Calibration of low activity 192Ir brachytherapy sources in terms of reference air kerma rate with large volume spherical ionization chambers

An air kerma rate calibration method for low dose rate 192Ir brachytherapy sources was elaborated using three different large volume spherical ionization chambers (PTW LS-10, NE 2551, Exradin A6) in a collaborative series of experiments between the University of Ghent (UG) and the Netherlands Measurements Institute (NMI). To this end these chambers were calibrated for x-ray qualities in the energy range 35 keV to 305 keV and for 137Cs and 60Co gamma rays. The results of these measurements are used to derive mean weighted calibration factors for the photon radiation of 192Ir brachytherapy sources. Furthermore, the effect of the finite chamber size on the effective measuring point and the correction for the contribution of scattered radiation were studied. Measurements on the same sets of sources show that the ionization chambers and the methods used yield results for the reference air kerma rates of 192Ir sources which agree within 0.42%. (author)