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

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Abstract
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 source, MicroSelectron high dose rate and pulsed dose rate sources, and a VariSource high dose rate source, as well as five other hypothetical cylindrical 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< 4h, 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 sources is suggested, and similarities and differences in the dose rate constant and radial dose function between these sources are explained.