Monte Carlo dosimetry of the VariSource high dose rate 192Ir source

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Abstract
The purpose of the work is to calculate basic dosimetry data for a VariSource high dose rate 192Ir source in water. These basic dosimetry data, expressed in the dose calculation formalism endorsed by the Interstitial Collaborative Working Group and AAPM Task group 43, include the dose rate constant, the radial dose function, and the anisotropy function. A modified version of the EGS4 Monte Carlo code was used to calculate (1) the transverse-axis dose distribution at radial distances from 0.1 to 14 cm, (2) the two-dimensional dose distribution for axial and radial distances from 0.1 cm to 10 cm, and (3) the air-kerma strength, for the VariSource high dose rate 192Ir source. From these Monte Carlo results the basic dosimetry data were derived. The calculated dose rate constant for the high dose rate source is 1.044 +/- 0.2% cGy h-1 per unit air-kerma strength. The anisotropy function exhibits 40%-60% deviations from isotropy at positions on the long axis. The radial dose function for the source is nearly identical to that for a microSelectron high dose rate 192Ir source, except at radial distances smaller than 0.5 cm where values for VariSource are 1.7%-2.8% smaller. These basic dosimetry data were compared with corresponding results from other authors for high and low dose rate 192Ir sources, as well as with Meisberger's fitting formula.