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ABSTRACT
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 192lr . This work has been carried out to calibrate the high dose rate (HDR) 192lr sources. Materials and Methods: An interpolation procedure, using calibrations above 1.25 MeV (60Co), and below 135 kV X-rays (61.1 keV), the exposure-weighted average energy 397 keV of 192lr is employed. Using Farmer ionization chamber HDR 192lr 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.
Keywords: Brachytherapy, calibration, ionization chamber, HDR.
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INTRODUCTION
High dose rate remote afterloading brachytherapy equipment, like Gamma Med plus manufactured by Varian Medical systems, are becoming more popular in Pakistan. At present there are six Gamma Med plus and one Varisource HDR system in Pakistan. The radionuclide used in these systems is 192Ir. The initial activity of these sources is round about 10 Ci. The half-life of 192Ir is 83.831 days. The short half-life of 192Ir HDR source requires changing the source at every interval of four months d-3). For brachytherapy sources vendors assign large uncertainties to their stated calibration values up to ± 10%. Independent verification as well as exact measurement of source strength is quite significant in order to assure the quality of brachytherapy treatment. To quote the report of American Association of Physicists in Medicine (AAPM) Task Group-40 M, "Each institution planning to...