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-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 beta and gamma sources ( (90)Sr and (192)Ir).

MATERIALS AND METHODS

An EGS4 Monte Carlo package was used to calculate dose in water for a (90)Sr (supplied by NOVOSTE) and an (192)Ir (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 mm(2)) 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 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 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 (90)Sr source than those for the (192)Ir source. Up to 30% dose reduction behind a plaque were observed for the (90)Sr source, while the dose reduction for the (192)Ir source was found to be negligible. 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 gamma source, the dose effect behind a plaque or a stent is practically negligible. These dosimetric differences between the beta and gamma sources in the presence of a calcified plaque or metallic stent should be considered in the dose prescription of intravascular brachytherapy.