Comparison of Radiation Dosimetry Effects of Metallic Biliary Stents

Abstract

Background and aims: Radiotherapy is utilized as treatment for hepatobiliary malignancies. This treatment is often done in conjunction with endoscopic stenting to relieve biliary obstruction. It is unclear how the different brands of metallic biliary stents may affect the local delivery of radiation. This study was designed to use the most accurate current dosimetry algorithm to evaluate the effect of biliary stents on radiation delivery. Methods: The EGS4/BEAM Monte Carlo system was modified and employed to calculate dose in water for a 192Ir HDR source and external beam photon irradiation, with or without the presence of a biliary stent. This technique was previously used to evaluate a single type of stent in our center. A dose pertubation factor (DPF), defined as the ratio of the doses with and without the presence of a stent, was introduced to quantify the effects. Dose enhancement is observed if the value of DPF is greater than 1.0. Three metal stents including a Luminex (Bard, Billerica, MA), Wallstent (Boston Scientific, Natick, MA), and Zilver (Wilson-Cook, Winston-Salem, NC) and one plastic stent, were studied. The metal stents were 10 mm diameter x 60 mm length. The plastic stent was a 10 Fr-7 cm prosthesis. The 192Ir source was placed along the central axis of the stents and dosimetry effect recorded. Calculation was also performed for 6 MV and 18 MV external photon beams. Results: For external beams, up to a 2% dose enhancement was observed in the upstream region (in front of the stent), while up to a 3 % dose reduction was seen in the downstream region (behind the stent) for all metal stents studied. For the 192Ir and the three metallic stents, there was a dose enhancement of a few percent (up to 7%) within 0.5 mm of the stent. There was negligible difference between the different brands of stents. For the plastic stent, dose effect can be ignored. Conclusions: Our model is the first to simultaneously evaluate these metal stents and apply the most accurate and highest resolution dosimetry techniques. This data would suggest that local radiation effect is enhanced by metal biliary stents. This could be considered when calculating doses for brachytherapy and upstream doses for external beam radiation. Response to radiation therapy in patients with metal stents should be clinically evaluated.