Dose characterization in the near-source region for two high dose rate brachytherapy sources.

High dose rate (HDR) 192Ir sources are currently used in intravascular brachytherapy (IVB) for the peripheral arterial system. This poses a demand on evaluating accurate dose parameters in the near-source region for such sources. The purpose of this work is to calculate the dose parameters for the old VariSource HDR 192Ir source and the new microSelectron HDR 192Ir source, using Monte Carlo electron and photon transport simulation. The two-dimensional (2D) dose rate distributions and the air kerma strengths for the two HDR sources were calculated by EGSnrc and EGS4 Monte Carlo codes. Based on these data, the dose parameters proposed in the AAPM TG-60 protocol were derived. The dose rate constants obtained are 13.119+/-0.028 cGy h(-1) U(-1) for the old VariSource source, and 22.751+/-0.031 cGy h(-1) U(-1) for the new microSelectron source at the reference point (r0 = 2 mm, theta = pi/2). The 2D dose rate distributions, the radial dose functions, and the anisotropy functions presented for the two sources cover radial distances ranging from 0.5 to 10 mm. In the near-source region on the transverse plane, the dose effects of the charged particle nonequilibrium and the beta-particle dose contribution were studied. It is found that at radial distances ranging from 0.5 to 2 mm, these effects increase the calculated dose rates by up to 29% for the old VariSource source, and by up to 12% for the new microSelectron source, which, in turn, change values of the radial dose function and the anisotropy function. The present dose parameters, which account for the charged particle nonequilibrium and the beta particle contribution, may be used for accurate IVB dose calculation.