Treatment of murine tumors using dual-frequency ultrasound in an experimental in vivo model

Acoustic inertia cavitation is the primary mechanism underlying sonochemical reactions and has potential for use in tumor treatment. In in vitro experiments that were performed previously and are thus not included in this paper, we found that the ultrasonically-induced chemical reactions are greatly accelerated when ultrasound is simultaneously applied at frequencies of 1 MHz and 150 kHz.. In this study, the in vivo anti-tumor effect of the simultaneous dual-frequency ultrasound at low level intensity (I(SPTA) <6 W/cm(2)) was investigated in a murine model of breast adenocarcinoma in Balb/c mice. The tumor-bearing mice were divided into five groups: those treated with combined dual-frequency ultrasound in continuous mode (1 MHz(con)+150 kH(zcon)) for 30 and 15 min (C and D), those treated with dual-frequency ultrasound in which the source of 1 MHz was in pulse mode (duty cycle of 80%) and that of 150 kHz was in continuous mode for 30 min (E), and untreated control and sham groups (A and B). The tumor growth parameters evaluated to assess delay include tumor volume, relative tumor volume, and T(5) and T(2), which are the time needed for each tumor to reach 5 and 2 times its initial volume, respectively. The survival period and percent of tumor growth inhibition ratio and were measured at various times after treatment. The results show that treatment with a combined continuous mode of 1 MHz(con)+150 kHz(con) and a pulse mode of 1 MHz(pl,80%)+150 kHz(con) effectively delayed tumor growth and increased the tumor growth inhibitory ratio compared to the sham group. When the tumor volume growth and relative volume of tumors in treated groups C, D and E were examined, an anti-tumor effect was observed in groups E and C. There is a significant difference between groups E and C and the sham group 12 d after treatment for tumor volume growth and 18 d after treatment for relative tumor volume (p < 0.05). The mean survival periods for animals in groups C and E were 16% and 17% more than the control group. T(5) and T(2) (in days) of groups E, C and B are 19.1 +/- 0.4 and 7.6 +/- 1.5, 17.2 +/- 0.6 and 6.3 +/- 2.1, 14.3 +/- 1.1 and 5.1 +/- 2.5 d, respectively. There was a significant difference between groups C and E and the sham group (B) for T(5) (p < 0.05), but for T(2), there was no significant difference between groups (p > 0.05). The tumor growth inhibition ratio in groups C, D and E was 23, 20 and 37% of that in group B, and those differences are statistically significant (p <0.05). It is concluded that sonodynamic therapy with combined dual-frequency ultrasound in a progressive wave mode can be useful for cancer therapy.