A Comparative Study of Biological Effects of Electrons and Co-60 Gamma Rays on pBR322 Plasmid DNA

We investigate the damage caused by 6 – 15 MeV electrons to pBR322 plasmid DNA and compare the break yield to that of Co-60 gamma rays to develop an understanding of the mechanisms of electron-induced DNA damage. Plasmids were chosen to allow for observation of DNA damage in isolation – unlike cells, plasmids have no repair mechanisms, so any damage remains unrepaired. We outline the set-up, analysis and results of plasmid irradiation experiments carried out at the Dalton Cumbrian Facility (UK) and the Christie NHS Foundation Trust (UK) in Feb and Apr 2019 respectively. The double-strand break (DSB) yield of each modality was determined to compare the efficacy of electrons to that of gamma rays with respect to DNA damage. INTRODUCTION AND AIMS A recent study by Cancer Research UK indicates that one in two people born after 1960 will suffer from cancer during their lifetime [1]. Radiotherapy in the UK is primarily carried out using 12 MV photons, with 40% of patients receiving radiotherapy as part of their treatment [2] typically in combination with surgery and/or chemotherapy. Developments in high-gradient linear accelerators [3-5] could allow Very High Energy Electron (VHEE) therapy (involving the use of 50-250 MeV electrons), to become a viable option for radiotherapy treatment [6,7]. By adapting high-gradient technology, medical linacs with accelerating gradients of ~100 MeV/m could be capable of producing 250 MeV electrons within current treatment facilities. VHEE therapy has several characteristics and potential advantages which make it an exciting area of radiotherapy research. Firstly, VHEEs have the potential to be used for treatment of deep-seated tumours – 200 MeV electrons can penetrate more than 30cm into tissue. Secondly, research at CERN’s CLEAR facility indicates that VHEEs show relative insensitivity to inhomogeneities [8], making them suitable for treatment of heterogeneous regions, e.g. the lung. As electrons are light compared to protons, they are more readily controllable using focusing and steering magnets, allowing rapid delivery. Sub-second treatment delivery could ‘freeze’ physiological motion, resulting in dose delivery with increased accuracy and reduced healthy tissue irradiation. In addition, these high dose rates (in excess of 40 Gy/s) appear to maintain tumour control while reducing radiation toxicity to healthy tissue [9-11]. The primary mechanism behind radiotherapy is DNA damage. Ionising radiation can cause several types of damage to DNA. The most difficult to repair are damages to one or both DNA strands – single-strand breaks (SSBs) and double-strand breaks (DSBs) respectively. If breaks are left unrepaired, or repaired incorrectly, the cell may be unable to function or replicate, potentially resulting in cell death. Here, we investigate DNA damage caused by 6 15 MeV electron irradiation of plasmid DNA. Plasmids are ringlike DNA structures found in bacteria. Plasmids were chosen over cells as they have no repair mechanisms pure DNA damage can be measured, as no DNA strand breaks will be repaired. This work is a prelude to cell irradiations – comparison of cell and plasmid irradiation will indicate the DNA damage repair (DDR) rate of irradiated cells. The resulting DSB yields were then compared with those caused by similar irradiation using Co-60 gamma rays. This study contributes to a wider investigation into the biological effects of VHEE, with the aim of producing a value for the Relative Biological Effectiveness (RBE) of VHEE – RBE is the ratio of doses required by two radiation modalities to cause the same level of biological effect. It is normalised with respect to Co-60 γ RBE. The following section outlines the setup of the plasmid irradiation experiments at the Dalton Cumbrian Facility (DCF) and the Christie. A section detailing the subsequent analysis techniques, mathematical models and method of calculating DSB yields will follow. The paper will conclude with a discussion of the experiment results and their fits to general models and an outline of future plasmid and cell irradiation studies.