Dosimetric perturbations of linear array of beta-emitter seeds and metallic stent in intravascular brachytherapy.

The radiation treatment with catheter-based beta-emitter sources is under clinical trials to prevent restenosis following interventional coronary procedures. There are still large uncertainties in the dose calculation due to the complicated treatment geometry. We present the Monte Carlo simulations to account for the dosimetric perturbations due to neighboring trained seeds, proximal/distal gold markers, and a stainless steel stent. A catheter-based beta-emitter system is modeled using the Monte Carlo code, MCNP4B. Dose distributions and dose rates are calculated in voxels (0.64x0.64x0.5 mm3) around the long cylindrical trains of 90Sr/Y source with and without the stent (at 1.92 mm from the source axis). For the total activity of 70 mCi (2.59x10(9) Bq), the dose around most of the source length (except for edge seeds and gold markers) varies from 40 to 0.23 cGy/s as the radial distance from the source axis (r) increases from 0.64 to 6.4 mm. At the prescription range of r = 1.5-4.0 mm, the dose gradient is very steep and the contribution of neighboring seeds to the dose is significant. The dose enhancement due to neighboring seeds (the so-called "train effect") varies from 9% to 64% as r increases from 0.64 to 5.2 mm. The doses at r = 2 mm from the last edge seed and the gold marker are about 80% and 40% of that of the nonedge seed (8.7 cGy/s), respectively. The dose enhancement due to the secondary electrons and the primary electrons scattered with the stent is shown to be about 9.3% in the voxel including the stent. However, as r increases beyond the stent (r = 2.0-6.4 mm), the dose is slightly reduced by 4%-12%, compared to that without the stent.