A technique for calibrating a high dose rate 192Ir brachytherapy source

The reference air kerma rate of an Ir-192 high dose rate brachytherapy source is determined based broadly on the International Atomic Energy Agency (IAEA) TECDOC 1274 code of practice. Since the primary standards dosimetry laboratory at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) does not maintain a standard at Ir-192 quality, the air kerma calibration coefficient of an IBA FC65-G Farmer type ionisation chamber is calculated using coefficients determined at 300 kV and Co-60 qualities. The methodology proposed by Mainegra-Hing and Rogers [1] at 250 kV and Cs-137 qualities is used. The validity of this approach is tested by performing Monte Carlo simulations to determine the chamber's air kerma calibration coefficient at Ir-192 quality. Very good agreement is obtained between values using these two methods. The reference air kerma rate is measured using the Farmer chamber in an in air jig. In addition the necessary correction factors are applied to the measured value. The reference air kerma rate determined in this way is compared to the value stated by the vendor of the Ir-192 source on the source calibration certificate. Differences are with one exception less than 1%. It is concluded that because of the agreement between the values from the methodology used in this study and the source calibration certificate values this methodology can be used clinically.