Assessment of Cancer Perfusion with Contrast-Enhanced Ultrasound Imaging and Relationship to Intratumoral Pressure Measures

It is well accepted that some breast cancers have abnormally high intratumoral pressures that physically impede systemic drug treatment efficacy. Owing in part to rapid blood vessel growth (angiogenesis) and lack of a functioning lymphatic system, these conditions lead to a chaotic microvascular network with higher permeability and these increased pressure levels. The objective of this study was to compare in vivo contrast-enhanced ultrasound (CEUS) image-derived measures of tumor perfusion with local intratumoral pressure measurements. Nine female athymic nude mice were implanted with breast cancer cells. Before and after microbubble (MB) dosing, nonlinear CEUS imaging was performed across the largest tumor cross-section. Local direct intratumoral pressure measurements were taken from within the ultrasound image plane and along the tumor periphery with a calibrated pressure catheter. Mice were then injected intravenously with fluorescent-conjugated tomato lectin to label all blood vessels with a patent blood supply. After euthanasia, tumors were excised for ex vivo analysis. Small region-of interests were manually selected from the CEUS images and matched to the locations where the pressure catheter tip was positioned. Time-intensity (TIC) curves were analyzed to calculate the wash-in rate (WIR) and peak enhancement (PE) parameters at each location. Pressure measurements and TIC-derived parameters were compared using linear regression analyses. Of the nine animals studied, 45 different in vivo pressure measurements were successfully performed and compared to CEUS imaging results. It was found that the WIR and PE parameters had an inverse relationship with direct pressure measurements ( $R^{2} > 0.63,\ p ). These results indicate that CEUS-derived perfusion parameters can provide surrogate information on intratumoral pressure levels and prognosis of systemic anticancer drug delivery.