Seminar: Photoacoustic monitoring of a nanobubble-enhanced radiation treatment to understand the effects of delayed nanobubble stimulation and subsequent delayed radiation delivery

Student: Helen Melino

Supervisor: Dr. Michael Kolios

Abstract

While tumour cell death is a goal of radiation therapy, it can be challenging to limit the radiation-induced effects to the tumour microenvironment (TME). Over the past 15 years, researchers have developed a combination of localized ultrasound (US) stimulation of lipid-shelled microbubbles (MBs) followed by radiation, as both treatments have been shown to induce endothelial cell dysfunction, ultimately leading to tumour cell death. This combination has increased the effectiveness of radiation therapy by 10-fold in vitro and is currently being used in clinical trials. Despite this success, the size of MBs (~1-8μm) limits their therapeutic effects to the vasculature. To overcome this, the current work proposes nanobubbles (NBs) as an alternative to MBs, as their small size (<600 nm) enables extravasation from the vasculature into the TME.

In a recently completed in vivo study, mice with solid prostate cancer tumours were treated by injection with NBs followed by US stimulation and subsequent single-dose radiation therapy. Variations in the time between NB injection and US stimulation (T1), time between US stimulation and radiation therapy (T2), and radiation dose (0 or 2 Gy) were applied to a total of 103 mice (4-6 mice/treatment combination). Variations in T1 are expected to affect the NB distribution throughout the tumour microenvironment. In contrast, variations in T2 are associated with the time required for downstream damage-associated molecular pathways, leading to the greatest changes in radiosensitivity. Radiofrequency (RF) data in both B-mode and photoacoustic (PA) modes were collected from tumours pre- and post-treatment. PA data were collected at 750 and 850 nm to assess treatment effectiveness by measuring changes in vascular integrity through quantitative PA (QPA) and spectral unmixing. QPA parameters include the midband fit (MBF) and spectral slope (SS), while spectral unmixing measures tumour oxygenation. These features are used to relate changes in the tumour blood to detectable PA biomarkers resulting from the combined treatment.

From this dataset, the values for non-zero T1 (10 or 40 min) were used in combination with varied non-zero T2 (1, 3, or 6 h) to understand whether an increased time delay for radiation delivery (T2) would cause greater tumour microenvironment remodeling. Changes in the PA signal at a radiation dose of 2 Gy were the most significant relative to the control groups for the combinations T1 = 10 min + T2 = 1 h and T1 = 40 min + T2 = 3 h. It was also found that the treatment’s effects on the PA signal depend on T1 when T2 > 0 h. Additionally, spectral unmixing of the PA data revealed that for the above treatments, the total blood flow through tumours increased 24 h after treatment while most other treatments demonstrated a decrease in total tumour blood. These results suggest the activation of damage-associated molecular patterns and the degree of vascular restructuring depend on the distribution of NBs within the tumour at the time of US activation, resulting in different lag times required to reach peak radiosensitivity.