Ultrasound-Stimulated Microbubbles Enhance Radiation-Induced Cell Killing

Background: There has been recent interest in the use of ultrasound-stimulated microbubbles (USMB) as a localised radiosensitiser, with in vivo studies to date demonstrating impressive results. In vitro results have focussed primarily on Human Umbilical Vein Endothelial Cells (HUVEC), and studies using other cell lines have shown varying results. This study aims to further investigate any increases in radiation-induced cell killing in vitro using HUVECs and two carcinoma cell lines; and to study the effect different radiation energies have on cells treated with ultrasound-stimulated microbubbles and radiation.Methods: HUVEC, Metastatic Follicular Thyroid Carcinoma cells (FTC-238) and Non-Small Cell Lung Carcinoma cells (NCI-H727) were treated using a combination of 1.6% v/v microbubbles (Definity®); ~90 seconds of 2MHz ultrasound with a mechanical index of 0.8; and 0-6Gy of kV or MV x-rays. Cell viability was measured using a cell viability assay 72 hours post-treatment, and normalised survival calculated relative to untreated controls. Statistical significance was determined using a Three-Way Analysis of Variance (ANOVA) after confirming equal variance existed across all groups.Results: All cells treated with combined USMB and radiation demonstrated decreased normalised survival, with statistically significant effects observed for the NCI-H727 cells. Statistically significant differences in the effect of kV and MV radiation were also observed for FTC-238 cells.Conclusions: USMB and Radiation Therapy significantly reduced the viability of NCI-H727 cells when compared to cells exposed to radiation treatment only. Decreased cell viability was also observed in HUVEC and FTC-238 cells, but this was not significant. Further studies using increased Microbubble concentrations may be required to achieve statistically significant results as reported elsewhere. Similarly, further research is needed to understand the differing effect of kV and MV x-rays on the survival of FTC-238 cells.