The aim of this study was to develop the calculation algorithm of source strength of Ir-192 source in terms of the absorbed dose to water instead of an apparent activity (Ci). For this work the Multi Purpose Brachytherapy Phantom(MPBP) was developed, which was designed to locate the source and the chamber precisely at a specific position inside the water phantom. The reference point of measurement was set at the 5 cm distance along the transverse axis of the source. For a brachytherapy source calibration, the absorbed dose to water calibration factor (N_D,W,Q) of an ionization chamber were determined and then apply standard protocols of absorbed dose to water. The calibration factor (N_D.W,Q) of the ion chamber (TM30013, PTW, Germany) was determined using the EGSnrcCPP Monte Carlo Code. The calculated calibration factor (N_D,W,Q) was 5.28 cGy/nC. The calculated factor was then used to determine the absorbed dose to water from which the air kerma strength for an Ir-192 source can be easily derived at the reference point (5 cm). The calculated air kerma strength showed discrepancies of -0.6% to +1.8% relative to the air kerma strength provided by the vendor. In this work we demonstrated that the air kerma strength (S_k) could be determined from the absorbed dose to water calibration factor for Ir-192 source. In addition, this source calibration method could be applied directly to the dose calculation formalism of AAPM report TG-43.

KeywordsAir-kerma strength, Apparent activity, Algorithm, Calibration factor