## 1 Introduction  
Worldwide, millions of people are diagnosed with cancer each year, and roughly every sixth death in the world is due to cancer [1]. In fact, cancer is responsible for 30% of all death in Canada, making it the number one leading cause of death in the nation [1]. According to the Public Health Agency of Canada, in 2015, about 2.1 million Canadians aged 12 years and older have been diagnosed with cancer at some point during their lifetime. This statistic has increased to 20%, approximately 1 in 5 Canadians, to be diagnosed with cancer by 70 years of age, making cancer the leading cause of death in Canada [1]. As the nation's population increases and the average life expectancy improves with the rise of new technologies, the number of cancer cases and deaths are becoming more prominent and expected to continue to grow over the coming decades [2]. Radiotherapy is one of the most common types of cancer treatment. The primary purpose of radiotherapy is to kill cancer cells by irradiating a specific dose of ionizing radiation onto the target tumour [3]. The radiation can either damage the cancer cells directly or indirectly at the deoxyribonucle acid (DNA) level. The most apparent consequence of the practice is unwanted damage to the surrounding healthy cells, causing side effects that are intensely unpleasant for the patient. Therefore, during treatment planning, limited radiation doses are given to minimize the undesirable radiation effects. Such limitation lowers the overall effectiveness of radiotherapy. Recently, with recent advances of heavy atom radiosensitizers as an effective dose enhancer for the tumour and contrast agent in medical imaging, a maximum radiation dose can be delivered to the tumour while keeping the dose at the surrounding normal tissue to a minimum [4, 5, 6]. Heavy atom radiosensitizers are adjunctive agents that make cancer cells more susceptible to radiation,designed to improve cancer cell killing while having much less of an effect on the surrounding healthy tissue [7]. Nanoparticles are defined as particles with a radius dimension of about between 1 and 100 nm. In many studies, gold nanoparticles have proven to be an excellent material for radiosentizing. This is due to the high atomic number (\(Z=79\)) of a gold nanoparticle, which can absorb more low-energy radiation (e.g., kV photon) than soft tissue, its biocompatibility, and it being easy to manufacture and be delivered to the patient [8; 9]. Since tumour requires large amounts of blood vessels to proliferate rapidly, cancer cells stimulate angiogenesis. As the tumour vasculature is leaky, it enables nanoparticles injected in the bloodstream to have an easier time entering the tumour tissues. This process can be explained by using the enhanced permeability and retention effect, which is the property by which molecules of certain sizes (usually liposomes, nanoparticles and macromolecular drugs) accumulate in tumour tissue much more than they do with normal tissues [10]. Once the nanoparticles are delivered into the tumour volume, they absorb high-energy radiation and emit secondary electrons through interactions between radiation and the cancer cell medium [11]. These additional electrons then damage the tumour DNA alongside ionizing radiation, adding more dose deposition at the tumour cells. To date, gold nanoparticle-enhanced radiotherapy has become a new cancer treatment modality with many dosimetry, preclinical and clinical studies published [12; 13; 14; 15; 16; 17]. Radiotherapy uses high-energy ionizing radiation or particles to damage cancer cells at the DNA scale to stop them from growing and dividing [18]. Depending on its ability to ionize matter such as the DNA strand, ionizing particles can be divided into two categories, namely, directly ionizing and indirectly ionizing particles. For directly ionizing particles, they deposit their energy in the tissue through direct Coulomb interactions between the charged particle and orbital electrons of atoms in the DNA molecules.