Combination of US hyperthermia and radiotherapy on a preclinical glioblastoma model

Abstract

In this work the effect of combining ultrasound (US) hyperthermia (HT) with radiotherapy (RT) was investigated. The treatment was applied to a GBM xenograft nude mouse model obtained by injecting $2\times {10}^6$ U87 luc+ cells. The combined treatment group received 6 Gy and HT at ${43}^{\circ }$ for 8 min. The ultrasound field was generated by a closed-loop computationally controlled system, consisting of a High Intensity Focused Ultrasound (HIFU) transducer with centre frequency 3.57 MHz, a power amplifier, a function generator and a MATLAB controller. A mechanical cone adaptor has been designed to use the HIFU beam at a pre-defined post-focal distance. Two thermocouples were placed between the mechanical cone and the mice skin to measure and control the temperature during the HT treatment. Radiotherapy was carried out by using a dedicated small animal image guided radiotherapy system. Measurements of tumor volume performed with a caliper showed good tumor control for the RT-HT group with respect to the RT or control groups for up to 21 days after treatment. The mean value of the normalized (before therapy) tumor volume was almost equal to 0.5 for two weeks after treatment with an increase to 1.5 at sacrifice. The control and HT groups showed a higher value of about 1.5 during the first two weeks and 3.5 at the end of the follow-up period. We concluded that the use of HT as a radiosensitizer can improve the outcome for glioblastoma treatments.

Figure 1

An example of the US transducer positioning (hand-guided) during in vivo HT treatment.

Figure 2

The plot shows an example of measured temperatures during the in vivo HT delivery.

Figure 3

The plot shows the normalized tumor volume measured at different time points after treatment. Statistical analysis between the RT and RT-HT groups has been performed using the Mann–Whitney test. A statistically significant difference ($p<0.05$) has been found from ten days after treatment.

Figure 4

The plot shows the total flux photons/seconds (p/s) values of a ROI placed over the tumor. Bioluminescence imaging was performed at different time points. A statistically significant difference ($p<0.05$) has been found for day 7 and 14 after treatment.

Figure 5

Immunohistochemical analysis show that Ki67 expression is higher in the untreated tumor (CTRL) and in the tumor treated with radiotherapy (RT) in comparison to the one treated with radiotherapy and hyperthermia (RT-HT).

Figure 6

Two thermocouples were used to recorded the temperature throughout for the temperature control during the in vivo experiments.

Figure 7

Peak positive pressure in MPa from the transducer, measured 30 mm from the focus, with the cone in place, and projected 15 mm from the focus, where the exposure was performed. For the measurements, the output power of the transducer was set at 4.2 W.

Figure 8

The image shows the two beams setup and dose distribution calculated using a Monte Carlo-based treatment planning system (SmART-ATP). The dose volume histogram is shown on the top left box.