Radiosensitization of Allogenic Subcutaneous C6 Glioma Model with Focused Ultrasound-Induced Mild Hyperthermia

Abstract

The radiosensitization potential of focused ultrasound (FUS)-induced mild hyperthermia was assessed in an allogenic subcutaneous C6 glioma tumor model in rats. Mild hyperthermia at 42 °C was induced in tumors using a single-element 350 kHz FUS transducer. Radiation was delivered with a small animal radiation research platform using a single-beam irradiation technique. The combined treatment involved 20 min of FUS hyperthermia immediately before radiation. Tumor growth changes were observed one week post-treatment. A radiation dose of 2 Gy alone showed limited tumor control (30% reduction). However, when combined with FUS hyperthermia, there was a significant reduction in tumor growth compared to other treatments (tumor volumes: control-1174 ± 554 mm³, FUS-HT-1483 ± 702 mm³, 2 Gy-609 ± 300 mm³, FUS-HT + 2 Gy-259 ± 186 mm³; ANOVA p < 0.00001). Immunohistological analysis suggested increased DNA damage as a short-term mechanism for tumor control in the combined treatment. In conclusion, FUS-induced mild hyperthermia can enhance the effectiveness of radiation in a glioma tumor model, potentially improving the outcome of standard radiation treatments for better tumor control.

Keywords: focused ultrasound; glioma; hyperthermia; radiosensitization; radiotherapy.

Figure 1

Numerical simulation of FUS propagation and the resulting temperature increase in tumors. (A) Left: simulation model composed of water, a skin layer (0.8 mm thick), a tumor (5 mm diameter), and soft tissues, with a superposition of the k-Wave simulation of acoustic pressure field from the focused 350 kHz transducer emitting a 1 MPa peak pressure in CW at focus, used as a heat source in BHTE simulations. Right: simulated pressure along the acoustical axis of the transducer, with dashed lines delimiting skin and tumor boundaries. (B) Temperature regulation in a tumor using the PID controller. For this proof-of-concept experiment, temperature was maintained at 42 °C for 5 min. (C) Left: simulated temperature distribution in the tumor. Right: simulated temperature along the acoustical axis of the transducer. (D) Simulated temperature over time at the center of the tumor, with an ON/OFF command of the transducer once the target temperature of 42 °C is reached. For these simulations, the values listed in Table 1 were used. (E). Left: estimated CEM43 distribution in the tumor. Right: estimated CEM43 along the acoustical axis of the transducer. (F) Image of the experimental set-up, including the transducer, the PID controller, and the thermocouple.

Figure 2

Allogenic subcutaneous C6 tumor models response to radiotherapy. Tumors were treated on day 5 post-implantation with either sham treatment (Control) or a single dose of 2 Gy or 5 Gy. At one week post-treatment, the mean tumor volumes were 1657 ± 397 mm³, 1160 ± 545 mm^3, and 595 ± 259 mm³ for the control (untreated, in blue), 2 Gy (in green) and 5 Gy (in red) groups, respectively. n = 3 per group. * indicate a p < 0.05.

Figure 3

Subcutaneous C6 tumor models response to RT (2 Gy), FUS hyperthermia (FUS-HT) or a combination of FUS-RT and RT. Tumors were treated on day 5 post-implantation, and they were monitored over the course of one week post-treatment. N = 8 to 10 per group. There was no significant difference between tumor volumes on the treatment day (mean tumor volumes on treatment day 108 ± 56 mm³ for control, 101 ± 45 mm³ for 2 Gy, 134 ± 48 mm³ for FUS-HT, 100 ± 36 for FUS-HT + 2 Gy, ANOVA p value = 0.45). * denotes p < 0.05.

Figure 4

Representative images of immunohistochemistry of tumors 24 h post-treatment. DNA double-strand break damage was detected by H2AX (γ-H2AX) immunohistochemistry.