Beta versus gamma for catheter-based intravascular brachytherapy: dosimetric perspectives in the presence of metallic stents and calcified plaques

Purpose: Both β and γ emitters are currently used in the catheter-based intravascular brachytherapy. The dosimetric effects due to the presence of metallic stents and calcified plaques have not been fully addressed. This work compares these effects for two most commonly used β and γ sources ( 90Sr and 192Ir). Materials and Methods: An EGS4 Monte Carlo package was used to calculate dose in water for a 90Sr (supplied by NOVOSTE) and an 192Ir (Supplied by BEST) source, with or without the presence of a calcified plaque or a metallic stent. Plaques of different shape (shell and disk), size and density, and two types of stainless-steel stents (ring or mesh stent) were studied. The ring stent consists of identical rings stacked along the long axis of the sources. The gap between two rings is 0.3 mm. The mesh stents are made of identical square (0.1 x 0.1 or 0.2 x 0.2 mm2) holes separated from each other by stainless-steel wire. The cross section of wire for both ring and mesh stents is 0.1 x 0.1 mm2. A dose perturbation factor (DPF), defined as the ratio of the doses with and without the presence of a plaque or a stent, was introduced to quantify the effects. A carefully chosen set of EGS4 transport parameters for the small geometry in question was used in the calculation. Results: The radial and axial dose distributions calculated in water were found to agree with the published measurements to within 3%. The dose perturbations due to the presence of calcified plaques or metallic stents were found far more significant for the 90Sr source than those for the 192Ir source. Up to 30% dose reduction behind a plaque were observed for the 90Sr source, while the dose reduction for the 192Ir source was found to be negligible. The dose enhancement inside a plaque was as high as 10% for the β source or 6% for the γ source. In the presence of a stent, the DPF was in the range of 1.15-0.75 for the β source, while it was almost equal to 1.0 for the γ source. Conclusion: The dose perturbation due to the presence of a calcified plaque or a metallic stent is significant for the β source. The dose reduction in the region beyond a plaque or a stent could be more than 20%. For the γ source, the dose effect behind a plaque or a stent is practically negligible. These dosimetric differences between the β and γ sources in the presence of a calcified plaque or metallic stent should be considered in the dose prescription of intravascular brachytherapy