Dose rate constants for 125I, 103Pd, 192Ir and 169Yb brachytherapy sources:: an EGS4 Monte Carlo study

An exhaustive revision of dosimetry data for Ir-192, I-125, Pd-103 and Yb-169 brachytherapy sources has been performed by means of the EGS4 simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. The absorbed dose rate per unit contained activity in a medium at 1 cm in water and air-kerma strength per unit contained activity for each seed model were calculated, allowing the dose rate constant (DRC) A to be estimated. The influence of the calibration procedure on source strength for low-energy brachytherapy seeds is discussed. Conversion factors for I-125 and Pd-103 seeds to obtain the dose rate in liquid water from the dose rate measured in a solid water phantom with a detector calibrated for dose to water were calculated. A theoretical estimate of the DRC for a Pd-103 model 200 seed equal to 0.669 +/- 0.002 cGy h(-1) U-1 is obtained. Comparison of obtained DRCs with measured and calculated published results shows agreement within 1.5% for Ir-192, Yb-169 and I-125 sources.