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

Purpose: Both beta and gamma emitters are currently used in the catheter-ba sed intravascular brachytherapy, The dosimetric effects due to the presence of metallic stents and calcified plaques have not been fully addressed. Th is work compares these effects for two most commonly used beta and gamma so urces (Sr-90 and Ir-192), Materials and Methods: An EGS4 Monte Carlo package was used to calculate do se in water for a Sr-90 (supplied by NOVOSTE) and an Ir-192 (Supplied by BE ST) source, with or without the presence of a calcified plaque or a metalli c 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 i dentical square (0.1 x 0.1 or 0.2 x 0.2 mm(2)) holes separated from each ot her by stainless-steel wire. The cross section of wire for both ring and me sh stents is 0.1 x 0.1 mm(2). 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 tr ansport parameters for the small geometry in question,vas used in the calcu lation Results: The radial and axial dose distributions calculated in,vater were f ound to agree with the published measurements to within 3%, The dose pertur bations due to the presence of calcified plaques or metallic stents were fo und far more significant for the Sr-90 source than those for the Ir-192 sou rce, Up to 30% dose reduction behind a plaque were observed for the 90Sr so urce, while the dose reduction for the Ir-192 source was found to be neglig ible. The dose enhancement inside a plaque was as high as 10% for the beta source or 6% for the gamma source, In the presence of a stent, the DPF was in the range of 1.15-0.75 for the beta source, while it was almost equal to 1.0 for the gamma source. Conclusion: The dose perturbation due to the presence of a calcified plaque or a metallic stent is significant for the beta source. The dose reduction in the region beyond a plaque or a stent could be more than 20%, For the g amma source, the dose effect behind a plaque or a stent is practically negl igible. These dosimetric differences between the beta and gamma sources in the presence of a calcified plaque or metallic stent should be considered i n the dose prescription of intravascular brachytherapy, (C) 2000 Elsevier S cience Inc.