Characterizing the Variations of Dose Delivery to Coronary Vessels for Sr-90/Y-90 IVBT Using Monte Carlo Simulations

Purpose: To quantify using Monte Carlo simulations the variations of dose delivery to coronary vessels undergoing treatment by Sr-90/Y-90 intravascular brachytherapy (IVBT). Material and Methods: The Novoste Beta-Cath 3.5 F Sr-90/Y-90 pure beta-emitting source and geometry were modeled in GATE, open-source Monte Carlo software based on the Geant4 toolkit. Source spectrum through the stainless-steel capsule and flexible jacket were verified by comparing the simulated percent depth dose curve with the manufacturer’s data sheet. Dose distribution to the nominal vessel geometry (3.2 mm in diameter, 1.1 mm wall thickness, no stent) were calculated, and the variations of dose delivery were characterized as the dose ratio with and without perturbations due to implanted metallic stents, calcified plaques, variable vessel diameter, and sources offset from center. Results: A dose reduction of up to 26% was found throughout the vessel due to the presence of a stent, dependent on stent material, metallic surface area, and strut thickness. The introduction of calcified plaque further reduced vessel doses by up to 40%. The doses given to the smallest vessels in their prescription category were 34% higher than those to the largest vessels. Random motion of the source from excess space inside the 3.5 F catheter leads to an up to 18% increase in dose depending on the size of the vessel. Conclusion: The dose effects from these often-unavoidable perturbations in the nominal vessel geometry are found to be significant and attempts should be made to account for these effects in accurate dose calculations. The authors recommend a refining of the current prescriptions based on vessel diameter, so the largest and smallest vessels have <20% dose variation.