Calibration of iridium-192 source by ionization chamber for high dose rate brachytherapy

Background: The effectiveness and safety of brachytherapy treatment is mainly concerned with the calibration of sources and their traceability to internationally accepted Standards. Secondary Standard Dosimetry Laboratory (SSDL) does not offer calibration of ionization chambers with gamma-ray spectrum of high dose rate source Ir-192. This work has been carried out to calibrate the high dose rate (HDR) Ir-192 sources. Materials and Methods: An interpolation procedure, using calibrations above 1.25 MeV (Co-60), and below 135 kV X-rays (61.1 keV), the exposure-weighted average energy 397 keV of Ir-192 is employed. Using Farmer ionization chamber HDR Ir-192 source has been calibrated by free in-air measurement technique and then the calibrated source has been used to calibrate well-type ionization chamber. Results: Difference between in-air measurement and that of manufacturer's remained within +/- 3%, and the difference between in-air measurement and well-type chamber remained within +/- 1%. Comparison between well-type measurements and manufacturer values shows differences less than +/- 2%. All these differences are within the acceptable tolerance limits. Conclusion: The method presented shows good degree of accuracy so that the traceability of calibration is assured. Installation of a new brachytherapy source needs to be calibrated by in-air measurement technique and then this calibrated source should be used for well chamber calibration. Brachytherapy community is suggested to calibrate their sources with this method.