Investigating Tumour Cell and Vascular Response to High-dose Irradiation Using Intravital Optical Imaging

Stereotactic body radiation therapy (SBRT), which involves conformal delivery of high doses of ionizing radiation, is gaining wider attention as an effective treatment for some solid cancers such as lung and liver cancer. More recently, SBRT has been investigated as a suitable treatment option for locally advanced pancreatic cancer patients. Although encouraging clinical data are emerging, controversy persists as to the underlying radiobiological mechanisms driving the response of solid tumours to SBRT. As clinical experience with SBRT for pancreatic and other cancer matures, it is increasingly important to elucidate the underlying radiobiological mechanisms in order to improve the efficacy of this treatment approach, either alone or in combination with conventional and emerging systemic therapies. Several preclinical studies suggest that the vasculature of solid tumours is significantly affected by high-dose irradiation, making it an important target for SBRT. However, the extent of radiation-induced vascular damage and its impact on the tumour microenvironment and the overall tumour response remain largely unclear. Such information may help to inform clinical practice involving SBRT and potentially guide the design of future SBRT-based clinical trials. The objective of my research project was to investigate the effects of SBRT-modeled high-dose irradiation on tumour cells, their vasculature and the tumour microenvironment in the in vivo setting using a pancreatic tumour model to better understand the mechanisms of action influencing the tumour response to SBRT. For this work, a novel combination of intravital optical microscopy techniques using transparent window chamber (WC) model and a small animal irradiator were developed. The techniques permitted dynamic, real-time, high-resolution imaging and simultaneous tracking of radiation response of fluorescent tumours, their vasculature and the microenvironment in vivo over prolonged periods. The work of this thesis presents the following: 1) development and experimental validation of a new preclinical platform integrating intravital microscopy and small animal microirradiation instrumentation to study radiation response in tumours in vivo, 2) improvement of the tumour model in the WC setup using a human pancreatic BxPC3 cell line, and characterization of normal and tumour vascular structures in the WC model, and 3) experimental results from in vivo studies assessing tumour and vascular response to irradiation doses ranging between 4-24 Gy in the pancreatic tumour model. The studies presented in this thesis revealed that high-dose irradiation causes significant but transient tumour vascular dysfunction leading to immediate modification of the tumour microenvironment by way of increased tumour hypoxia. The results indicate a novel therapeutic opportunity, for example the use of hypoxia-activated agents, which can be exploited as a combination therapy with SBRT to improve therapeutic outcome. This novel therapeutic combinations may be tested in the future using the experimental platform established in this thesis. The results also suggest that there could be clinical value in developing methods to monitor SBRT response in solid tumours, including using real-time vascular and/or hypoxia imaging following treatment, in order to time most appropriately the delivery of treatments for maximum therapeutic efficacy.

Advisor: DaCosta, Ralph
Wilson, Brian