Influence of source geometry and materials on the transverse axis dosimetry of 192Ir brachytherapy sources

Monte Carlo dose rates on the transverse axis in water and air kerma strengths normalized to unit source activity were calculated for a low dose rate steel-clad 192Ir source. MicroSelectron high dose rate and pulsed dose rate 192Ir sources, and a VariSource high dose rate 192Ir source, as well as five other hypothetical cylindrical 192Ir source designs. Based on these results, the dependence of dose rate and air kerma strength on source geometry and materials was analysed. Source geometry and attenuation in the core material are the important factors determining basic dosimetric characteristics. Core length, h, only affects the dose rate on the transverse axis at radial distances r < 4 h, over which the dose rate decreases nonlinearly with increasing core length. By comparison, core diameter, d, influences the air kerma strength and dose rate at all radial distances; these decrease linearly with increasing core diameter. Based on their dosimetric characteristics, pertinent dosimetry modelling for four actual 192Ir sources is suggested, and similarities and differences in the dose rate constant and radial dose function between these sources are explained.