Self-absorption correction for 32P, 198Au and 188Re stents: dose point kernel calculations versus Monte Carlo.

Abstract

Monte Carlo simulations of dose distributions around radioactive stents are very time intensive. Thus, in order to calculate the dose distribution around a 188Re stent, we chose to test a point kernel method, a method which is known to be faster but the accuracy of which has not been established for this application. The dose point kernel method, which takes into account absorption in the strut material (=self-absorption), was based on different beta-emitting point source distributions in water by itself and surrounded by steel spheres of different thicknesses. This information was input into an integration routine that modeled either a Palmaz-Schatz or Multilink stent. The dose distributions around 198Au and 32P stents calculated with the dose point kernel method were compared to those calculated using EGS4 and MCNP 4B Monte Carlo methods. The resulting correction for self-absorption in steel was distance dependent and averaged 1.12 for 32P and 1.25 for 198Au stents. The dose point kernel method gave nearly identical results to these full Monte Carlo simulations and was thus used to calculate the dose distributions around a 188Re stent. Although 188Re has a half-life of only 17 hours, it is posited to be useful for radioactive restenosis prevention, given that a recently developed rapid electrodeposition procedure allows stents to be made radioactive, at predetermined activities, within 15 minutes. The dose point kernel calculations of a 188Re-coated Multilink stent were compared to its radiochromic film measurements. The dose fall-off agreed with the calculations within 5% over 0.4 to 3.5 mm from the stent surface. The dose point kernel method is a valuable tool to determine depth dose distributions around activated stents taking into account the detailed geometry and the self-absorption in the struts. It not only requires much less processing time than Monte Carlo methods, but also allows the use of higher resolutions in modeling the geometry, which leads to more accurate self-absorption correction factors.