Calibration of low activity¹⁹²Ir 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 192 Ir brachytherapy sources was elaborated using three different large volume spherical ionization chambers (PTW LS-10, NE 2551, Exradin A6). To this end these chambers were calibrated for X-ray qualities in the energy range 35 keV to 305 keV and for 137 Cs and 60 Co gamma rays. The results of these measurements are used to derive mean weighted calibration factors for the photon radiation of 192 Ir brachytherapy sources. Furthermore, the effect of the finite chamber size on the effective measuring point and the correction for the contribution of scattered radiation where 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 192 Ir sources which agree within 0.42%.