Room: Exhibit Hall
Purpose: Using water phantom with a Farmer chamber for calibrating Ir-192 high dose rate (HDR) brachytherapy source without an indigenously designed jig and to determine the accuracy and suitability of this method for routine source calibrations.
Methods: The Ir-192 HDR brachytherapy source (Elekta AB, Sweden) was calibrated using two different procedures, one using the well-type ionization chamber(Standard Imaging Inc, Middleton WI), the result of which were set as a reference. The other procedure was in-air calibration method using a chamber(FC-65G, Scanditronix/Wellhofer) with an empty water phantom(Blue Phantom2, IBA Dosimetry, USA). Firstly, a needle were fixed 5cm up to the chamber on one side of the water phantom. Secondly, the radiation source was sent to the tip of the needle, then the cross-line and in-line profiles were scanned using the scan function of water phantom software. Thirdly, the chamber origin position were reset on the maximum signal position of each profile curve, rescan profiles until the two curves overlap together and centered on the source. Lastly, the reference air kerma rate (RAKR) of the source was determined using the7-distance technique, the ionization chamber was moved downward to various distances (10, 15, 20, 25, 30, 35 and 40 cm) from the origin position, along the Z direction of water phantom. Measurements were repeated ten times over two half-life of the source.
Results: The RAKR determined using the two calibration methods both show good agreement with the manufacturer stated value. The average differences in percentage between the two procedures mentioned above are 1.53 Â± 0.19 %and1.11 Â± 0.30%, respectively.
Conclusion: Using water phantom provides an accurate positioning of the source and chamber without additional indigenously designed source calibration jig for in-air measurements. Methods of calibration discussed in this study are effective to be used for routine calibration purposes.