The accuracy of the averaging procedure proposed by Goetsch et al. (Med. Phys. 18(3), (1991) 462-467) for calibrating
192
Ir HDR brachytherapy sources is investigated. Quantities required for obtaining air kerma calibration factors N k
for a Farmer ionization chamber, irradiated by 250 kV x-ray beams and
137
Cs,
192
Ir and
60
Co photons, have been calculated using the EGSnrc simulation system. Use of a medium filtered x-ray beam (E ave
= 146 keV) and removing the influence of A wall by averaging over N k
A wall
predicts an N k
value for the
192
Ir HDR brachytherapy source, which is 1.1% higher than the Monte Carlo value. This method agrees within
0.2% with the N k
Monte Carlo estimate for a heavily filtered x-ray beam (E ave
= 205.6 keV). A simpler averaging of just the calibration factor N k
has been found to improve the accuracy to 0.7% and 0.02% for the medium and the heavily filtered x-ray beams and this also avoids use of the invalid linear extrapolation technique of determining A wall
. The energy dependence of the calibration factor N k
is mainly governed by the wall to air mass energy absorption coefficient ratios rather than by the correction factor for attenuation and scatter in the chamber walls A wall
. An air-kerma weighted averaging technique of the chamber response (Verhaegen et
al. Phys. Med. Biol. 37(11), (1992) 2071-2082) produces an N k
calibration factor that is within 0.02% of the Monte Carlo estimate. If the average is taken over the calibration factor, the agreement is still excellent (0.03%).