibration coefficients for well-type ionization chambers agree to within ± 1 %, with a total uncertainty of 2.4%. For eBT sources, calibration coefficients derived from the University of Wisconsin free-air chamber agree with coefficients derived from the NIST standards to within ± 1 % as well. An estimate of the expanded uncertainty in each case will also be described. Comparison of the University of Wisconsin standard with other primary laboratories will also be presented, along with general characteristics of well-type ionization chambers. Calibration comparison data for several HDR ~(192)Ir sources has been demonstrated to agree within ± 1 %. [2] The agreement worldwide is also within this value. The 2.4 % uncertainty covers all of the various manufacturer sources and results in confidence in the calibration and measurement in the clinic using a well chamber. Electronic brachytherapy source calibrations will require a modification in the standard protocol as proposed by DeWerd et al. [3]. Clinical physicists can be assured that HDR ~(192)Ir calibrations traceable to primary standards dosimetry laboratories around the world are very similar and accurate.