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

The reference air kerma rate of an ¹⁹²Ir 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 ¹⁹²Ir quality, the air kerma calibration coefficient of an IBA FC65-G Farmer type ionisation chamber is calculated using coefficients determined at 300 kV and ⁶⁰Co qualities. The methodology proposed by Mainegra-Hing and Rogers [1] at 250 kV and ¹³⁷Cs 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 ¹⁹²Ir 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 ¹⁹²Ir 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.