Microbeam

Gold nanoparticles (GNPs) have been demonstrated as effective radiosensitizing agents in a range of preclinical models using broad field sources of various energies. This study aimed to distinguish between these mechanisms by applying subcellular targeting using a soft X-ray microbeam in combination with GNPs. DNA damage and repair kinetics were determined following nuclear and cytoplasmic irradiation using a soft X-ray (carbon K-shell, 278 eV) microbeam in MDA-MB-231 breast cancer and AG01522 fibroblast cells with and without GNPs. To investigate the mechanism of the GNP induced radiosensitization, GNP-induced mitochondrial depolarisation was quantified by TMRE staining, and levels of DNA damage were compared in cells with depolarised and functional mitochondria. Differential effects were observed following radiation exposure between the two cell lines. These findings were validated 24 hours after removal of GNPs by flow cytometry analysis of mitochondrial depolarisation. This study provides further evidence that GNP radiosensitisation is mediated by mitochondrial function and it is the first report applying a soft X-ray microbeam to study the radiobiological effects of GNPs to enable the separation of physical and biological effects. Radiotherapy has rapidly progressed in recent decades and has become one of the most therapeutically and cost effective tools in cancer treatment. While the key driver of radiotherapy developments is the delivery of improved physical dose distributions 1 , there is increasing interest in the combination of sophisticated dose delivery techniques with new imaging tools introducing new concepts as 'biological target volume' , 'molecular imaging' and 'theranostics' to radiobiologically targeted therapy 2. Interest in the use of radiotherapy contrast agents was stimulated by early studies finding elevated levels of damage in tissues after contrast enhanced medical imaging, indicating that the presence of a high-Z material can increase radiation damage 3. This is attributed to the high photoelectric cross-section of these materials which means that high Z materials absorb substantially more energy per unit mass than soft tissue (between 10–150 times for kV photons) and which translates to an increase in local dose 4. While these dose enhancing effects were undesirable in imaging, there was interest in applying them to improve tumour cell killing in therapy. Early attempts were made to achieve radiation dose enhancements using gold (Z = 79) in the form of foil or microspheres 5 as pioneering work for the use of gold as a radiosensitisers, but this was limited by delivery challenges. However, nanoparticles (NPs) have been shown 6 to inherently become trapped in tumour tissues due to the poorly formed leaky tumour vasculature allowing nanoparticles to pass into the tumour volume and become internalised. This specific feature gives Gold Nanoparticles (GNPs) the potential to be used as tumour-specific radiosensitisers either directly 6 or by modifying them with tumour-targeting antibodies 7. The effectiveness of these particles was initially explained purely in terms of dose-modifying effects. Previous work 8 has calculated the amount of GNPs required within a tumour to significantly enhance dose deposition based on the ratio of the mass energy absorption coefficients of gold and soft tissue. Calculations have suggested delivering 1% of gold by mass to the tumour would mean the doubling of the amount of energy deposited

In order to study the radiobiological effects of low dose radiation, microbeam irradiation facilities have been developed in the world. This type of facilities now becomes an essential tool for studying bystander effects and relating signaling phenomena in cells or tissues. This review introduces you available microbeam facilities in Japan and in China, to promote radiobiology using microbeam probe and to encourage collaborative research between radiobiologists interested in using microbeam in Japan and in China.