Quantification of dose perturbation by plaque in vascular brachytherapy

Background Dose prescription and reporting in vascular brachytherapy (VBT) is based on the assumption that the vessel wall is water equivalent, which does not consider a possible dose perturbation by plaque. As the extent of this perturbation is unknown, we aimed to quantify dose attenuation by atherosclerotic plaque for beta- and gamma-radiation. Material and methods The dose delivered from Strontium-90/Yttrium-90 (Sr-90/Y) and Iridium-192 (Ir-192) sources with and without human peripheral arteries (Sr-90/Y: n = 38, Ir-192: n = 7) surrounding the respective delivery catheter was determined with radiochromic films. Plaque and vessel wall thickness were measured using light microscopy. From the ratio-attenuated doseunattenuated dose (dose perturbation factor: DPF) we determined averaged attenuation coefficients for atherosclerotic plaque (mu(P)) and the residual part of the vessel wall (mu(W)) by regression analysis based on the function DPF = exp(-mu(P) * plaque thickness -mu(W) * residual wall thickness). Results Attenuation in case of Ir-192 was less than the measurement uncertainties. For beta-radiation correlation was found by discrimination between calcified and noncalcified plaque. Classifying noncalcified plaque as normal arterial tissue, the regression coefficient was r = 0.845 at mu(P)= 0.5356 mm(-1) and mu(W) = 0.0663 mm(-1). Conclusions Vascular brachytherapy with beta radiation in calcified arteries results in significant dose attenuation within the vessel wall, which can be calculated on knowing the vascular morphometry. Thus, plaque thickness should be taken into account in treatment planning and retrospective analyses.