Gold nanorods targeting tumor vasculature: Monte Carlo evaluation of the radioenhancement effects

Gold nanoparticles targeted to tumor vasculature can raise the dose to endothelial cells, and the enhancement depends on their position relative to the vessel wall. Despite growing interest in endothelial radiosensitization, intravascular and immediate perivascular dose enhancement has not been resolved at the level of individual endothelial cells for different nanoparticle locations. Earlier vascular dosimetry scored thin wall layers, analytic endothelial slabs, or tissue-region averages. This study quantifies the dose enhancement ratio (DER) of gold nanorods (AuNRs) in individual endothelial cells and in the perivascular region for three AuNR locations. Track-structure Monte Carlo simulations in TOPAS modeled a 10 um long, 16 um diameter microvessel segmented into six endothelial cells. AuNRs were placed homogeneously in the lumen, on the inner vessel wall (vascular-targeted), or on the outer vessel wall (extravasated) at concentrations of 5 to 50 mg Au/g water within the vessel volume under 50 and 100 kVp X-rays. All DERs were corrected for the lack of charged-particle equilibrium (CPE) inherent to microscale simulations. Inner wall-attached AuNRs produced the highest microvessel-averaged DERs, reaching approximately 3.8 at 50 mg Au/g water and approximately 1.96 at 18 mg Au/g water under 50 kVp. The DER increased linearly with AuNR concentration and was higher at 50 kVp than at 100 kVp. Including AuNRs outside the simulated segment (infinite vessel approximation) increased endothelial DERs by 5 to 10 percent. In the perivascular region, the DER was highest within the first 10 um, where outer wall-attached AuNRs gave the highest values. The results establish a physical dose enhancement basis for endothelial and perivascular cells and can guide the design of vascular-targeted radiosensitization.